Chemical peeling, also known as chemexfoliation or chemical exfoliation, is a procedure where a chemical substance applied to the skin causes controlled destruction of the epidermis with or without part of the dermis, leading to skin regeneration and remodeling. Chemical peels can be used to treat various skin conditions, such as acne vulgaris, photodamage, pigmentary disorders, and scars. Also, dermatologists and facial plastic surgeons commonly employ a chemical peel as a therapeutic or cosmetic intervention to enhance appearance either alone or in combination with other techniques, such as laser or dermabrasion.[rx][rx] Chemical peels are commonly classified based on their depth of skin penetration into superficial, medium, and deep peels. The factors affecting the depth of peeling, and thus the degree of its therapeutic effects, include the properties of the chemical agent used (e.g., concentration and pH), the physician’s application technique, and the patient’s skin condition and sensitivity.
Anatomy and Physiollogy
A basic comprehension of the layers composing skin is crucial to understanding the different types of chemical peeling and its depth of action.[rx] The skin consists of three layers, epidermis, dermis, and subcutaneous tissue (hypodermis).
- Epidermis represents an avascular layer that consists of 4 layers (superficial to deep): stratum corneum, stratum granulosum, stratum spinosum, and stratum basale. The only exceptions to that are the palms of the hands and soles of the feet where a fifth layer called the stratum lucidum lies below the stratum corneum. Keratinocytes (squamous cells) are the predominant cells in all those layers, except the stratum basale layer where cuboidal stem precursor cells exist in a single layer. Keratinocytes produce keratin, which is a protein present in skin, hair, and nails.
- Stratum corneum is a protective layer consisting of 15 to 30 layers of highly keratinized dead cells that shed and are replaced by layers below.
- Stratum granulosum has squamous cells with a “grainy” appearance due to the increased amount of keratin protein.
- Stratum spinosum layer consists of cells of which have prominent processes that interlock the cells together via desmosomes. These desmosomes ensure the integrity of the bond between the squamous skin cells. Langerhans macrophage cells are found interspersed in this layer.
- Stratum basale is at the base of the epidermal layer connecting it to the dermis. In this layer are melanocytes and Merkel cells which are responsible for producing melanin pigment and for sensory stimulation of touch respectively.
- The dermis consists of connective tissue that contains structures including hair follicles, sebaceous glands, nerves, and blood vessels.
- The papillary layer consists of finger-like projections (also known as dermal papillae) intertwining with the stratum basale and connecting the epidermal and dermal layers.
- The reticular layer lies just beneath the papillary layer and has elastin and collagen fibers, which give skin its elasticity and tensile strength. This layer is divided based on depth into the upper, mid, and lower reticular dermis.
- The subcutaneous tissue (i.e., hypodermis layer) is the deepest layer of skin and consists of loose connective tissue and fat.
Chemical peels are commonly classified based on their depth of skin penetration into superficial, medium, and deep peels. Superficial peels penetrate the stratum corneum of the epidermis and are further sub-classified by some authors into very light (to stratum spinosum) and light (to stratum basale). Medium peels penetrate the papillary dermis to the upper reticular dermis, while the deep peels’ effect reaches the mid-reticular dermis.
When performing a chemical peel, proper patient evaluation, and execution of a comprehensive treatment plan can produce safe, reliable, and satisfactory outcomes. Various indications for using chemical peeling as a technique for skin resurfacing exist, including:
- Facial rejuvenation of the aging skin to address issues such as enlarged pores and rhytides
- Inflammatory disorders including acne vulgaris, pseudofolliculitis barbae, rosacea, and post-acne scarring
- Pigmentary disorders such as melasma, ephelides or freckles, lentigines, and post-inflammatory pigmentation
- Epidermal proliferation and pre-cancerous lesions including sebaceous and actinic keratosis
Contraindications to chemexfoliation include:
- History of an allergic reaction to a peeling agent
- Ongoing infection, open lacerations or wounds in the area of the peel
- Recent isotretinoin use in the last 6 months (particularly relevant to medium and deep peels)
- Patients who are pregnant or breastfeeding
- Patients with psoriasis, atopic dermatitis, exposure to radiation therapy, or who had recent facial surgery are generally not candidates for chemical peeling.
- Special considerations are also necessary for patients with a known propensity for keloid formation or poor wound healing with uncontrolled diabetes or immunosuppression, as well as patients classified as Fitzpatrick III-VI as they may be prone to aberrant pigmentation or dyschromasia (see Preparation section below)
The first and most important equipment required for performing chemical peeling is the actual peeling agent. Before delving into listing the various types of solutions and acids used for this purpose, a swift review of basic chemistry is necessary to understand some of the commonly used terms. One important concept is the pKa of the peeling agent, which is the pH at which 50% of the chemical is in a free acid state. When choosing the type of peel used, a lower pKa correlates with a stronger peel. Indeed, there are many types of peeling agents available.
