Category Archive Eye & Vision Care

Cranial Nerve Six/Cranial nerve six (CN VI) also known as the abducens nerve, is one of the nerves responsible for the extraocular motor functions of the eye, along with the oculomotor nerve (CN III) and the trochlear nerve (CN IV).

The sixth cranial nerve runs a long course from the brainstem to the lateral rectus muscle. Based on the location of an abnormality, other neurologic structures may be involved with the pathology related to this nerve. Sixth nerve palsy is frequently due to a benign process with full recovery within weeks, yet caution is warranted as it may portend a serious neurologic process. Hence, early diagnosis is often critical for some conditions that present with sixth nerve palsy. This article outlines a simple clinical approach to sixth nerve palsy based on its anatomy.

The abducens (also called abducent) nerve (CN VI) is the last of the three visual-motor nerves. It is a brainstem structure that is located in the dorsal aspect of the pons, deep to the facial colliculus in the 4th ventricle (rhomboid fossa). As the fibers of CN VI emerge from the pons, they travel ventrally, to leave the brain parenchyma. It eventually arrives at the preoptic region where it will innervate the lateral rectus muscle, which arises from the third preoptic myotome.

Structure of Cranial Nerve Six

Unlike the oculomotor nerve and the trochlear nerve, the abducens nerve is a purely somatic nerve, meaning the nerve has no sensory function. Its main function is to carry general somatic efferent nerve axons to innervate the lateral rectus muscle which then abducts the eye on the ipsilateral side. It is also secondarily involved in innervation of the contralateral medial rectus muscle by way of the medial longitudinal fasciculus so that both eyes move laterally in a coordinated manner.[rx][rx][rx]

The abducens nerve leaves the brainstem at the junction of the pons and the medulla, medial to the facial nerve. It runs upwards and forwards from this position to reach the eye.

The nerve enters the subarachnoid space when it emerges from the brainstem. It runs upward between the pons and the clivus and then pierces the dura mater to run between the dura and the skull through Dorello’s canal. At the tip of the petrous part of the temporal bone, it makes a sharp turn forward to enter the cavernous sinus.^[1] In the cavernous sinus, it runs alongside the internal carotid artery. It then enters the orbit through the superior orbital fissure and innervates the lateral rectus muscle of the eye.

Nucleus

The abducens nucleus is located in the pons, on the floor of the fourth ventricle, at the level of the facial colliculus. Axons from the facial nerve loop around the abducens nucleus, creating a slight bulge (the facial colliculus) that is visible on the dorsal surface of the floor of the fourth ventricle. The abducens nucleus is close to the midline, like the other motor nuclei that control eye movements (the oculomotor and trochlear nuclei).

Motor axons leaving the abducens nucleus run ventrally and caudally through the pons. They pass lateral to the corticospinal tract (which runs longitudinally through the pons at this level) before exiting the brainstem at the pontomedullary junction.

Nerves of Cranial Nerve Six

The nerve itself can be divided into four distinct portions: the nucleus, the cisternal portion, the cavernous sinus portion, and the orbital portion. The abducens nucleus resides in the dorsal pons, ventral to the floor of the fourth ventricle, and just lateral to the medial longitudinal fasciculus. About forty percent of the axons project through the ipsilateral medial longitudinal fasciculus to cross over to the contralateral medial rectus subnucleus to eventually innervate the contralateral medial rectus muscle. The abducens nucleus is supplied by the pontine branches of the basilar artery.

Of all the cranial nerves, the abducens nerve has the longest intracranial course. It is located in the pons on the floor of the fourth ventricle, at the same level as the facial colliculus. In fact, the axons of the facial nerve loop around the posterior aspect of the abducens nucleus. This will be of clinical significance later. The nerve originates from the caudal, dorsal pontine below the fourth ventricle. After the fibers emerge from the nucleus, they course superiorly and then anteriorly before the majority of the axons leave the brainstem at the junction of the pontine and the medulla (i.e., the pontomedullary groove) caudal and medial to both the facial nerve and the vestibulocochlear nerve in most cases.

The nerve then travels through the subarachnoid space and crosses the upper edge of the tip of the petrous part of the temporal bone towards the clivus within a fibrous sheath called Dorello’s canal and enters the dura inferior to the posterior clinoid process. Because it is anchored in Dorello’s canal, the nerve is prone to stretching when intracranial pressure is increased due to multiple causes discussed later. It then enters the cavernous sinus (along with the oculomotor nerve, the trochlear nerve, and the first branch of the trigeminal nerve (V1), following lateral to the internal carotid artery and medial to a lateral wall of the sinus before following the sphenoidal fissure and entering the orbit through the superior orbital fissure within the tendinous ring to reach its destination at the lateral rectus muscle.

Muscles of Cranial Nerve Six

The abducens nerve functions to innervate the ipsilateral lateral rectus muscle and partially innervate the contralateral medial rectus muscle (at the level of the nucleus – via the medial longitudinal fasciculus).

• https://www.ncbi.nlm.nih.gov/books/NBK545167/
• https://www.ncbi.nlm.nih.gov/books/NBK384/
• https://www.ncbi.nlm.nih.gov/books/NBK482283/
• https://www.ncbi.nlm.nih.gov/books/NBK556126/
• https://www.ncbi.nlm.nih.gov/books/NBK539823/
• https://www.ncbi.nlm.nih.gov/books/NBK11036/
• https://www.ncbi.nlm.nih.gov/books/NBK507820/
• https://www.ncbi.nlm.nih.gov/books/NBK539729/
• https://www.ncbi.nlm.nih.gov/books/NBK406/
• https://www.ncbi.nlm.nih.gov/books/NBK406/
• https://www.ncbi.nlm.nih.gov/books/NBK537244/
• https://www.ncbi.nlm.nih.gov/books/NBK470534/
• https://www.ncbi.nlm.nih.gov/books/NBK537126/
• https://www.ncbi.nlm.nih.gov/books/NBK542213/
• https://www.ncbi.nlm.nih.gov/books/NBK542213/
• https://en.wikipedia.org/wiki/Trochlear_nerve

The Occipital Nerves are a group of nerves that arise from the C2 and C3 spinal nerves. They innervate the posterior scalp up as far as the vertex and other structures as well, such as the ear.[rx] There are three major occipital nerves in the human body: the greater occipital nerve (GON), the lesser (or small) occipital nerve (LON), and the third (or least) occipital nerve (TON).

Structure and Function

Greater occipital nerve

The GON is the biggest purely afferent nerve that arises from the medial division of the dorsal ramus of the C2 spinal nerve. It goes backward between the C1 and C2 vertebrae and traverses between the inferior capitis oblique and semispinalis capitis muscles from underneath the suboccipital triangle.[rx] Rarely does the GON travel within the inferior oblique. While traveling to the subcutaneous layer, the GON is found to pierce the semispinalis capitis muscle in most cases, and in some cases, the trapezius and the inferior oblique.[rx] This complex involvement with the nearby musculature may make the GON a potential source of nerve compression, entrapment, or irritation. The GON then perforates the aponeurotic fibrous layer of the trapezius and the sternocleidomastoid to travel to the scalp and the superior nuchal line.[rx] The GON also traverses along the occipital artery after passing through the semispinalis capitis. The GON innervates the skin of the back of the scalp up to the vertex of the skull, the ear, and the skin just above the parotid gland.[rx]

Lesser occipital nerve

The LON originates from the ventral rami of the C2 and C3 spinal nerves and goes to the occipital region along the posterior margin of the sternocleidomastoid muscle.[rx] It pierces the deep cervical fascia close to the cranium and travels upward. Near the cranium, it penetrates the deep cervical fascia and goes superiorly above the occiput to innervate the skin and communicate with the GON. [rx] The LON has three branches: the auricular, mastoid, and occipital branches. The LON divides into medial and lateral segments between the inion and intermastoid line.[rx] The LON innervates the scalp in the lateral region of the head behind the ear and the cranial surface of the ear.[rx]

Third occipital nerve

The TON is a superficial medial branch of the dorsal ramus of the C3 spinal nerve and is thicker compared to other medial branches.[rx] The dorsal ramus of the C3 spinal nerve divides into lateral and medial branches. The medial division further divides into superficial and deep branches, of which the superficial division is named the TON. The TON travels through the dorsolateral surface of the C2-C3 facet joint.[rx] Based on a study by Tubbs et al., the TON was found to send out small branches that travel across the midline and interact with the contralateral TON in 66.7% of patients.[rx] The TON also perforates the splenius capitis, trapezius, and semispinalis capitis. It then communicates with the GON and innervates the region of the skin below the superior nuchal line after innervating the semispinalis capitis. The TON also innervates the facet joint between the C2 and C3 spinal nerves and a portion of the semispinalis capitis.[rx]

Blood Supply and Lymphatics

The scalp is highly vascularized and is characterized by having many arterial anastomoses. Most of the blood supply comes from the external carotid arteries.[rx] With regards to the occipital region of the scalp, the vascularization is via the occipital artery and the posterior auricular arteries.[rx] Within the auriculomastoid sulcus, the posterior auricular artery travels superficially and separates into three branches: the mastoid, auricular, and transverse nuchal arteries.[rx] The LON is found to be close to the occipital artery. According to Kemp et al., the LON was found to be situated 2.5cm lateral to the occipital artery above the occiput.[rx] Also, according to Lee et al., who studied the topography of the LON in 20 sides of 10 heads from fresh cadavers, branches from the occipital artery communicated with the LON in 55% of samples.[rx] Among these samples, 45% of samples had the occipital artery crossing the LON at a single location while 10% of samples had the occipital artery communicating with the LON via a helical intertwining relationship. The researchers also found a fascial band as a compression point in 20% of samples.[rx]

The GON is also closely associated with the occipital artery in that after the GON perforates the semispinalis capitis, it travels with the occipital artery that is medial to the nerve.[rx] The GON may have a much more intimate relationship than previously thought. According to a study conducted by Janis et al., in which the researchers analyzed the topographic relationship between the GON and occipital artery in fifty samples of 25 posterior necks and scalps from cadavers, the GON, and occipital artery were found to cross each other in 54% of samples.[rx] Among samples where there was an intersection between the GON and the occipital artery, these crossings could differ from intersecting each other at a single point (29.6%) to intertwining with each other in a helical fashion (70.4%). These crossings were usually discovered in the tunnel of the trapezius caudal to the occipital protuberance but were also present above the occipitalis.[rx] These findings may be useful for migraine patients, as many of these patients report having pulsatile symptoms, and their headaches may contain a vascular component. Many researchers have proposed that the intersections between the GON and occipital artery may be responsible for these symptoms.[rx] Furthermore, another study by Shimizu et al. discovered the occipital artery and GON intersected in the nuchal subcutaneous layer, and the GON was always more superficial to the occipital artery at the point of intersection.[rx] They postulated the intimate relationship between the GON and occipital artery might be a contributing factor for occipital neuralgia (ON).[rx]

Nerves

As mentioned previously, the GON arises from the medial branch of the dorsal ramus of the C2 spinal nerve and innervates the skin of the back of the scalp up to the vertex of the skull, the ear, and the skin just above the parotid gland.[rx] When the GON is over the occiput, it communicates with the LON laterally and the TON. The LON comes from the ventral rami of the C2 and C3 spinal nerves and provides innervation to the scalp in the lateral region of the head behind the ear.[rx] The LON also transmits a branch to the GON as it goes above the occiput near the cranium.[rx] It also communicates with the mastoid division of the greater auricular nerve. The TON originates from the medial branch of the dorsal ramus of the C3 spinal nerve and innervates the facet joint between the C2 and C3 spinal nerves and a portion of the semispinalis capitis.[rx] Its cutaneous division also innervates the skin below the occiput. The TON also communicates with the GON and innervates the region of the skin below the superior nuchal line.[rx]

Muscles

Greater occipital nerve

As stated previously, the GON traverses between the inferior capitis oblique and semispinalis capitis muscles from underneath the suboccipital triangle.[rx] Rarely does the GON travel within the inferior oblique. While traveling to the subcutaneous layer, the GON is found to pierce the semispinalis capitis muscle in most cases, and in some cases, the trapezius and the inferior oblique.[rx] For the treatment of GON entrapment neuropathy, the regions where the GON traverses between the atlas and the axis, the GON courses between the obliquus capitis inferior and semispinalis capitis, or the GON perforates the semispinalis capitis and the trapezius, which are potential areas of GON irritation and entrapment.[rx] These zones could be affected by other medical issues, such as whiplash injuries and posture imbalances, and could serve as possible origins of ON.[rx] However, there are many physiological variants of the GON, which will be a topic in the following section.

