Chronic Effects of Sunlight
(See also Overview of Effects of Sunlight.)
Chronic exposure to sunlight ages the skin (photoaging, dermatoheliosis, extrinsic aging), primarily by causing destruction of skin collagen due to various biochemical and DNA disruptions. Skin changes include both fine and coarse wrinkles, rough leathery texture, mottled pigmentation, lentigines (large frecklelike spots), sallowness, and telangiectasia.
Actinic keratoses are precancerous changes in skin cells (keratinocytes) that are a frequent, disturbing consequence of many years of sun exposure. People with blonde or red hair, blue eyes, and skin type I or II are particularly susceptible.
Actinic keratoses are usually pink or red, poorly marginated, and feel rough and scaly on palpation, although some are light gray or pigmented, giving them a brown appearance. They should be differentiated from seborrheic keratoses, which increase in number and size with age. Seborrheic keratoses tend to appear waxy and stuck-on but can take on an appearance similar to actinic keratoses. Close inspection usually reveals distinguishing characteristics of the lesion. Unlike actinic keratoses, seborrheic keratoses also occur on non–sun-exposed areas of the body and are not premalignant.
The incidence of squamous cell carcinoma and basal cell carcinoma in fair, light-skinned people is directly proportional to the total annual sunlight in the area. Such lesions are especially common among people who were extensively exposed to sunlight as children and adolescents and among those who are chronically exposed to the sun as part of their profession or recreational activities (eg, athletes, farmers, ranchers, sailors, frequent sunbathers). Sun exposure also substantially increases the risk of malignant melanoma.
Treatment begins with preventive efforts to minimize UV light exposure—avoiding the sun and tanning beds and wearing protective clothing and sunscreen.
Various combination therapies, including chemical peels, 5-fluorouracil (5-FU), topical alpha-hydroxy acids, imiquimod, photodynamic therapy, and tretinoin, have been used to reduce carcinogenic changes and improve the cosmetic appearance of chronically sun-damaged skin. These therapies are often effective in ameliorating superficial skin changes (eg, fine wrinkles, irregular pigmentation, sallowness, roughness, minor laxity) but have a much less pronounced effect on deeper changes (eg, telangiectasias). Many ingredients are used in OTC cosmetic products without significant evidence that they improve chronic changes of the skin caused by sunlight.
There are several options, depending on the number and location of lesions:
If only a few actinic keratoses are present, especially if large, cryotherapy (freezing with liquid nitrogen) or curettage with electrocautery are rapid and highly effective treatments.
If there are too many lesions for cryotherapy or curettage with electrocautery, topical 5-FU applied to the affected area nightly or twice daily for 2 to 6 wk often clears the majority of lesions. Several strengths and formulations of 5-FU are commercially available. Many patients tolerate 0.5% 5-FU cream applied once/day for 4 wk on the face better than stronger concentrations. Actinic keratoses on the arms may require stronger concentrations, such as 5% cream.
Topical 5-FU causes an inflammatory reaction, with redness, scaling, and burning, often affecting areas with no visible actinic keratoses. The intensity of this unsightly and uncomfortable reaction correlates with the effectiveness of treatment. However, if the reaction is too uncomfortable, application may be suspended for 1 to 3 days and, if necessary, can be suppressed with topical corticosteroids. Topical 5-FU has few significant adverse effects except for this reaction. 5-FU should not be used to treat basal cell carcinomas, except those shown by biopsy to be superficial.
Photodynamic therapy involves topical application of a systemic photosensitizer (eg, aminolevulinate or methyl aminolevulinate), followed by light of a specific wavelength that causes preferential damage of photodamaged skin vs normal skin. After treatment, the skin appears similar to how it would look after a mild to moderate sunburn. Significant benefits include the ability to treat multiple lesions at one time and a shorter downtime (when skin appears red, scaly, and irritated) compared to topical creams such as imiquimod and 5-FU.
The topical immunomodulator imiquimod is often used for treatment of actinic keratoses and superficial basal cell carcinomas. Imiqimod stimulates the immune system to recognize and destroy these precancerous skin lesions and early skin cancers. Duration of treatment for actinic keratoses is about 12 to 16 wk. A newer topical gel, ingenol mebutate, can be applied for 2 to 3 days to treat actinic keratoses and has the advantage of a short course of therapy. The skin reactions to imiquimod and ingenol mebutate are similar to those of 5-FU, with redness, scaling, and crusting occurring in most patients.
For treatment of skin cancers, see Cancers of the Skin.