Some people develop a rash because their skin is sensitive to sunlight; this is known as photosensitivity. Patients may not associate their skin complaint with the light. It is not always the bright summer sun which is responsible; some people also react to winter daylight, and very sensitive people may even be affected by fluorescent lamps indoors.
Photosensitivity occurs for a variety of reasons:
Although most people with the common skin conditions psoriasis and atopic dermatitis (eczema) find sun exposure or ultraviolet light treatment helpful, about 10% report they cause flare-ups.
Photosensitizing medications: Photosensitizing medications can result in unexpected sunburn or a dry, bumpy or blistering rash on exposed skin (face, neck, and arms, backs of hands and often lower legs and feet). The rash may or may not be itchy. They can also result in onycholysis (nail plate lifting off the nail bed). The most common medications causing photosensitivity are diuretics, tetracycline antibiotics, and anti-inflammatory agents.
Photo contact dermatitis: Skin contact with various chemicals may result in a toxic or allergic reaction on sun-exposed skin. Photo contact dermatitis most often arises from tar products, fragrances, and sometimes sunscreens.
Blisters and brown streaks may occur from touching certain plants followed by sun exposure. This is known as phytophotodermatitis. The most common causes are vegetables (celery, parsley) and their leaves (parsnips, carrots), fruit (figs, citrus) and weeds (e.g. hogweed) that contain photosensitizing psoralen chemicals.
Cutaneous lupus erythematosus: Cutaneous lupus erythematosus (LE) is an autoimmune disease that most often affects young adult women. Cutaneous LE can be provoked by sunlight, but it is actually more common in dark skinned than in fair skinned people. Sunscreens do not totally prevent it.
In the most common form, discoid LE, unsightly red scaly patches develop which leave white scars. Discoid LE predominantly affects the cheeks and nose, but sometimes involves the upper back, V of neck, and backs of hands. Bald patches can develop if the hair follicles are involved. Discoid LE may affect the lips causing ulcers and scaling.
A few people with cutaneous LE also have systemic lupus erythematosus (SLE). They may have a rash on all sun exposed skin, mouth ulcers, and diffuse hair thinning. SLE may affect joints, kidneys, lungs, heart, liver, brain, and blood. Management of cutaneous LE should always include careful sun protection.
Actinic prurigo: Actinic prurigo affects mostly female children prone to atopic dermatitis (eczema), especially American Indians. They develop intensely itchy exposed skin and lips throughout the year. They usually fail to realize the role of sunlight, but must be persuaded to cover up when outdoors.
Solar urticaria: Solar urticaria is a rare form of physical urticaria (hives) in which skin swells within minutes of exposure to ultraviolet radiation. The reaction lasts up to an hour and can be very disabling. Oral antihistamines are helpful, but rarely prevent the reaction altogether.
There is enough UV rays to cause a rash on photosensitive skin between 10 a.m. and 5 p.m. even on a cloudy day. Bright surfaces, like snow, concrete and sand, reflect UV rays and can nearly double the amount that gets to the skin. Confine summer excursions out of doors to early in the morning or late in the evening. Sun protection is needed whatever the weather. It is needed even if you sit in the shade. Protect yourself in the car and house too; UVA can pass through window glass.
There are two basic ways of protecting your skin from the damaging effects of UV rays:
UV ray-absorbing film can be applied to windows at home or in the car. Unguarded fluorescent daylight lamps can occasionally provoke a rash. Ordinary tungsten light bulbs are usually all right. It is perfectly safe to watch television.
For the most severely light sensitive patients, normal activities may be severely curtailed. Some find night work and sleep during the day, others put up with the rash. Nearly always, medications in the form of ointments or tablets can help to a variable extent.
Porphyria refers to a group of genetic diseases in which there are excessive porphyrins in the blood or tissues. Porphyrins are chemicals involved in synthesis of the red pigment in blood cells.
Affected individuals develop fragile skin, sores (erosions), blisters (vesicles and bullae), and tiny cysts (milia) on the sun-exposed areas i.e. the backs of the hands and the forearms. Some people develop mottled brown patches around the eyes and increased facial hair. The urine characteristically fluoresces pink.
Sunscreens do not block out the responsible wavelengths in sunlight, so patients must cover up when outdoors.
Polymorphic light eruption: Polymorphic light eruption (PMLE) is a common form of photosensitivity. Most often, there are crops of small irritable pink or red bumps on the arms. Other areas may be involved, particularly the chest and lower legs, but the face is usually spared.
PMLE may recur each spring, provoked by being outside for several hours on a sunny day. If further sun exposure is avoided, the rash settles in a few days. It may or may not recur next time the sun shines on the skin.
In most individuals, there is a hardening as the summer progresses and more sun can be tolerated without a rash appearing. However, this does not always occur, and some very sensitive individuals even develop PMLE in the winter.
Besides careful sun protection, PMLE may need treatment with topical or oral steroids or an anti-malarial drug. A course of ultraviolet radiation in early spring may harden the skin preventing the rash arising as a result of natural sun exposure.