Lesser occipital nerve

With regards to the LON, the area where the LON traverses from behind the sternocleidomastoid, the area where the LON ascends along the posterior margin of the sternocleidomastoid, and the area where the LON intersects with the nuchal line have been found to serve as potential compression points.[rx] This article will cover the physiological variants of the LON in the following section.

• https://www.ncbi.nlm.nih.gov/books/NBK545167/
• https://www.ncbi.nlm.nih.gov/books/NBK539843/
• https://www.ncbi.nlm.nih.gov/books/NBK537063/
• https://www.ncbi.nlm.nih.gov/books/NBK482317/
• https://www.ncbi.nlm.nih.gov/books/NBK536921/
• https://www.ncbi.nlm.nih.gov/books/NBK406/
• https://www.ncbi.nlm.nih.gov/books/NBK406/
• https://www.ncbi.nlm.nih.gov/books/NBK470534/
• https://www.ncbi.nlm.nih.gov/books/NBK537126/
• https://www.ncbi.nlm.nih.gov/books/NBK542213/
• https://www.ncbi.nlm.nih.gov/books/NBK542213/

Third Cranial Nerve (CN III) /Oculomotor Nerve is the third cranial nerve (CN III). It enters the orbit via the superior orbital fissure and innervates extrinsic eye muscles that enable most movements of the eye and that raise the eyelid. The nerve also contains fibers that innervate the intrinsic eye muscles that enable pupillary constriction and accommodation (ability to focus on near objects as in reading). The oculomotor nerve is derived from the basal plate of the embryonic midbrain. Cranial nerves IV and VI also participate in the control of eye movement.^[rx]

The oculomotor nerve (the third cranial nerve; CN III) has three main motor functions:

• Innervation to the pupil and lens (autonomic, parasympathetic)
• Innervation to the upper eyelid (somatic)
• Innervation of the eye muscles that allow for visual tracking and gaze fixation (somatic)

Like all other nerve fibers in the human body, the oculomotor nerve can become impaired in disease states which can lead to lifelong impairment in normal vision. Dysfunction can also be indicative of more serious underlying diseases, such as an aneurysm or a neoplasm.

Structure of Third Cranial Nerve (CN III)

The oculomotor nerve originates from 2 nuclei in the midbrain[rx]:

• Oculomotor nucleus
• Accessory parasympathetic nucleus (Edinger-Westphal nucleus)

The oculomotor nerve exits the brainstem near midline at the base of the midbrain just caudal to the mammillary bodies. It passes through the cavernous sinus and proceeds through the supraorbital fissure to reach the orbit of the eye

The third cranial nerve has both somatic and autonomic fibers. Somatic (voluntary) nerve fibers are bundled deep inside the nerve, while the autonomic (involuntary) fibers surround the somatic fibers around the outside of the nerve. Knowing the spatial layout of these fibers will help one understand the various forms of presentation in third nerve palsies.

The oculomotor nerve originates from the third nerve nucleus at the level of the superior colliculus in the midbrain. The third nerve nucleus is located ventral to the cerebral aqueduct, on the pre-aqueductal grey matter. The fibers from the two-third nerve nuclei located laterally on either side of the cerebral aqueduct then pass through the red nucleus. From the red nucleus fibers then pass via the substantia nigra exiting through the interpeduncular fossa.

On emerging from the brainstem, the nerve is invested with a sheath of pia mater, and enclosed in a prolongation from the arachnoid. It passes between the superior cerebellar (below) and posterior cerebral arteries (above), and then pierces the dura mater anterior and lateral to the posterior clinoid process, passing between the free and attached borders of the tentorium cerebelli.

It traverses the cavernous sinus, above the other orbital nerves receiving in its course one or two filaments from the cavernous plexus of the sympathetic nervous system, and a communicating branch from the ophthalmic division of the trigeminal nerve. As the oculomotor nerve enters the orbit via the superior orbital fissure it then divides into a superior and an inferior branch.^[rx]

Superior branch

The superior branch of the oculomotor nerve or the superior division, the smaller, passes medially over the optic nerve. It supplies the superior rectus and levator palpebrae superioris.

Inferior branch

The inferior branch of the oculomotor nerve or the inferior division, the larger, divides into three branches.

• One passes beneath the optic nerve to the medial rectus.
• Another, to the inferior rectus.
• The third and longest runs forward between the inferior recti and lateralis to the inferior oblique.
• From the third one, a short thick branch is given off to the lower part of the ciliary ganglion and forms its short root.

All these branches enter the muscles on their ocular surfaces, with the exception of the nerve to the inferior oblique, which enters the muscle at its posterior border.

Nuclei

The oculomotor nerve (CN III) arises from the anterior aspect of the mesencephalon (midbrain). There are two nuclei for the oculomotor nerve:

• The oculomotor nucleus originates at the level of the superior colliculus. The muscles it controls are the striated muscle in levator palpebrae superioris and all extraocular muscles except for the superior oblique muscle and the lateral rectus muscle.
• The Edinger-Westphal nucleus supplies parasympathetic fibers to the eye via the ciliary ganglion, and thus controls the sphincter pupillae muscle (affecting pupil constriction) and the ciliary muscle (affecting accommodation).

Sympathetic postganglionic fibres also join the nerve from the plexus on the internal carotid artery in the wall of the cavernous sinus and are distributed through the nerve, e.g., to the smooth muscle of superior tarsal (Mueller’s) muscle.

Somatic (voluntary) functions of the oculomotor nerve include elevation of the upper eyelid via innervation of the levator palpebrae superioris muscle. Other essential functions include coordination of eye muscles for visual tracking and gaze fixation. These functions of eye movement occur through innervation of four eye muscles:

• Superior rectus muscle – elevates the eye while looking straight ahead (primary position)
• Medial rectus muscle – adducts the eye from a primary position
• Inferior rectus muscle – moves the eye down from a primary position
• Inferior oblique muscle – elevates the eye when the eye is adducted from a primary position 

Blood Supply of Third Cranial Nerve (CN III)

The somatic and autonomic components of the oculomotor nerve have differentiated vascular supplies. The vasa vasorum supplies the inner somatic (voluntary) nerve fibers while pia mater blood vessels supply the outer autonomic nerve fibers.[rx]

Lymphatic drainage of the orbit of the eye is not yet well understood. There is morphological evidence for lymphatic structures within the lacrimal gland and arachnoid; however, distinctive lymphatic structures within other regions of the orbit are currently unknown.[rx]

Both the oculomotor nucleus and the Edinger-Westphal nucleus are located in the medial midbrain, which is supplied by the paramedian branches of the upper basilar artery and the proximal posterior cerebral artery.

Nerves Supply of Third Cranial Nerve (CN III)

The oculomotor nerve helps to adjust and coordinate eye position during movement. Several movements assist with this process: saccades, smooth pursuit, fixation, accommodation, vestibular-ocular reflex, and optokinetic reflex.

Saccades are rapid, jerky motions of the eye. This type of motion typically occurs when moving a gaze between objects. When tracking an object, we use smooth pursuit to keep our eyes focused on an object as it moves. When we want to stare at an object, we fixate on that object. Control of each of these actions is by either vestibule-ocular or optokinetic reflexes.

The vestibule-ocular reflex adjusts eye position during fast movements of the head. Head motion activates cells within the semicircular canals. Information about the motion is transmitted to the ipsilateral vestibular nucleus and forwarded to the oculomotor nucleus. Therefore, if the head moves to the left, the eyes move to the right to keep the gaze steady.

The optokinetic reflex adjusts eye position in response to changes in the visual field. This reflex initiates from visual information. Information about the visual field is transmitted from the parieto-occipital eye field through the pontine nuclei to the vestibulo-cerebellum. It then moves through the vestibular nuclei to the oculomotor nucleus where eye movement initiates. This reflex allows the eyes to follow large objects in the visual field.

The last action is accommodation. This action helps to keep the gaze focused when both vestibular and visual stimuli change. It requires the interaction of circuits between the vestibule-ocular and optokinetic reflexes.[rx][rx]

Deficits within the peripheral or central vestibular system will result in inappropriate quick gaze deviation movements of the eyes “nystagmus.” The direction of the last phase of the nystagmus can assist clinicians in the diagnosis of a patient’s pathology.

Muscles Attachment of Third Cranial Nerve (CN III)

The oculomotor nerve controls several muscles:

• Levator palpebrae superioris – raises the upper eyelid
• Superior rectus muscle – rotates the eyeball backward, “looking up”
• Medial rectus muscle – adducts the eye, “looking towards your nose”
• Inferior rectus muscle – rotates the eyeball forwards, “looking down”
• Inferior oblique muscle – rotates the eyeball backward when the eye is adducted
• Ciliary muscle – controls lens shape to focus on up-close objects
• Sphincter pupillae – constricts the pupil

Control of other eye muscles is by the trochlear (CN IV) and abducens (CN VI) nerves.

The trochlear nerve (4th cranial nerve) controls:

• Superior oblique muscle – rotates the eyeball forward when the eye is adducted

The abducens nerve (6th cranial nerve) controls:

• Lateral rectus muscle – abducts the eye, “looking towards your ear on the same side”

Function

There are two primary functions of the autonomic parasympathetic (involuntary) oculomotor nerve. It constricts the pupil (miosis) by innervating the smooth muscle (sphincter pupillae) near the pupil. It also innervates the ciliary muscles. These muscles connect the iris to the choroid. Contraction of the muscle alters the curvature of the lens which allows individuals to focus the lens on near objects.[rx]

The oculomotor nerve includes axons of type GSE, general somatic efferent, which innervate skeletal muscle of the levator palpebrae superioris, superior rectus, medial rectus, inferior rectus, and inferior oblique muscles.(innervates all the extrinsic muscles except superior oblique and lateral rectus.)

