Photosensitivity is any increase in the reactivity of the skin to sunlight.
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 and 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.
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 itchingrash 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.
Diseases of several kinds increase skin sensitivity.