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DermNet NZ


Facts about the skin from DermNet New Zealand Trust. Topic index: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z



Photosensitivity

Some people are sensitive to sunlight; this is known as photosensitivity. Photosensitivity may produce a rash, which is known by the general term, photodermatosis.

Patients may not associate their skin complaint with exposure to light. It is not always the bright summer sun which is responsible; some people also react to sunlight in winter, and very sensitive subjects may even be affected by fluorescent lamps indoors.

Photosensitivity Photosensitivity Photosensitivity
Photosensitivity

Ultraviolet Radiation

Sunlight contains both ordinary visible light and shorter invisible light rays called ultraviolet radiation (UVR). UVR can produce tanning but also causes burning and skin cancer.

UVR is divided into UVB (short wavelength rays that cause sunburn and tan) and UVA (longer wavelength tanning rays). Patients can be sensitive to one kind of sunlight (i.e. only to UVB, UVA or visible light) or to a wider range of radiation. The most common photosensitivity is to UVA.

Cause of photosensitivity and photodermatoses

Photosensitivity occurs for a variety of reasons. These can be classified into the following groups:

Photodermatoses Causes
Idiopathic photodermatoses Cause is unknown but exposure to UV light produces a clearly defined disease entity. These include:
Exogenous photodermatoses Photosensitivity is caused by the introduction of an external agent that is applied topically or administered internally. These agents are called photosensitisers and include:
  • Medicines e.g. amiodarone, tetracyclines
  • Contact with plant, vegetable, fruit, chemicals, fragrances, dyes, disinfectants
Metabolic photodermatoses Photosensitivity is caused by a metabolic defect or imbalance of a body chemical. The most common disorders of this type are porphyrias , in which there are increased porphyrins in the skin.
Photoexacerbated dermatoses Photosensitivity is caused by a pre-existing disease or skin. These include conditions such as:
Genetic photodermatoses Photosensitivity is caused by a pre-existing genetic disorder, e.g.:

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.

How do you confirm the skin is photosensitive?

Photosensitivity can be confirmed by phototests – artificial light from various different sources is shone on small areas of the skin to see whether the rash can be reproduced, or if sunburn occurs more easily than expected.

Photosensitivity induced by contact with certain items can be tested by photopatch tests. Adhesive patches containing known photosensitizing materials are applied to the upper back, removed after two days, and light is shone on the area. The reaction is observed two days later.

Sun protection

Find out when sun protection is needed in your area (spring, summer and autumn). UVR is present in significant quantities in New Zealand between September and April. During these months, the NZ MetService releases a daily regional Sun Protection Alert. There is enough UVR 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 UVR and can nearly double the amount that gets to the skin.

There are two basic ways of protecting your skin from the damaging effects of UVR:

Unfortunately, photosensitive patients often find it difficult to find a sunscreen they can tolerate. Contact allergy or contact photodermatitis to the sunscreening chemicals themselves can occur, although this is uncommon, particularly benzophenone or butyl methoxy dibenzoylmethane, and in the past, para-aminobenzoic acid (PABA). Patch and photopatch tests will identify which ones are safe for you.

Other measures

Oral antioxidants may provide some extra protection, particularly Polypodium leucotomas.

UVR-absorbing film can be applied to windows at home or in the car (e.g. Bonwyke DermaGard ). Masks can be made to cover the face for trips outside too (clear ones are available), but not surprisingly, only the most disabled patients are prepared to wear these.

Unguarded fluorescent daylight lamps can occasionally provoke a rash, because they may produce some ultraviolet radiation (UVA). Ordinary tungsten light bulbs are usually alright. 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.

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If you have any concerns with your skin or its treatment, see a dermatologist for advice.