The nerve also includes axons of type GVE, general visceral efferent, which provide preganglionic parasympathetic to the ciliary ganglion. From the ciliary ganglion postganglionic fibers pass through the short ciliary nerve to the constrictor papillae of the iris and the ciliary muscles.

• https://www.ncbi.nlm.nih.gov/books/NBK545167/
• https://www.ncbi.nlm.nih.gov/books/NBK539843/
• https://www.ncbi.nlm.nih.gov/books/NBK537063/
• https://www.ncbi.nlm.nih.gov/books/NBK482317/
• https://www.ncbi.nlm.nih.gov/books/NBK536921/
• https://www.ncbi.nlm.nih.gov/books/NBK406/
• https://www.ncbi.nlm.nih.gov/books/NBK406/
• https://www.ncbi.nlm.nih.gov/books/NBK470534/
• https://www.ncbi.nlm.nih.gov/books/NBK537126/
• https://www.ncbi.nlm.nih.gov/books/NBK542213/

Oculomotor Nerve is the third cranial nerve (CN III). It enters the orbit via the superior orbital fissure and innervates extrinsic eye muscles that enable most movements of the eye and that raise the eyelid. The nerve also contains fibers that innervate the intrinsic eye muscles that enable pupillary constriction and accommodation (ability to focus on near objects as in reading). The oculomotor nerve is derived from the basal plate of the embryonic midbrain. Cranial nerves IV and VI also participate in the control of eye movement.^[rx]

The oculomotor nerve (the third cranial nerve; CN III) has three main motor functions:

• Innervation to the pupil and lens (autonomic, parasympathetic)
• Innervation to the upper eyelid (somatic)
• Innervation of the eye muscles that allow for visual tracking and gaze fixation (somatic)

Like all other nerve fibers in the human body, the oculomotor nerve can become impaired in disease states which can lead to lifelong impairment in normal vision. Dysfunction can also be indicative of more serious underlying diseases, such as an aneurysm or a neoplasm.

Structure

The oculomotor nerve originates from 2 nuclei in the midbrain[rx]:

• Oculomotor nucleus
• Accessory parasympathetic nucleus (Edinger-Westphal nucleus)

The oculomotor nerve exits the brainstem near midline at the base of the midbrain just caudal to the mammillary bodies. It passes through the cavernous sinus and proceeds through the supraorbital fissure to reach the orbit of the eye

The third cranial nerve has both somatic and autonomic fibers. Somatic (voluntary) nerve fibers are bundled deep inside the nerve, while the autonomic (involuntary) fibers surround the somatic fibers around the outside of the nerve. Knowing the spatial layout of these fibers will help one understand the various forms of presentation in third nerve palsies.

The oculomotor nerve originates from the third nerve nucleus at the level of the superior colliculus in the midbrain. The third nerve nucleus is located ventral to the cerebral aqueduct, on the pre-aqueductal grey matter. The fibers from the two-third nerve nuclei located laterally on either side of the cerebral aqueduct then pass through the red nucleus. From the red nucleus fibers then pass via the substantia nigra exiting through the interpeduncular fossa.

On emerging from the brainstem, the nerve is invested with a sheath of pia mater, and enclosed in a prolongation from the arachnoid. It passes between the superior cerebellar (below) and posterior cerebral arteries (above), and then pierces the dura mater anterior and lateral to the posterior clinoid process, passing between the free and attached borders of the tentorium cerebelli.

It traverses the cavernous sinus, above the other orbital nerves receiving in its course one or two filaments from the cavernous plexus of the sympathetic nervous system, and a communicating branch from the ophthalmic division of the trigeminal nerve. As the oculomotor nerve enters the orbit via the superior orbital fissure it then divides into a superior and an inferior branch.^[rx]

Superior branch

The superior branch of the oculomotor nerve or the superior division, the smaller, passes medially over the optic nerve. It supplies the superior rectus and levator palpebrae superioris.

Inferior branch

The inferior branch of the oculomotor nerve or the inferior division, the larger, divides into three branches.

• One passes beneath the optic nerve to the medial rectus.
• Another, to the inferior rectus.
• The third and longest runs forward between the inferior recti and lateralis to the inferior oblique.
• From the third one, a short thick branch is given off to the lower part of the ciliary ganglion and forms its short root.

All these branches enter the muscles on their ocular surfaces, with the exception of the nerve to the inferior oblique, which enters the muscle at its posterior border.

Nuclei

The oculomotor nerve (CN III) arises from the anterior aspect of the mesencephalon (midbrain). There are two nuclei for the oculomotor nerve:

• The oculomotor nucleus originates at the level of the superior colliculus. The muscles it controls are the striated muscle in levator palpebrae superioris and all extraocular muscles except for the superior oblique muscle and the lateral rectus muscle.
• The Edinger-Westphal nucleus supplies parasympathetic fibers to the eye via the ciliary ganglion, and thus controls the sphincter pupillae muscle (affecting pupil constriction) and the ciliary muscle (affecting accommodation).

Sympathetic postganglionic fibres also join the nerve from the plexus on the internal carotid artery in the wall of the cavernous sinus and are distributed through the nerve, e.g., to the smooth muscle of superior tarsal (Mueller’s) muscle.

Somatic (voluntary) functions of the oculomotor nerve include elevation of the upper eyelid via innervation of the levator palpebrae superioris muscle. Other essential functions include coordination of eye muscles for visual tracking and gaze fixation. These functions of eye movement occur through innervation of four eye muscles:

• Superior rectus muscle – elevates the eye while looking straight ahead (primary position)
• Medial rectus muscle – adducts the eye from a primary position
• Inferior rectus muscle – moves the eye down from a primary position
• Inferior oblique muscle – elevates the eye when the eye is adducted from a primary position 

Blood Supply and Lymphatics

The somatic and autonomic components of the oculomotor nerve have differentiated vascular supplies. The vasa vasorum supplies the inner somatic (voluntary) nerve fibers while pia mater blood vessels supply the outer autonomic nerve fibers.[rx]

Lymphatic drainage of the orbit of the eye is not yet well understood. There is morphological evidence for lymphatic structures within the lacrimal gland and arachnoid; however, distinctive lymphatic structures within other regions of the orbit are currently unknown.[rx]

Both the oculomotor nucleus and the Edinger-Westphal nucleus are located in the medial midbrain, which is supplied by the paramedian branches of the upper basilar artery and the proximal posterior cerebral artery.

Nerves

The oculomotor nerve helps to adjust and coordinate eye position during movement. Several movements assist with this process: saccades, smooth pursuit, fixation, accommodation, vestibular-ocular reflex, and optokinetic reflex.

Saccades are rapid, jerky motions of the eye. This type of motion typically occurs when moving a gaze between objects. When tracking an object, we use smooth pursuit to keep our eyes focused on an object as it moves. When we want to stare at an object, we fixate on that object. Control of each of these actions is by either vestibule-ocular or optokinetic reflexes.

The vestibule-ocular reflex adjusts eye position during fast movements of the head. Head motion activates cells within the semicircular canals. Information about the motion is transmitted to the ipsilateral vestibular nucleus and forwarded to the oculomotor nucleus. Therefore, if the head moves to the left, the eyes move to the right to keep the gaze steady.

The optokinetic reflex adjusts eye position in response to changes in the visual field. This reflex initiates from visual information. Information about the visual field is transmitted from the parieto-occipital eye field through the pontine nuclei to the vestibulo-cerebellum. It then moves through the vestibular nuclei to the oculomotor nucleus where eye movement initiates. This reflex allows the eyes to follow large objects in the visual field.

The last action is accommodation. This action helps to keep the gaze focused when both vestibular and visual stimuli change. It requires the interaction of circuits between the vestibule-ocular and optokinetic reflexes.[rx][rx]

Deficits within the peripheral or central vestibular system will result in inappropriate quick gaze deviation movements of the eyes “nystagmus.” The direction of the last phase of the nystagmus can assist clinicians in the diagnosis of a patient’s pathology.

Muscles

The oculomotor nerve controls several muscles:

• Levator palpebrae superioris – raises the upper eyelid
• Superior rectus muscle – rotates the eyeball backward, “looking up”
• Medial rectus muscle – adducts the eye, “looking towards your nose”
• Inferior rectus muscle – rotates the eyeball forwards, “looking down”
• Inferior oblique muscle – rotates the eyeball backward when the eye is adducted
• Ciliary muscle – controls lens shape to focus on up-close objects
• Sphincter pupillae – constricts the pupil

Control of other eye muscles is by the trochlear (CN IV) and abducens (CN VI) nerves.

The trochlear nerve (4th cranial nerve) controls:

• Superior oblique muscle – rotates the eyeball forward when the eye is adducted

The abducens nerve (6th cranial nerve) controls:

• Lateral rectus muscle – abducts the eye, “looking towards your ear on the same side”

Function

There are two primary functions of the autonomic parasympathetic (involuntary) oculomotor nerve. It constricts the pupil (miosis) by innervating the smooth muscle (sphincter pupillae) near the pupil. It also innervates the ciliary muscles. These muscles connect the iris to the choroid. Contraction of the muscle alters the curvature of the lens which allows individuals to focus the lens on near objects.[rx]

The oculomotor nerve includes axons of type GSE, general somatic efferent, which innervate skeletal muscle of the levator palpebrae superioris, superior rectus, medial rectus, inferior rectus, and inferior oblique muscles.(innervates all the extrinsic muscles except superior oblique and lateral rectus.)

The nerve also includes axons of type GVE, general visceral efferent, which provide preganglionic parasympathetic to the ciliary ganglion. From the ciliary ganglion postganglionic fibers pass through the short ciliary nerve to the constrictor papillae of the iris and the ciliary muscles.

• https://www.ncbi.nlm.nih.gov/books/NBK545167/
• https://www.ncbi.nlm.nih.gov/books/NBK539843/
• https://www.ncbi.nlm.nih.gov/books/NBK537063/
• https://www.ncbi.nlm.nih.gov/books/NBK482317/
• https://www.ncbi.nlm.nih.gov/books/NBK536921/
• https://www.ncbi.nlm.nih.gov/books/NBK406/
• https://www.ncbi.nlm.nih.gov/books/NBK406/
• https://www.ncbi.nlm.nih.gov/books/NBK470534/
• https://www.ncbi.nlm.nih.gov/books/NBK537126/
• https://www.ncbi.nlm.nih.gov/books/NBK542213/

The ophthalmic artery is the first branch of the internal carotid artery. It comes off just distal to the cavernous sinus. The ophthalmic artery gives off many branches, which supply the orbit, meninges, face, and upper nose. When the ophthalmic artery is occluded, it can compromise vision. The ophthalmic artery comes off the internal carotid artery on the medial side of the anterior clinoid process and traverses anteriorly through the optic canal and just lateral to the optic nerve.[rx][rx][rx]

The following are branches of the ophthalmic artery:

• The first branch of the ophthalmic artery is the central retinal artery that runs in the dura mater of the optic nerve. It then moves further along and supplies the inner layers of the retina.
• The second and the largest branch of the ophthalmic artery is the lacrimal artery. It also enters the orbit and traverses along the superior edge of the lateral rectus muscle. It supplies the eyelids, lacrimal gland, and conjunctiva.
• The ophthalmic artery gives off several posterior ciliary arteries that pass through the sclera and supply the posterior uveal tract. Because the posterior ciliary vessels are end vessels, sudden occlusion can produce infarction in the region of the choroid.
• The ophthalmic artery also gives off the inferior and superior muscular vessels that supply the extraocular muscles. The supraorbital artery is also a branch of the ophthalmic artery and passes through the supraorbital foramen to supply the skin of the forehead and Levator palpebrae muscle.
• Other branches of the ophthalmic artery include the ethmoid arteries, medial palpebral vessels, and terminal branches.