- Alpha hydroxy acids (AHA) are a fruit-derived family of carboxylic acid. In this group, the hydroxyl group links to the alpha part of the chemical. Examples to AHAs are glycolic acid from sugar cane, lactic acid from milk, and citric acid from citrus fruits.
- Beta hydroxy acids (BHA) on the other hand, have their hydroxyl group attached to the second carbon atom and include salicylic acid. A popular solution containing equal parts (14 grams) of salicylic acid, lactic acid 85%, resorcinol (a phenol derivative), and a 100 mL of ethanol is Jessner solution.
- Trichloroacetic acid (TCA) is a widely used peeling agent with varying effects according to concentration and whether it was part of a solution.
- Phenol, which is an aromatic hydrocarbon, represents a powerful chemical used in deeper peels. For example, one can mix phenol 88% with hexachlorophene, soap, croton oil, and distilled water to create the Baker-Gordon formula.
The following are examples of common agents listed with its corresponding depth of effect:
- Very light peels: 10 to 20% TCA, low-potency glycolic and salicylic acid, and retinoic acid
- Light peels: 20 to 30% TCA, Jessner solution, and 40 to 70% glycolic acid
- Medium peels: combinations of 35% TCA with either Jessner’s solution (credit to Monheit) or 70% glycolic acid (credit to Coleman)
- Deep peels: greater than 50% TCA, croton oil-phenol combinations (e.g., Baker-Gordon formula)
Each agent has a slightly different mechanism of action, not within the scope of this review, but the principles are generally the same. Epidermolysis, particularly of the stratum corneum provides smoother skin. Peels that reach the basal layer containing melanocytes will improve uneven pigmentation. When peels affect the papillary or upper to the mid-reticular dermis, this aids with reducing rhytides with deposition of elastin and collagen.
For patient safety, resuscitation equipment needs to be available in a well-ventilated procedure room. Personnel must wear gloves. Application brushes, cotton swabs, and gauze should be available as well. Cardiac monitoring must be available if using phenol, given the risk of cardiotoxicity.
A clinician trained in the application and management of skin disorders requiring chemical peeling generally performs this procedure with a nurse present to assist. An anesthesiologist must be present if using general anesthesia or a phenol peel.
Before considering this procedure, obtaining informed consent is necessary. Photo documentation is encouraged after dedicated patient consent for comparison. Post-peel expectations require discussion with the patient. A complete history and physical examination must exclude any contraindications for the procedure as listed above.
The first step is choosing the appropriate depth of peel according to the patient’s skin type and condition requiring treatment. One commonly used classification of skin phototypes is the Fitzpatrick classification. It classifies patients into six different complexion colors/types by their reaction to sun exposure:
- Type I: very fair skin, blue eyes, freckles, and light hair. Skin burns with sun exposure and does not tan
- Type II: fair skin, hazel/green/blue eyes, and light hair. Can tan with difficulty, but frequently burns
- Type III: white skin, any eye, or hair color. Can tan gradually, mild burn sometimes with sun exposure
- Type IV: brown or Mediterranean skin, tan easily and do not routinely burn
- Type V: dark brown or Middle-eastern skin, tan very easily and infrequently burn
- Type VI: black skin, almost never burns and tans very easily
Generally, light or very light peels are appropriate for all Fitzpatrick skin types. The perfect patient for medium or deep chemexfoliation is a female with fair skin and blue eyes (i.e., Fitzpatrick I). Fitzpatrick III-VI patients may be prone to aberrant pigmentation or dyschromasia.
Another way to categorize skin is by using the Glogau classification, which assesses photoaging:
- Type I: early aging changes with no to minimal wrinkling
- Type II: smile lines form (i.e., wrinkles in motion)
- Type III: Wrinkles at rest
- Type IV: widespread wrinkles
Special considerations in male patients are often needed as they tend to have thicker skins with less predictable peeling outcomes. One way to reliably predict the result is with pre-peel spot testing, though this is not a routine process; nevertheless, some clinicians elect to perform pre-peel testing to help select the proper peel for the patient.
The clinician should perform priming should to facilitate effective peeling. Adequate priming should be started 2 to 4 weeks before the procedure and involves the application of a topical agent on the skin to thin the stratum corneum layer, which increases the depth of penetration. A commonly prescribed priming agent is all-trans retinoic acid or tretinoin (0.025 to 0.05%) cream. Other options include salicylic and glycolic acids at 5 to 10% concentration. For patients susceptible to hyperpigmentation, hydroquinone (2 to 4%) cream can be utilized pre- and post- peel to decrease post-inflammatory pigmentary reactions by targeting and inhibiting tyrosinase in melanocytes. In patients with a history of herpes simplex viral infection, a course of acyclovir merits consideration, though some authors routinely prescribe prophylactic antivirals as well.
