Xeroderma pigmentosum

Definition

Photosensitivity is any increase in the reactivity of the skin to sunlight.

Description

The skin is a carefully designed interface between our bodies and the outside world. It is infection-proof when intact, nearly waterproof, and filled with protective mechanisms. Sunlight threatens the health of the skin. Normal skin is highly variable in its ability to resist sun damage. Natural skin pigmentation is its main protection. The term photosensitivity refers to any increase beyond what is considered normal variation.

Causes & symptoms

There are over three dozen diseases, two dozen drugs, and several perfume and cosmetic components that can cause photosensitivity. There are also several different types of reaction to sunlight--phototoxicity, photoallergy, and polymorphous light eruption. In addition, prolonged exposure to sunlight, even in normal skin, leads to skin aging and cancer. These effects are accelerated in patients who have photosensitivity.

Diseases of several kinds increase skin sensitivity.

• Phototoxicity is a severely exaggerated reaction to sunlight caused by a new chemical in the skin. The primary symptom is sunburn, which is rapid and can be severe enough to blister (a second degree burn). The chemicals associated with phototoxicity are usually drugs. The list includes several common antibiotics--quinolones, sulfonamides, and tetracyclines; diuretics (water pills); major tranquilizers; oral diabetes medication; and cancer medicines. There are also some dermatologic drugs, both topical and oral, that can sensitize skin.
• Photoallergy produces an intense itching rash on exposure to sunlight. Patients develop chronic skin changes (lichen simplex) as a result of scratching. Some of the agents that cause phototoxicity can also cause photoallergy. Some cosmetic and perfume ingredients, including one of the most common sunscreens--para-amino benzoic acid (PABA)--can do this.
• Polymorphous light eruption resembles photoallergy in its production of intensely itching rashes in sunlight. However, this condition lessens with continued light exposure, and so is seen mostly in the spring. Also, there does not seem to be an identifiable chemical involved.

• A hereditary disease called xeroderma pigmentosum includes a defect in repair mechanisms that greatly accelerates skin damage from sunlight.
• A family of metabolic diseases called porphyrias produce chemicals (porphyrins) that absorb sunlight in the skin and thereby cause damage.
• Albinos lack skin pigment through a genetic defect and are thus very sensitive to light.
• Malnutrition, specifically a deficiency of niacin known as pellagra, sensitizes the skin.
• Several diseases like acne, systemic lupus erythematosus, and herpes simples (fever blisters) decrease the resistance of the skin to sun damage.

Diagnosis

The pattern of appearance on the skin, a history of drug or chemical exposure, and the timing of the symptoms often suggests a diagnosis. A skin biopsy may be needed for further clarification.

Treatment

Removal of the offending drug or chemical is primary. Direct sunlight exposure should be limited. Some people must avoid sunlight altogether, while others can tolerate some direct sunlight with the aid of sunscreens.

Prevention

A sunscreen with an SPF of 15 or greater protects most skin from damage. Protective clothing such as hats are highly recommended in addition.

Key Terms

Biopsy

Surgical removal of tissue for examination.

Further Reading

For Your Information

Books

• Bickers, David R. "Photosensitivity and other reactions to light." In Harrison's Principles of Internal Medicine. edited by Anthony S. Fauci, et al. New York: McGraw-Hill, 1998, pp. 329-333.
• Harber, Leonard C. "Abnormal responses to ultraviolet radiation: drug induced photosensitivity." In Dermatology in General Medicine, edited by Thomas B. Fitzpatrick, et al. New York: McGraw-Hill, 1993 pp. 1677-1689.

Gale Encyclopedia of Medicine. Gale Research, 1999.