The orbital arteries include the ciliary arteries, central retinal artery, and muscular arteries.[rx]

• Long Posterior Ciliary Arteries – The long posterior ciliary arteries (1 to 2) travel near the optic nerve and pierce the posterior sclera to supply the choroid and ciliary muscle before joining the major arterial circle of the iris.[rx] The major arterial circle of the iris distributes branches to the iris and ciliary body.
• Short Posterior Ciliary Arteries – The number of short posterior ciliary arteries vary per individual, often ranging between 6 to 12 arteries that branch off the ophthalmic artery as it crosses the optic nerve medially. These arteries supply the ciliary processes and optic disk. The arterioles branching from the posterior ciliary arteries supply the choroid. The perpendicular terminal arterioles supply choriocapillaris, the blood supply to Bruch’s membrane and outer retina.[rx]
• Anterior Ciliary Arteries – There are seven anterior ciliary arteries that branch from the muscular arteries and run with the extraocular muscles. The anterior ciliary arteries supply the rectus muscles, conjunctiva, and sclera before joining the long posterior ciliary arteries to form the major arterial circle of the iris.[rx] Each rectus muscle receives its vascular supply from two anterior ciliary arteries, except the lateral rectus which receives blood supply from only one anterior ciliary artery.
• Central Retinal Artery – It is the first branch of the ophthalmic artery. It is a terminal branch supplying the inner layer of the retina, and its occlusion can cause sudden visual loss. It travels inferiorly and within the optic nerve sheath to supply the inner two-thirds of the retina. The artery further divides into superior and inferior arcades, which form the blood-retina barrier.
• Muscular branches – The two muscular branches of the ophthalmic artery that supply extraocular muscles include the medial and lateral muscular branches. The medial artery being larger than the lateral muscular branch.

When there is occlusion of the ophthalmic artery, it can result in an ischemic syndrome. Amaurosis fugax is a condition associated with temporary, painless loss of vision due to either an embolic phenomenon or hypoperfusion. Emboli to the ophthalmic artery usually originate from the carotid artery bifurcation. One may visualize Hollenhorst bodies (a.k.a., Eickenhorst plaques) in the retina during fundoscopic evaluation. When there is a sudden, painless loss of vision in one eye, it is recommended that one obtain a duplex ultrasound of the neck to assess the carotid artery for atherosclerotic plaques.[rx][rx][rx]

Nerves

The ophthalmic artery is intimately associated with the optic nerve (e.g., Cranial Nerve II). Additional important neurovascular structures in proximity to the ophthalmic artery include the following structures:

Nerves

• Lacrimal Nerve (Cranial Nerve Va)
• Frontal Nerve (Cranial Nerve Va)
• Trochlear Nerve (Cranial Nerve IV)
• Superior Division of the Oculomotor Nerve (Cranial Nerve IIIs)
• Nasociliary Nerve (Cranial Nerve Va)
• Inferior Division of the Oculomotor Nerve (Cranial Nerve IIIi)
• Abducens Nerve (Cranial Nerve VI)
• Ganglionic Branches (from Pterygopalatine Ganglion to Maxillary Nerve)
• Infraorbital Nerve (Cranial Nerve Vb)
• Zygomatic Nerve (Cranial Nerve Vb)

Veins

• Superior Ophthalmic Vein
• Inferior Ophthalmic Veins
• Infraorbital Vein

Arteries

• Infraorbital Artery

Muscles

As mentioned previously, the ophthalmic artery continues medially as the superior and inferior muscular branches. These important branch vessels originate either directly from the ophthalmic artery or from a separate trunk that subsequently divides into the superior and inferior branches. These branches provide blood supply to the extraocular muscles.

• https://www.ncbi.nlm.nih.gov/books/NBK545167/
• https://www.ncbi.nlm.nih.gov/books/NBK539843/
• https://www.ncbi.nlm.nih.gov/books/NBK537063/
• https://www.ncbi.nlm.nih.gov/books/NBK482317/

The orbits are bony structures of the skull that house the globe, extraocular muscles, nerves, blood vessels, lacrimal apparatus, and adipose tissue. Each orbit protects the globe, while the supportive tissues allow the globe to move in three dimensions (horizontal, vertical, and torsional).[rx][rx] The anatomy of the orbit is a complex topic vital for understanding the communication between the eye and the central nervous system and the potential for the spread of malignancy or infection. Certain surgical emergencies, such as severe fractures, are often intricate because of the delicate anatomy of the orbit and its contents.[rx][rx] The following article will provide insight into the structure and function of the different components of the orbit and will explain the importance of understanding orbital anatomy and physiology in relation to pathology.

Structure and Function

The orbits are symmetrical paired structures separated by the nasal cavity and paranasal sinuses. Seven bones form each orbit: frontal, sphenoid, maxillary, zygomatic, palatine, ethmoid, and lacrimal. The orbital roof is formed by the lesser wing of the sphenoid bone and the frontal bone. The lateral wall comprises the greater wing of the sphenoid bone and zygomatic bone. The medial orbital wall comprises the lacrimal, ethmoid, maxillary, and lesser wing of the sphenoid bones. Finally, the orbital floor comprises the maxillary, palatine, and zygomatic bones.[rx][rx] The lateral wall is the strongest of the four orbital walls.[rx] The walls of the orbit function as a physical barrier from blunt trauma to the eye, an anchor for muscles and ligaments to attach, and additionally serve as a window for neuro vasculature to travel through.

Connective tissue structures within the orbit aid in support and protection of the orbital contents. Orbital fat, which surrounds the extraocular muscles and the globe itself, serves as a cushion and facilitates the movement of the eye. The orbital septum is a connective tissue structure that acts as an anterior border between the facial skin and fat and the orbital contents, impeding the spread of infection into the orbit.[rx]

The lacrimal gland, a secretory gland comprising acini and ducts, produces tears and maintains the microenvironment of the eye. The location of the main lacrimal gland is near the lateral aspect of the anterior orbital roof, and it comprises two lobes, the orbital and palpebral lobes. Each lobe contains a duct that opens into the superior conjunctival fornix. The accessory lacrimal gland is smaller and is found within the lamina propria of the conjunctiva, with ducts opening onto the conjunctival surface.[rx]

Blood Supply and Lymphatics

The ophthalmic artery is a branch of the internal carotid artery that courses through the optic canal of the sphenoid bone. Once it enters the orbit, the ophthalmic artery has pierced the dura of the optic nerve where it continues anteriorly. It supplies the central retinal artery (which enters the globe), the lacrimal artery (supplying the lateral rectus muscle, lacrimal gland, eyelids, temporal fossa, and cheeks), and the superior and inferior muscular arteries (which supply the superior rectus and superior oblique muscles, as well as the inferior rectus and medial rectus muscles, respectively).

The infraorbital artery, a branch of the maxillary artery, and the infraorbital vein, which drains into the pterygoid plexus, course through the inferior orbital fissure through the infraorbital canal alongside the infraorbital nerve. These vessels emerge anteriorly from the infraorbital foramen of the maxilla.[rx][rx]

The superior and inferior ophthalmic veins course through the superior orbital fissure. These veins communicate with the facial veins anteriorly and the cavernous sinus posteriorly. Their exact course is variable, see the “Physiologic Variants” section below.

The eyelids and bulbar conjunctiva use the orbital lymphatic system to drain the preauricular nodes. Controversy exists over the clinical significance of the lymphatic system surrounding the optic nerve and lacrimal gland, however.[rx]

Nerves

The infraorbital nerve, a branch of the trigeminal nerve that provides sensation to the maxillary region of the face, courses anteriorly from the inferior orbital fissure. This fissure is located at the posterior aspect of the orbit and meets the infraorbital canal of the orbital floor. The infraorbital nerve courses through the canal and emerges facially from the infraorbital foramen of the maxilla.

The superior orbital fissure allows for the passage of cranial nerves (CN) originating from the cranial fossa to enter the orbit. CN III (oculomotor nerve), CN IV (trochlear nerve) and CN VI (abducens nerve) innervate extraocular muscles, while the first division of CN V (ophthalmic branch), provides sensation to the upper face, mucous membranes, and scalp. CN III innervates the superior rectus muscle, medial rectus muscle, inferior rectus muscle, and inferior oblique muscle. CN IV provides innervation to the superior oblique muscle, and CN VI innervates the lateral rectus muscle.

The optic canal is located medially to the superior orbital fissure and transmits the optic nerve (CNII). The optic nerve transmits visual input from the retina to the brain.[rx]

Muscles

The levator palpebrae superioris muscle, which receives nerve supply from CN III, elevates the upper eyelid. It is superior to the superior rectus muscle at the roof of the orbit, and these two muscles join in a common aponeurosis anteriorly. This intimate muscular relationship explains why the eye elevates as the upper eyelid is retracted.

The extraocular muscles and their actions are as follows:[rx]

• Superior rectus: Elevates, adducts and rotates medially
• Medial rectus: Adducts
• Inferior rectus: Depresses, adducts and rotates laterally
• Lateral rectus: Abducts
• Superior oblique: Depresses, abducts, and rotates medially
• Inferior oblique: Elevates, abducts, and rotates laterally

• https://www.ncbi.nlm.nih.gov/books/NBK545167/
• https://www.ncbi.nlm.nih.gov/books/NBK539843/
• https://www.ncbi.nlm.nih.gov/books/NBK537063/
• https://www.ncbi.nlm.nih.gov/books/NBK482317/

The optic canal is a funnel-like structure as part of the sphenoid bone that extends from the optic foramen to the orbital apex, the posterior-most end of the orbit. The orbital apex consists of the optic canal and the superior orbital fissure. The superior orbital fissure is bordered superomedially by the lesser wing of the sphenoid bone and inferolaterally by the greater wings of the sphenoid bone. The superior orbital fissure is the largest opening that connects the orbit with the middle cranial fossa.[rx] The optic canal connects the orbit to the middle cranial fossa and transmits the optic nerve, ophthalmic artery, meningeal sheaths, and sympathetic nerve fibers.

The optic nerve, also known as the cranial nerve II, transmits visual signal from the retina to the visual cortex. The ophthalmic artery, the first branch of the internal carotid artery, arises distal to the cavernous sinus and supplies mainly the orbit but also other structures in the face and meninges.