The clinician must stress the importance of protection against UV rays (pre and post-treatment) to the patient. Patients are advised to avoid procedures that could traumatize the skin such as dermabrasion and waxing. On the day of the procedure, they should be instructed to cleanse their skin and not to apply any skincare or makeup products.
Before starting the chemical peeling procedure, the patient’s skin requires cleaning with a degreasing agent such as isopropyl alcohol or acetone for the removal of any residue or makeup. They should be in a supine position with the head of the bed elevated to 45 degrees. A hair cap should be worn to keep the hair away from the treatment field. The clinician can apply petroleum jelly to areas of potential pooling of the chemical such as the nasolabial folds and lateral canthi. The eyes should be covered with an appropriate shield. General anesthesia is not routinely required but may be a consideration for deep peels. Oral analgesics can be administered, such as acetaminophen.
The peeling agent can be applied on the thicker skin regions first (i.e., chin, nose, cheeks, and forehead) followed by the thinner skin regions (i.e., around the eyes and mouth). A brush, gauze, or a wooden spatula can be used to apply the chemical in evenly distributed strokes, the choice of which application method will depend on the desired effect and chemical peel used. Peel-specific application nuances exist for each type and require experience until achieving mastery. As an example, gauze sponges can be more abrasive and more suitable for liquid formulations. Overlaps in brushstroke coverage should be avoided as it may cause overconcentration of the agent and an uneven outcome. The edge of the treatment site can be feathered to blur demarcation lines.
Certain areas requiring special attention include:
- Thicker keratoses: these areas tend to pick up solution unequally and thus frost unevenly; therefore, additional applications with vigorous rubbing may be needed to maximize penetration
- Wrinkled skin: should be stretched to ensure an even coating into troughs and folds
- Peri-oral rhytids: may require application with the wooden end of the cotton-tipped applicator, with extension to the vermilion of the lip
- Deep wrinkles (expression lines): should be treated as the surrounding skin since these areas are best treated with either neuromodulator and/or filler
- Eyelid skin: delicate and careful application is paramount so that excess peel solution does not come into the eyes or come into contact with tears; a semi-dry applicator should be gently rolled onto the lids and periorbital skin, with care to remain at least 3 mm from the lid margin;
Post-application, specific agents (primarily TCA) can induce a reaction called frosting. It has three levels based on the degree of protein denaturation. In level, I frosting (superficial peel), erythema, and patchy areas of frosting occur. Level II (medium peel) represents a white coat of frosting with some erythema being visible beneath, while level III (deep peel) exists when a white plaque-like formation with almost no erythema visible.
After peel application, cool saline compresses can be applied to help neutralize the solution (glycolic acid) and/or provide patient comfort. “Vinegar soaks” using 0.25% acetic acid facilitates healing, as does the application of a non-scented, bland emollient 3 to 5 times a day to the desquamated areas after soaks. Desquamation usually concludes within several days (depending on the depth of the peel) and is followed by intense erythema which gradually subsides over 1 to 2 weeks thereafter. After 24 hours, the patient may shower and use a non-detergent face wash. Protection against sun exposure and applying sunscreen requires re-emphasis. The patient should defer use of any makeup/concealer until the skin has healed.
A general rule-of-thumb when thinking about the risks of chemexfoliation is that the deeper the peel correlates with increased therapeutic effects but also more significant risks. Certain patients are more vulnerable to complications, including darker skin individuals with a known history of scar formation. Exposure to isotretinoin would also negatively affect peeling outcomes. As once can infer, patient selection and counseling remain pivotal to maximize results and lessen complications. Strict adherence to the pre- and post-peel care instructions, especially sun avoidance, is important to optimize results as well.
Complications for chemical peeling can be classified into immediate (minutes to hours) or delayed (days to weeks):
- Immediate complications include skin edema, burning, and itching sensations, and blistering. Complications requiring emergent management include anaphylactic reactions, inadvertent ocular mucosal splashes, and cardiac arrhythmia with phenol application. Hence, liver and kidney function testing is encouraged before phenol peeling to reduce the chance of toxicity. A rare complication specific to salicylic acid is acute toxicity with salicylism, where the patient develops tinnitus, nausea, and shortness of breath.
- Delayed complications include bacterial, viral, or fungal infections. Chemoprophylaxis can be a consideration with medium and deep peels. Scarring can occur as well, the risk factors for which are skin “picking,” recent isotretinoin use, exposure to therapeutic radiation, history of keloid formation, and aggressive peeling in thin-skinned regions such as the neck. Prolonged erythema, acne, and milia eruptions are potential risks, but these tend to be self-limited. Patients with Fitzpatrick skin types IV to VI, estrogen use, pregnancy, or excessive sun exposure may be more prone to post-inflammatory hyperpigmentation. Hypopigmentation and lines of demarcation can result from deep peels in particular. Lastly, incomplete responses can occur, which heightens the importance of managing expectations.
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