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:
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.
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Vision loss: Symmetric or asymmetric
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Slow versus rapid onset
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Family history of cancer
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History of radiation
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Cardiovascular risk factors: hypertension, peripheral vascular disease, tobacco use
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Metabolic disease
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Autoimmune history
Physical Exam
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Snellen chart: Visual acuity
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Funduscopic exam
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Slit-lamp examination: Evaluate retina, retinal arteries and veins, cornea, fovea, optic cup
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Visual field test: Help differentiate central vs. peripheral visual loss
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Ishihara’s test: Evaluate if a color deficit is present
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Tonometry: Evaluate intraocular pressure
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Extraocular eye movements
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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]
References
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