Blood Supply and Lymphatics

Understanding blood supply to the optic nerve is vital as the optic nerve is a vulnerable structure to compression within the limited space of the optic canal. The main vascularization of the optic nerve comes from the superior hypophyseal arteries and ophthalmic artery.[rx]

The hypophyseal arteries mainly supply the intracranial and intracanalicular part of the optic nerve while the ophthalmic artery supplies the intraorbital portion of the optic nerve through the long ciliary arteries and the central retinal artery.[7]

A critical structure passing through the optical canal is the ophthalmic artery. The ophthalmic artery is the first main branch of the internal carotid artery. It originates from the distal dural ring intracranial, intracanalicular, and intraorbital sections. During its course, it runs inferolateral relative to the optic nerve within the optic canal.[rx]

Branches of the ophthalmic artery include the following blood vessels[rx][rx]:

• Central retinal artery: supplies the inner layer of the retina
• Long posterior ciliary artery: two branches arise that supply the iris through the circulus arteriosus major
• Short posterior ciliary artery: branches pierce the sclera to supply the choroid and the ciliary process.
• Lacrimal artery: supplies the lacrimal gland and the anterior portion of the eyeball and a portion of the eyelid
• Anterior ethmoidal artery: gives rise to the anterior meningeal artery. It supplies ethmoidal air cells and the periosteum
• Posterior ethmoidal artery: supplies the ethmoidal air sinuses and part of the nasal mucosa and septum
• Supraorbital artery: supplies part of the orbit and face – terminal branches: include the supratrochlear (or frontal) artery and the dorsal nasal artery, which supply the forehead and scalp

Understanding the lymphatic drainage of the various tissues in the eye are crucial in studying conditions that involve dysfunctional lymphatic systems, including inflammatory diseases, metastatic cancers, transplant rejection, lymphatic malformation, and surgical complications.

While most body tissues have an embedded lymphatic drainage system, the ocular lymphatic structure has a heterogeneous appearance. While the cornea, lens, retina, ciliary body, choroid, and sclera are mostly lymphatic-free, other tissues are not. Optic nerve sheath is considered a lymphatic-rich ocular structure.[rx] This area is rich with LYVE-1 (lymphatic vessel endothelial hyaluronan receptor-1).[rx]

Nerves

Another crucial structure passing through the optical canal is the optic nerve. The optic nerve is the second cranial nerve surrounded by the cranial meninges and responsible for the transmission of sensory information for vision. The retinal ganglion cells receive impulses from the rods and cones and subsequently converge to form the optic nerve. After its formation, it leaves the bony orbit, passes through the optic canal and the sphenoid bone to enter the cranial cavity and run along the middle cranial fossa.

Within the middle cranial fossa, both optic nerves converge to form the optic chiasm. The following optic tracts emerge from the optic chiasm:

• Left optic tract: transmits signal from the left temporal retina and the right nasal retina
• Right optic tract: transmits signal from the right temporal retina and the left nasal retina

All optic tracts synapse at the lateral geniculate nucleus in the thalamus. Axons project from this region into two major optic radiation tracts[rx]:

• Upper optic radiation: these fibers travel through the parietal lobe and carry information from the superior retinal quadrants, which correspond to the inferior visual field quadrants.
• Lower optic radiation: these fibers travel through the temporal lobe and carry information from the inferior retinal quadrants, which correspond to the superior visual field quadrants

Each optic tract travels to its corresponding cerebral hemisphere to reach the lateral geniculate nucleus (LGN), a relay system located in the thalamus; the fibers synapse here.

Axons from the LGN then carry visual information via a pathway known as the optic radiation. The pathway itself divides into the following[rx]:

• Upper optic radiation – carries fibers from the superior retinal quadrants (corresponding to the inferior visual field quadrants). It travels through the parietal lobe to reach the visual cortex.

• Lower optic radiation– also known as the Meyers loop, carries fibers from the inferior retinal quadrants (corresponding to the superior visual field quadrants). It travels through the temporal lobe, via a pathway known as Meyers loop, to reach the visual cortex.

These optic radiations project to the visual cortex, where sensory data is processed and interpreted.

Muscles

There are seven extraocular muscles within the ocular orbit[rx]:

• Levator palpebrae superioris and superior tarsal muscle: levator palpebrae superioris innervated by the oculomotor nerve; superior tarsal muscle innervated by the sympathetic nervous system; both elevate the upper eyelid
• Superior rectus: innervated by the oculomotor nerve; the primary function is the elevation of the eyeball.
• Inferior rectus: innervated by the oculomotor nerve; the primary function is a depression of the eyeball.
• Medial rectus: innervated by the oculomotor nerve; the primary function is the adduction of the eyeball.
• Lateral rectus: innervated by the abducens nerve; the primary function is the abduction of the eyeball.
• Superior oblique: innervated by the trochlear nerve; the primary function is depression, abduction, and medial rotation of the eyeball.
• Inferior oblique: innervated by the oculomotor nerve; the primary function is elevation, abduction, and lateral rotation of the eyeball.

References

Optic nerve sheath decompression more commonly known as optic nerve sheath fenestration (ONSF), is a surgical procedure performed to decompress the optic nerves and relieve vision-threatening papilledema in the setting of elevated intracranial pressure (ICP). Performance of this procedure is generally in cases of pseudotumor cerebri syndrome (PTC), where patients are rapidly losing vision from significantly elevated ICP, or in cases of progressive vision loss in patients who do not respond to medical therapy or are non-adherent with therapy. The exact mechanism by which fenestration relieves papilledema is not entirely understood at this time; however, both decompression of the periodic subarachnoid space with filtration of cerebrospinal fluid (CSF) out of the subarachnoid space through the dural opening as well as scarring of the surgical site that prevents further accumulation of CSF are felt to be contributing factors.[rx] Optic nerve sheath decompression does not treat the source of the elevated ICP itself, but the procedure does diminish the associated visual sequelae.

Anatomy and Physiology

The optic nerve is distinctive among cranial nerves in that it is a central nervous system (CNS) white matter tract that shares a similar developmental origin with the CNS. As such, the optic nerve covering is the same outer connective tissue membranes as the brain: the pia, arachnoid, and dura mater. The pia lies on the surface of the brain and follows along the brain’s gyri and sulci. The arachnoid mater lies just above the pia mater, and CSF flows in the subarachnoid space (SAS), between the pia and the arachnoid. The dura mater is a strong connective tissue covering that overlies the other arachnoid and pia mater, encasing the brain and spinal cord. The SAS of the optic nerve is contiguous with that of the brain via the suprasellar cistern, allowing the circulation of CSF between the intracranial SAS and the optic nerves in a 3-compartment system. As a result, increases in ICP can be transmitted to the optic nerves and can cause optic disc edema known, in this clinical setting, as papilledema. Optic nerve sheath fenestration is a surgical procedure performed to relieve perioptic pressure in cases of elevated ICP causing papilledema.

The subarachnoid space of the human optic nerve is not homogenous but possesses a complex system of arachnoid trabeculae and septa that subdivide the subarachnoid space between the arachnoid and pia mater. The trabeculae, septa, and pillars vary in their density as well as their arrangement depending upon their specific location within the different portions of the optic nerve. They appear to be more densely and tightly packed in the immediate retrobulbar segment of the optic nerve than the more distal mid-orbital and canalicular segments. This variability may play a role in the CSF dynamics between the subarachnoid space of the optic nerve and the suprasellar cistern that connects it to the intracranial space and may contribute to the understanding of the pathophysiology of papilledema and the sometimes variable response seen to ONSF.[rx]

Increased ICP can result from a multitude of CNS disease states, including intracranial hypertension, venous sinus thrombosis, CNS infections, inflammatory diseases, subarachnoid hemorrhage, and intracranial malignancies or other lesions causing obstructive hydrocephalus. Vision loss in the setting of papilledema is thought to occur as a result of the disruption of axonal transport and intraneuronal ischemic damage to the optic nerve.[rx][rx] An increase in CSF pressure increases tissue pressure on the optic nerve axons and disrupts the gradient between intraocular and retrolaminar pressure. This results in defective axonal transport, followed by intra-axonal swelling, which is seen as papilledema. This swelling can secondarily compress small arterioles to the nerve, consequently causing intraneuronal ischemia. Clinically, this manifests as vision loss, initially as blind spot enlargement, followed by loss of peripheral and then central vision and field loss. Ultimately, optic nerve atrophy ensues in the setting of chronically elevated ICP and papilledema, often with profound vision loss.

Indications of Optic Nerve Sheath Decompression

Indications for ONSF

As stated earlier, the indication for optic nerve sheath fenestration is in the setting of vision loss due to elevated ICP causing papilledema. Increased ICP can result from a multitude of CNS disease states. The most common of these include PTC syndrome, cerebral venous sinus thrombosis, and intracranial tumors causing ICP elevation from decreased CSF outflow. Other less common and historical indications are discussed as well.

Pseudotumor Cerebri Syndrome (Idiopathic Intracranial Hypertension)

ONSF is most frequent in the setting of PTC, also known as benign intracranial hypertension. It is a syndrome of elevated ICP in the absence of an underlying CNS pathology. Idiopathic intracranial hypertension (IIH) is a subset of PTC syndrome in which no secondary cause (e.g., transverse sinus stenosis, venous sinus thrombosis, corticosteroid use, a hormonal abnormality, etc.) is present. I most commonly present in women of childbearing age who are obese or have recently gained weight. Patients commonly present with symptoms of elevated ICP including headaches and pulsatile tinnitus as well as visual symptoms of transient visual obscurations and diplopia from unilateral or bilateral cranial nerve (CN) VI palsies.[rx] Severe visual loss from progressive field constriction is typically a late finding associated with significantly elevated ICP and can lead to blindness in 5% of patients.[rx] Non-invasive treatment measures include weight loss, acetazolamide, and furosemide.

Diagnostic criteria for IIH in adults and children were revised in 2013 by Friedman et al.[rx] Definitive IIH will exhibit the following:

• Papilledema
• Normal neurologic exam, except for cranial neuropathies
• Neuroimaging will show one or more of the following:

  • Typical female and obese patient: MRI will show normal brain parenchyma and no meningeal enhancement without hydrocephalus, masses, or structural lesions
  • Other types of patients: normal MRI and magnetic resonance venography
  • If MRI is unavailable or unobtainable, CT imaging may is an option.
• Normal CSF composition
• Elevated opening lumbar puncture pressure (greater than 250 mm CSF in adults and greater than 280 mm CSF in children [250 mm CSF in non-sedated or non-obese child])

If papilledema is not present, IIH can be diagnosed if 2 to 5 of the above are present and if it is also unilateral or bilateral CN VI palsy. If both papilledema and CN VI palsy are absent, it is suggestive of IIH (but not positively diagnosed). If 2 to 5 of the above are present, and the patient has at least 3 of the following on neuroimaging: empty sella; flattening of the posterior globe; distension of the periodic SAS with or without a tortuous optic nerve; and/or transverse venous sinus stenosis.[rx]

Optic nerve sheath fenestration is an attempt to preserve visual function in patients with PTC who present with vision-threatening papilledema or who have not responded adequately to or cannot tolerate maximal medical therapy. It is an essential tool in the acute setting for patients presenting with “malignant” PTC (extremely high opening pressure, evidence of optic neuropathy, and poor prognosis if treated with medical therapy alone).[rx] Serial lumbar punctures (LP) are not recommended to reduce ICP in this clinical setting given the rapid reaccumulation of CSF and return of elevated ICP. CSF diversion procedures in the form of ventriculoperitoneal shunting (VPS) or lumboperitoneal shunting (LPS) can also be used to decrease ICP. While they can be effective at reducing papilledema and preventing vision loss, there is the attendant risk of brain and spinal surgery and the not uncommon complication of shunt failure. ONSF and CSF diversion procedures may both be necessary for patients with malignant PTC syndrome, as well as those with severe vision loss and ICP elevation in PTC refractory to conservative management.

Unilateral ONSF can result in papilledema resolution in both eyes; however, in most cases of severe papilledema and vision loss, bilateral ONSF is required. A review of unilateral ONSF shows that the unfenestrated eye typically shows less papilledema reduction than the operated eye.[rx] Some patients may also experience an improvement in their headaches, but ONSF is not primarily for this indication. Repeat ONSF may be necessary on patients with recurrent visual disturbance or further visual deterioration after surgery.[rx][rx][rx] A ventriculoperitoneal shunt is preferable in most cases in this setting; however, scarring of the optic nerve sheath from prior surgery makes repeat fenestration more technically challenging, and there is a potentially higher risk of complications.

Cerebral vein thrombosis or stenosis

Obstruction of venous drainage from the cerebral venous system secondary to thrombosis or stenosis can result in increased ICP and papilledema from decreased CSF outflow. This condition can present very similarly to PTC and IIH but tends to have a more rapid onset and more severe visual loss from significant ICP elevation; it is not limited to the demographics of PTC syndrome and should be suspected in non-obese women or men with symptoms and signs of PTC syndrome. It presents in patients with prothrombotic states, including pregnancy, use of oral contraceptives, Factor V Leiden mutations, Factor XII deficiency, G20210A mutations, antithrombin III, and protein S deficiency. Therapies include anticoagulation, stenting, or thrombectomy of the cerebral sinus, and CSF diversion procedures. ONSF can also be performed to protect visual function in patients with vision-threatening papilledema. The European Federation of Neurological Sciences (EFNS) 2010 guidelines appropriates ONSF in cases of serial lumbar punctures, acetazolamide, and/or VPS do not prevent the progression of visual loss in these conditions.[rx][rx]

Intracranial masses

Intracranial masses/tumors create increased ICP primarily by restricting the flow of CSF through the ventricles or outflow of CSF and blood from the brain via the dural venous sinuses. Optic nerve sheath fenestration is a viable option for patients in whom ICP develops rapidly and causes profound vision loss and in whom complete resection of the mass is not possible with resulting persistent intracranial hypertension. Performing ONSF can dramatically increase the quality of life in these patients by preserving their vision. Of all indications for ONSF, this has the poorest visual prognosis, as the surgery is generally a therapeutic option in late-stage patients with the intracranial illness.

Other Indications for ONSF

Cryptococcal meningitis – up to 40% of patients develop ocular disease; papilledema is the most common manifestation of CNS infection and may result in a profound visual loss for patients who survive. Cryptococcal meningitis usually results in a very high organism burden in the CSF and marked inflammation; this leads to increased ICP from the possible aggregation of cryptococcal capsular polysaccharide and the resulting CSF outflow obstruction through arachnoid granulations.[rx] By some physicians, consideration of early ONSF is essential to avoid visual impairment. It is a reasonable consideration to practice caution before performing invasive surgical procedures on an infected optic nerve. However, studies show that postoperative orbital infections had not occurred even when histopathological studies revealed numerous cryptococci.[rx][rx] This is true even if ONSF takes place before antifungal medication administration.[rx][rx] Studies have shown that patients with papilledema from cryptococcal meningitis should be considered for ONSF early in the course, even if the infection is at an active stage in the disease.[rx][rx]

The local disease of the optic nerve

• Traumatic optic neuropathy (TON): this can cause elevated intraorbital pressure. It commonly manifests early as an optic nerve hematoma. The treatment of TON is controversial, and a large prospective study showed no difference in outcomes whether treatment for TON was with observation, corticosteroids, or optic nerve decompression.[rx]
• Optic nerve tumors: the most common optic nerve tumors are gliomas and meningiomas. Visual improvement with ONSF in patients with meningiomas has mixed results in reported cases. Other optic nerve tumors reported having visual improvement after ONSF include bilateral infiltration with T-cell non-Hodgkin lymphoma and metastatic breast cancer.[rx]
• Optic nerve drusen: this may uncommonly cause central and peripheral visual loss through an unknown mechanism. A few small studies showed some improvement in the management of visual loss via ONSF. To date, no clear recommendations exist on the treatment of optic nerve drusen with ONSF. Further studies are warranted.[rx][rx]

Contraindications of Optic Nerve Sheath Decompression

Optic nerve sheath fenestration should not be performed in patients on chronic anticoagulation due to the attendant risk of bleeding into the orbit. The procedure should typically be avoided in patients with CNS infections due to the potential for seeding of the orbit with the infectious organism. It is typically not performed in patients with mild or moderate vision loss related to ICP elevation, as they should receive a full trial of medical therapy prior to the consideration of surgical intervention. While headache relief can result in up to 50% of patients [rx], it is not indicated for the management of a headache in patients with PTC syndrome and patients with ICP elevation without optic disc edema or vision loss.

Technique

Surgical Approaches

De Wecker was the first to describe optic nerve sheath fenestration in 1872. There are several surgical approaches for accessing the orbit for fenestration of the optic nerves; however, the final goal is to create a window or a series of slits in the optic nerve sheath just behind the globe to release CSF under pressure causing compression of the nerve.

Ophthalmologists typically perform the surgery, after training in orbital surgery and/or neuro-ophthalmology. Based on a 2015 survey of ophthalmologists who perform ONSF, the three most commonly utilized surgical approaches are the medial transconjunctival approach (59%), the superomedial lid crease incision (31%), and lateral orbitotomy (10%).[rx] The choice of surgical procedure employed is surgeon-specific, depending on individual surgical training and comfort level with their chosen technique.

All procedures are performed under general anesthesia and magnification either with loupes or a surgical microscope. Surgical preparation and draping should take place in the usual fashion for ophthalmic plastic surgery. Avoidance of injury to the posterior ciliary arteries that course along the surface of the optic nerve sheath and supply the optic nerve and choroid is critical to prevent ischemic optic neuropathy or choroidal infarction.

Medial transconjunctival approach

The medial transconjunctival approach is the most widely used approach for exposure and fenestration of the optic nerve. It was first described in 1973 by Galbraith and Sullivan.[rx] One significant benefit of this approach is that it offers quick access to the retro-orbital optic nerve without creating an incision in the skin. However, it requires disinsertion of the medial rectus muscle that can result in postoperative strabismus and diplopia.

Technique

After the establishment of general anesthesia and patient preparation, a lid speculum is placed. A limbal peritomy is made nasally, and dissection is performed down to bare sclera where the medial rectus muscle is identified. The medial rectus muscle is isolated with muscle hooks and secured with a 6-0 double-armed polyglactin at the insertion. The muscle is transected from the globe anterior to the suture. A traction suture may be passed through the muscle stump at the insertion so that the globe may be abducted for enhanced exposure. Continuous attention to pupil size and reaction to light is critical in the monitoring of optic nerve integrity throughout the rest of the procedure. Gentle blunt dissection is carried down along the medial aspect of the globe and into the intraconal space using the surgeon’s orbital retractor of choice. Once the optic nerve is visualized, the operating microscope is brought over the field. A fine, long forceps, such as a myringotomy or bayonet, is used to grasp the optic nerve sheath. Sharp scissors are used to incise the optic nerve sheath at least 1 mm away from the insertion of the nerve. Alternatively, a long fine-cutting instrument, such as a super sharp paracentesis knife, may be used to make the first cut. A gush of CSF indicates successful full-thickness penetration of the sheath. Additional cuts with the scissors are made until a window of tissue is excised and removed. All orbital instruments are removed, and the medial rectus muscle is reattached at its insertion. Hemostasis is confirmed, and the conjunctiva is closed with the surgeon’s suture of preference.

Farris and Lai described an alternative to this procedure in 2014. THeir approach entails a transconjunctival approach to the medial intraconal space; however, instead of detaching the medial rectus muscle, this muscle and the superior rectus are engaged on bridle sutures that are used to rotate the globe inferiorly and temporally. The orbit is then entered with an orbital retractor in the space between these muscles to access and fenestrate the optic nerve as described above.

Superomedial lid crease incision

Pelton and Patel described this surgical approach in 2001.[rx] Historically, it has found wide utilization by orbital surgeons for access to the medial intraconal space for removal of intraorbital lesions. By creating the incision in the crease of the upper eyelid, the scar is well-hidden and is appealing for some patients for cosmetic considerations. It is favored by some orbital surgeons because it can be performed more quickly than the classical medial transconjunctival approach, affords a direct angle of approach to the optic nerve, and does not require disinsertion of any of the extraocular muscles. Additionally, an operative microscope is usually unnecessary due to the more direct approach to the nerve as compared to the medial orbitotomy approach. The dissection takes place medial to the levator aponeurosis and optic nerve, and as such, there is little danger of ptosis, ciliary ganglion injury, or strabismus. The constraining factor with this approach is the increased distance from the incision site to the optic nerve, and it is relatively more technically challenging to perform.

Technique

The lid crease is marked, and local anesthetic with epinephrine is injected subcutaneously. After the establishment of general anesthesia and patient preparation, an incision is made through the medial one-half to two-thirds of the crease over the previously made marking. Sharp dissection is carried through the orbicularis and the septum either with cutting cautery or Wescott scissors. The nasal and central (preaponeurotic) fat pads are identified, and careful blunt dissection is performed between them into the orbit between the levator aponeurosis and superior oblique muscle. Dissection is carried out inferiorly and posteriorly toward the back of the globe in the loose areolar orbital fat until the optic nerve is strummed and then visualized. Continuous attention to pupil size and reaction to light is critical for the monitoring of optic nerve integrity throughout the rest of the procedure. Neurosurgical cottonoids may then be used to pack around the nerve. An operating microscope may be brought over the field at this point per surgeon preference or loupes may be utilized for magnification. A fine long forceps, such as a myringotomy or bayonet, is used to grasp the optic nerve sheath. An incision is then made in the optic nerve sheath at least 1 mm away from the insertion of the nerve using the surgeon’s instrument of choice, taking care to avoid contact with the nerve itself. A gush of CSF indicates successful full-thickness penetration of the sheath. Additional cuts with the scissors are made until a window of tissue is excised and removed. Hemostasis is confirmed, and all instruments are removed from the orbit. The skin is closed with 6-0 plain gut sutures.

Lateral orbitotomy approach

This approach was first described for use in ONSF in 1872 by De Wecker and then subsequently in 1988 by Tse et al. and Patel and Anderson who performed the lateral approach without removal of the lateral orbital wall.[21] The benefits include an excellent perpendicular view of the optic nerve without the requirement of muscle disinsertion. However, it can potentially result in increased operating times and injury to the ciliary ganglion. Additionally, an external incision is required.

Technique

After the establishment of general anesthesia and patient preparation, a lateral canthotomy with cantholysis is performed. A traction suture is placed through the cut edge of the upper eyelid and the lid margin. This process is repeated for the lower eyelid. Finally, a third traction suture can be placed at the insertion of the lateral rectus muscle. The globe is adducted using the traction suture. The conjunctiva is incised with Wescott scissors as far laterally as possible, taking care not to injure the lacrimal gland or lateral rectus muscle belly. Blunt dissection with small malleables is carried out between the lacrimal gland and the lateral rectus muscle, directed posterior to the globe. It is often necessary to cut through any intermuscular septa that may be present before the intraconal compartment becomes accessible. Blunt dissection is continued through the intraconal fat until the optic nerve is identified. Neurosurgical cottonoids may then be used to pack around the nerve. As focus shifts to the optic nerve, an operating microscope may be brought over the field at this point per surgeon preference or loupes may be utilized for magnification. A fine, long forceps, such as a myringotomy or bayonet, is used to grasp the optic nerve sheath. An incision is then made in the optic nerve sheath at least 1 mm away from the insertion of the nerve using the surgeon’s instrument of choice, taking care to avoid contact with the nerve itself. A gush of CSF indicates successful full-thickness penetration of the sheath. Additional cuts with the scissors are made until a window of tissue is excised and removed. Hemostasis is confirmed, and the canthotomy and skin are closed using the surgeon’s preferred technique.

A revised lateral approach is another technique that obviates the need to traverse the intraconal fat by following the curve of the globe with the posterior dissection. In this way, it is similar to the medial transconjunctival approach but takes advantage of the additional space afforded by the anatomy.[rx]

Complications

Outcomes and Complications of ONSF for PTC/IIH

Optic nerve sheath fenestration is most commonly performed in the setting of IIH; therefore, most of the data published on postoperative complications and outcomes are in this clinical context. There are a variety of reported complications, most of which are transient and minor. There is no published comparison of outcomes and complications between different ONSF techniques.

In 2017, Kalyvas et al. looked at ONSF efficacy, complications, and the associated costs of surgical procedures for IIH. There were 525 ONSF procedures performed on 341 patients, with an average follow-up time of 42.3 months. The medial approach was performed in 342 eyes, the lateral in 53 eyes, combined in 3 eyes, and superomedial lid incision in 1 eye. They concluded that ONSF could reduce papilledema and improve vision. Disc swelling improved in 95% of patients, visual acuity improved in 67%, and visual fields showed improvement in 64%. However, ONSF was less efficacious in headache relief (41%). Approximately 11% required a second fenestration procedure even after initially improving.[23] In another study, 95% of patients who underwent ONSF had improved visual acuity and visual fields with no reported intraoperative complications. Mean follow-up was 18.7 months, and postoperative complications were ocular misalignment (6%) and corneal delle (0.8%).[24][16]

Interestingly, a retrospective chart review published in 2011 by Alsuhaibani et al. showed bilateral improvement of papilledema and vision with unilateral ONSF.[7][24] A search of the literature on outcomes of ONSF for IIH overwhelmingly reveals marked improvement in greater than 90% of patients in nearly every study, particularly in cases with acute papilledema.

In a 2008 prospective study in India that compared ONSF for papilledema from IIH vs. cerebral venous thrombosis, there was an improvement in 94% of patients in optic disc edema and vision with minimal and transient complications. These complications included tonic pupil (13.4%), diplopia (3.4%), and orbital cellulitis in 1 patient. Visual worsening was found in 2 eyes at 1 month postoperatively that failed to improve despite repeat fenestration.[25]

Fonseca et al. compared ONSF to CSF diversion procedures. They found that CSF shunting was superior to ONSF regarding visual improvement. However, ONSF patients in this study had worse preoperative papilledema and visual acuity. There were no complications reported, but 21% of ONSF patients eventually required CSF shunting.[26][24]

Additional studies on repeat fenestrations for papilledema also show significant improvement in visual function despite requiring multiple surgeries. There were two studies published in 1991 that looked at repeat ONSF procedures. In Kelman et al., 12 patients required repeated decompression with concurrent functioning lumboperitoneal shunts. All 12 patients showed improvement in visual function without surgical complications.[27] Spoor et al. studied 13 of 53 IIH patients with acute papilledema and visual loss who recently underwent ONSF. Eleven of 13 patients showed visual improvement even after secondary or tertiary decompressions.[28][24]

In general, visual outcomes tend to be worse the longer a nerve has been subjected to elevated perioptic ICP causing irreversible loss of axonal function. The earlier the intervention can be instituted to prevent progressive vision loss, the better.

Although not employed solely for the treatment of headaches, ONSF may reduce headache in over half of patients with IIH undergoing the procedure.[9][17] The mechanism for the improvement of headaches after ONSF is unclear.

The most feared and serious complication of ONSF is complete visual loss, which occurs either as a complication of the procedure or therapeutic failure of an uncomplicated fenestration despite improvement of the appearance of the optic nerve. The risk of blindness from ONSF is rare and reportedly 1 to 2% based on a large series study.[29][30] Blindness occurs from damage to the posterior ciliary arteries causing arterial occlusion or ischemic neuropathy and extraocular causes such as retrobulbar hemorrhage/hematoma formation. Optic nerve sheath cysts and pseudomeningoceles have also developed after ONSF and can present with postoperative vision changes, pain, and proptosis.[31] Ptosis, strabismus, or ciliary ganglion injury may also occur; however, the deficits related to these occurrences are often transient and minimal and are related to the surgical approach used for ONSF. The medial transconjunctival approach requires detachment of the medial rectus, which can result in exotropia and diplopia; however, the double vision is typically transient. Due to the close proximity of the posterior ciliary nerves and ciliary ganglion, the medial and lateral approaches may result in a tonic pupil.[29][32] Larger studies are required to determine the incidence of these complications.

Other potential complications of ONSF include the following:

• Conjunctiva and sclera: chemosis, globe perforation, conjunctival bleeding
• Cornea: corneal dellen, corneal ulceration
• Anterior chamber: acute angle-closure glaucoma, microhyphema
• Iris: tonic pupil
• Retina and choroid: branch retinal artery occlusion, central retinal artery occlusion, choroidal ischemia/infarction, chorioretinal scarring
• Optic nerve: optic nerve cysts/pseudomeningoceles, traumatic optic neuropathy
• Posterior ciliary nerve and ciliary ganglion: diplopia, ptosis, mydriasis
• Orbit: orbital apex syndrome, infection, hemorrhage/hematoma
• Extraocular muscles: strabismus, diplopia

Considering that visual loss is a very rare complication and others are relatively minor and transient, ONSF is an attractive option for the treatment of vision loss in PTC/IIH. This is especially true when considering the risks of alternative treatments, such as CSF diversion procedures and bariatric surgery.

• https://www.ncbi.nlm.nih.gov/books/NBK559130/
• https://www.ncbi.nlm.nih.gov/books/NBK499979/
• https://www.ncbi.nlm.nih.gov/books/NBK532877/
• https://www.ncbi.nlm.nih.gov/books/NBK534104/
• https://www.ncbi.nlm.nih.gov/books/NBK560583/
• https://www.ncbi.nlm.nih.gov/books/NBK538300/
• https://www.ncbi.nlm.nih.gov/books/NBK557853/
• https://www.ncbi.nlm.nih.gov/books/NBK430868/
• https://www.ncbi.nlm.nih.gov/books/NBK554479/
• https://www.ncbi.nlm.nih.gov/books/NBK560577/

Optic Nerve Compression results in progressive, and often painless, vision loss. In features of anterior and posterior compressive optic neuropathy. We next review the common causes of compressive optic neuropathy, which include orbital tumors (e.g., optic nerve sheath meningioma, optic glioma, and capillary hemangioma), orbital infection, orbital inflammation, intracranial tumors (e.g., pituitary macroadenoma, meningioma, and craniopharyngioma), aneurysm, and thyroid eye disease. We then review the workup for compressive optic neuropathy and discuss the various imaging options. Lastly, we discuss the clinical features, imaging findings, management options, and prognosis for visual recovery for patients with optic nerve sheath meningioma.

Any intrinsic or extrinsic compression anywhere along the optic nerve can produce compressive optic neuropathy (CON). Other than damage by compression, optic nerve damage can occur as a result of demyelination, ischemia, metabolic, and traumatic insult.[rx] The most common sign is a slow progressive monocular visual loss, sometimes associated with headaches. Bilateral visual loss can result from midline lesions (pituitary adenoma, craniopharyngioma, meningioma, giant aneurysms) or bilateral orbital lesions (thyroid disease, sarcoidosis). It is essential to correctly identify the cause of the CON as the differential diagnosis is broad, and management varies accordingly.

The optic nerve has over 1 million nerve fibers.[rx][rx] This quantity of fibers demonstrates the complexity and importance that the visual system has had in our evolution. The visual pathway starts in the retina and ends in the visual cortex at the occipital lobe. The retina consists of two functional parts: the optic part and the non-visual retina. The optic part of the retina consists of the neural and pigmented layer. In contrast, the non-visual retina is an extension of the pigmented layer and ends in the ciliary and iridial parts of the retina.

Pathophysiology

The optic nerve is part of the central nervous system (CNS). The mammalian CNS lacks the ability for regeneration and axonal growth. When the optic nerve is exposed to axonal damage, glial scars are formed that limits the diffusion of growth factors.[rx] Inhibitory proteins of myelin like Nogo and myelin-associated glycoprotein, low expression of growth factors, and lack of laminin are also some factors that hinder the ability for re-growth.[rx] The more proximal the damage is to the eye, the quicker the apoptosis of retinal ganglion cells will be. Apoptosis will lead to a cascade of p53 that will result in further cell death.

A CON can occur by compressing the vascular supply and causing ischemia to the nerve or directly causing mass effect upon the axons, thereby impairing axonal transport and signal transmission. The areas most susceptible to compression are where the nerve passes through small bone structures like the orbital apex and optic canal.

Optic nerve compression seen in exophthalmos secondary to thyroid disease occurs due to the enlargement of extraocular muscles due to the proliferation of fibroblasts, increased extracellular matrix, and adipocyte proliferation and differentiation.[rx][rx]

Causes of Optic Nerve Compression

CON can be produced by an extrinsic or intrinsic lesion, causing a mass effect anywhere along the optic nerve.[rx] Rarely, an intrinsic lesion of the optic nerve (optic nerve glioma) can cause a slow compression damaging its axons. Many disorders can compress the optic nerve and can be categorized as follows:

1. Infectious

   • Aspergilloma[rx][rx]
2. Inflammatory

   • Idiopathic orbital inflammation-pseudotumor[rx]
   • Sarcoidosis[rx]
   • Thyroid orbitopathy-Graves disease[rx][rx]
   • IgG4 pachymeningitis[rx][rx][rx][rx]
3. Vascular

   • Aneurysm[rx]
   • Carotid-cavernous fistula[rx]
   • Lymphangioma[rx][rx]
   • Orbital varix[rx][rx]
   • Ectatic internal carotid artery[rx][rx]
4. Traumatic

   • Hematoma[rx][rx]
   • Fracture[rx]
5. Neoplastic

   • Hemangioma[rx][rx]
   • Schwannoma[rx]
   • Meningioma[rx][rx]
   • Glioma (Neurofibromatosis Type 1)[rx]
   • Lymphoma[rx]
   • Sarcoma[rx]
   • Metastatic[rx]
   • Pituitary adenoma[rx]
   • Craniopharyngioma[rx]
   • Dermoid[rx]
6. Bone tumors/lesion

   • Fibrous dysplasia[rx][rx]
   • Paget’s disease[rx]
   • Osteoma[rx]
   • Osteopertrosis[rx][rx]
   • Hyperostosis[rx]
   • Langerhans cell histiocytosis[rx]
7. Other

   • Mucocele[rx][rx][rx]
   • Granulomatous meningitis[rx]

Diagnosis of Optic Nerve Compression

History and Physical

Patients with CON usually present with chronic progressive vision loss. It can be in one or both eyes. They can present with headaches, nausea, vomiting, diplopia, dyschromatopsia, exophthalmos, afferent pupillary defect, photophobia, red-eye, or unexplained weight loss. Sudden or rapid visual loss are rare except for traumatic cases. These cases usually have a blunt trauma or a penetrating injury. The nerve can be injured at any part, but the orbital apex and the optic canal are the most susceptible areas to damage.

It is imperative to obtain a good history and physical exam to help narrow the broad differential diagnosis.

• Vision loss: Symmetric or asymmetric
• Slow versus rapid onset
• Family history of cancer
• History of radiation
• Cardiovascular risk factors: hypertension, peripheral vascular disease, tobacco use
• Metabolic disease
• Autoimmune history

Physical Exam

• Snellen chart: Visual acuity
• Funduscopic exam
• Slit-lamp examination: Evaluate retina, retinal arteries and veins, cornea, fovea, optic cup
• Visual field test: Help differentiate central vs. peripheral visual loss
• Ishihara’s test: Evaluate if a color deficit is present
• Tonometry: Evaluate intraocular pressure
• Extraocular eye movements
• Proptosis

Evaluation

A full neurological examination followed by a complete ophthalmological evaluation should be performed. The exam will give a baseline visual acuity and monitor progression or improvement.

The eye with optic nerve compression will have reduced visual acuity. It can also show a deficiency in color vision (dyschromatopsia), which can be evaluated by using the Ishihara test plate. Proptosis or resistance to manual pressure, suggest an intraorbital lesion. Ocular motility abnormalities are checked. The optic disc can be atrophic or edematous but can also appear healthy. Optociliary shunt vessels can be seen due to obstruction in the venous return.

Laboratory studies include complete blood count, comprehensive metabolic panel, lipid panel, thyroid-stimulating hormone, T3, T4, luteinizing hormone, anti-thyroid antibodies, thyroid releasing hormone, follicle-stimulating hormone, prolactin, adrenocorticotropin hormone, insulin growth factor-1, cortisol, bone-specific alkaline phosphatase, and prostate-specific antigen. The angiotensin-converting-enzyme is usually elevated in more than half of the patients with active sarcoidosis.

Brain and orbit magnetic resonance imaging will show in detail the optic nerves, para sellar area, and orbital contents. Fat suppression images are needed to demonstrate enhancing lesions inside the orbit. Lesions involving the orbital bones should be examined using a computed tomographic (CT) scan of the head and orbit. It will demonstrate orbital fractures and concomitant injuries in traumatic cases. Ultrasonography can be used for intraorbital biopsy of anterior lesions.

Treatment of Optic Nerve Compression

The first step in management is to treat the underlying condition. Corticosteroids are beneficial for inflammation (sarcoidosis) and thyroid disease CON. For these conditions, withdrawal of the steroid treatment can cause acute deterioration of vision. Surgical orbital decompression can help CON caused by thyroid ophthalmoplegia.[rx][rx][rx] For tumors intimately attached to the optic nerve, like optic nerve meningiomas, surgery can cause further loss of vision. Radiation therapy is beneficial for aggressive recurrent tumors and those in areas adjacent to cranial nerves and eloquent brain. It can also be used for surgically difficult to reach tumors like the cavernous sinus. Radiation can cause irreversible optic nerve damage; therefore, it has to be used judiciously.

In traumatic cases, conservative treatment is appropriate in patients with mild deficits as spontaneous improvement is possible. Steroids have no benefit for trauma. Surgery is used for patients with radiological evidence of compression.[rx][rx] Direct compression of the optic nerve by bone fragments or a subperiosteal hematoma is usually treated surgically.[rx][rx] However, surgery carries the risk of complications such as postoperative cerebrospinal fluid leak and meningitis.[rx]

• https://www.ncbi.nlm.nih.gov/books/NBK559130/
• https://www.ncbi.nlm.nih.gov/books/NBK499979/
• https://www.ncbi.nlm.nih.gov/books/NBK532877/
• https://www.ncbi.nlm.nih.gov/books/NBK534104/
• https://www.ncbi.nlm.nih.gov/books/NBK560583/
• https://www.ncbi.nlm.nih.gov/books/NBK538300/
• https://www.ncbi.nlm.nih.gov/books/NBK557853/
• https://www.ncbi.nlm.nih.gov/books/NBK430868/
• https://www.ncbi.nlm.nih.gov/books/NBK554479/
• https://www.ncbi.nlm.nih.gov/books/NBK560577/

Causes of Optic Nerve Cysts

The cysts may be caused by a variety of etiologies including idiopathic, associated with meningiomas, meningoceles, arachnoid cysts, neuroepithelial cysts, and post-traumatic or postoperative cysts. Specifically, optic nerve sheath meningocele is a dilation of the optic nerve sheath, and expansion of the subarachnoid or subdural cerebrospinal fluid (CSF)-filled space around the optic nerve. An arachnoid cyst is a benign proliferation of normal fibrovascular tissue of the leptomeninges around the optic nerve. Neuroepithelial cysts are rare lesions that typically present in the ventricles or cerebral parenchyma. They have rarely been reported in the intracranial portion of the optic nerve.[rx]

Cysts may also be iatrogenic. For example, Naqvi et al.[rx] reported 2 cases of cysts that formed after optic nerve sheath fenestration.

Diagnosis of Optic Nerve Cysts

Histopathology of optic nerve sheath meningoceles and arachnoid cysts shows normal meninges.[rx]

A colobomatous optic nerve cyst is composed of 2 layers. The inner layer has retinal architecture and is derived from primitive neuroretinal tissue. The outer layer has vascularized connective tissue continuous with the sclera.[rx]

Mehta et al.[rx] reported on a case of a neonate with a neuroepithelial cyst that was resected. Histopathology showed the cyst was lined by simple cuboidal epithelium without cilia or goblet cells. No neural tissue was present; however, immunohistochemical staining was positive for S100, a marker of neural tissue.[rx]

History and Physical

Patients with optic nerve cysts may be asymptomatic or present with nonspecific orbital or neurological findings. In the setting of arachnoid cysts, visual acuity can range from 20/20 to NLP.

Garrity et al.[rx] reported in a case series of thirteen patients with optic nerve sheath meningocele who presented with a headache, decreased vision, proptosis, afferent pupillary defect, enlarged blind spot, optic disc edema, shunt vessels at optic disc, and tortuous retinal veins.

Colobomatous cyst often involves the optic nerve itself. As the lesion grows, typically inferiorly, it may produce a palpable mass behind the lower eyelid. In some cases, lowering the lower eyelid can reveal a dark uveal pigmentation to the mass. Bilateral cases may be associated with systemic diseases such as chiasmal glioma, polycystic kidney, trisomy, or Edward syndrome.[rx]

Evaluation

Lesions that can be localized on history and physical exams to the optic nerve or brain should be imaged with the brain and or orbital MRI with and without contrast. Imaging can elucidate the size of the cyst, consistency, and effect on surrounding structures with imaging. Typically arachnoid cysts and meningoceles will have a signal intensity equal to that of CSF, dark on T1 and bright on T2, without enhancement after intravenous (IV) contrast administration.[rx]

Optic nerve sheath meningoceles appear as tubular-cystic enlargement of the optic nerve and optic nerve sheath complex on CT and MRI. On coronal MRI both the optic nerve and sheath appear dilated in a “bull’s eye” pattern. Off-axis sagittal views are the best for showing the widening of the meninges with a fluid-filled space.[rx]

In children with microphthalmos, orbital ultrasound should be used to determine the organization of ocular structures. US can also be used to visualize cysts in these patients. MRI is more helpful than CT in characterizing the content of the cyst, which is usually similar to vitreous or CSF. MRI also be used to visualize the relationships or connections, communications between the cyst, nerve, and globe. Visual potential can be evaluated with retinal electrophysiology.[rx]

Treatment of Optic Nerve Cysts

Given the variety of etiologies of optic nerve cysts, treatment is not uniform or established in the literature.

Our understanding of treatment is mostly based on case studies or case series. The main concern with surgical resection or drainage of these cysts is that it can be associated with significant morbidity due to the risk of optic nerve transection. Additionally, surgeons have to ensure that the cyst itself is not a coloboma in an eye with stable vision as drainage can result in drainage of intraocular contents.

Saari et al.[rx] reported an interesting case of arachnoid cyst of the intraorbital optic nerve in 1977 before imaging was available. The patient presented with slight pain on eye movements, and transient attacks of blurred vision, optociliary shunt vessels, significant optic disc edema with hemorrhages and macular edema, and shallowing of the anterior chamber all in the left eye only. The patient had a lumbar puncture with an opening pressure of 150 mm H20. The lesion was thought to be a meningioma. Optic nerve sheath fenestration showed forceful and voluminous egress of CSF. Histopathology of the resected optic nerve sheath was normal. Nine months after the procedure, the left eye was blind with significant optic nerve atrophy. The authors recommend prompt diagnosis and drainage of arachnoid cysts to preserve vision.

Naqvi et al.[rx] reported 2 cases of post optic nerve sheath fenestration optic nerve cysts. The patients had loculated CSF surrounded by fibrous proliferation at the site of previous optic nerve sheath fenestration. One patient presented with pain, proptosis, and visual loss 9 months after the initial procedure. The other patient presented with vision decline and choroidal folds. Both patients underwent repeat fenestration during which the closed sheath with fibrosis and outpouching was visualized. Larger windows were created in the sheaths.

For optic nerve sheath meningoceles, Lunardi et al.[rx] reviewed 31 cases and recommended early surgical management of optic nerve sheath decompression in patients who present with a rapid decrease in visual acuity over 3 to 6 months. They endorsed the improvement of visual function with minimal morbidity.

Mehta et al.[rx] reported a case of neuroepithelial cyst that presented in a 6 week old with proptosis, exotropia, RAPD, and normal anterior and posterior segment ophthalmic exam. The cyst was drained and resected by anterior orbitotomy through an upper eyelid crease incision. The patient’s presenting signs including RAPD resolved postoperatively. The authors iterate that the goal of treatment in patients who present with optic nerve cysts is to prevent or reverse vision loss.

Colobomatous cysts can be aspirated if they are cosmetically unacceptable. In many cases, if the cyst returns after repeat aspirations, the eye and cyst may be removed, and the socket may be fitted with prosthetics.[rx]

• https://www.ncbi.nlm.nih.gov/books/NBK559130/
• https://www.ncbi.nlm.nih.gov/books/NBK499979/
• https://www.ncbi.nlm.nih.gov/books/NBK532877/
• https://www.ncbi.nlm.nih.gov/books/NBK534104/
• https://www.ncbi.nlm.nih.gov/books/NBK560583/
• https://www.ncbi.nlm.nih.gov/books/NBK538300/
• https://www.ncbi.nlm.nih.gov/books/NBK557853/
• https://www.ncbi.nlm.nih.gov/books/NBK430868/
• https://www.ncbi.nlm.nih.gov/books/NBK554479/
• https://www.ncbi.nlm.nih.gov/books/NBK560577/