La photosensibilité (Pour les professionnels)

If you are sensitive to sunlight, you may have a condition known as photosensitivity.

• Photosensitivity is often defined as the amount to which a person reacts upon receiving light. There are many conditions which can give rise to sensitivity of the skin to a light source. It is said that people with particular skin types are more sensitive to direct sunlight and develop sunburn as a response.

• Photosensitivity manifests by the development of a rash (photodermatosis). Photodermatosis happens after exposure to sunlight or to any other bright light. Sunlight contains both ordinary visible light (photons) and shorter invisible light rays called ultraviolet radiation (UVR). Ultraviolet light is what causes tanning but also causes burning and skin cancer.

• Ulltraviolet light is divided into two main types: UVA and UVB (UVC* is a third type). UVA consists of longer wavelength (315-400nm) and gives is the principal wavelength involved in tanning while UVB (280-315nm) consists of shorter wavelength rays which cause sunburn and tan. So far, the most common photosensitivity is to UVA.

*UVC (100-280nm)

• There are many reasons why a person may become sensitive to sunlight. Many medications make the skin more sensitive to sunlight: examples include tetracycline and sulfonamide antibiotics, drugs for heart problems such as amiodarone etc… There are also medical conditions which lead to increased light sensitivity. Patients with systemic lupus erythematosus experience skin symptoms after sunlight exposure; some types of porphyria are aggravated by sunlight. A rare hereditary condition xeroderma pigmentosum increases the risk of UV-light-exposure-related cancer by increasing photosensitivity (due to an impaired protein involved to fix DNA). Other genetic photodermatosis include Bloom syndrome and Rothmund Thomson syndrome.

• In some cases, photosensitivity may be idiopathic, meaning that the cause is unknown but exposure to UV light produces some forms of photodermatosis. Examples include polymorphic light eruption, juvenile spring eruption, actinic prurigo, solar urticaria, chronic actinic dermatitis (actinic reticuloid), hydroa vacciniforme and pseudoporphyria.

• Photodermatosis can be induced by intake or application of external agents (photosensitization) There is another type of skin reaction to visible light known as exogenous photodermatoses. Locally, Photodermatosis can be caused by plants, vegetables, fruits, chemicals, fragrances, dyes, and disinfectants.

• A type of photodermatosis, known as metabolic photodermatoses, 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. Examples of these are porphyria cutanea tarda, erythropoeitic protoporphyria, variegate porphyria and erythropoeitic porphyria (Gunther’s disease).

• Photoexacerbated dermatosis is caused by a pre-existing disease or skin. These include conditions such as lupus erythematosus (especially subacute and systemic forms), dermatomyositis, Darier’s disease, rosacea, pemphigus, atopic dermatitis and psoriasis.

• 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.

• Photosensitivity can be confirmed by a phototest. In this test, 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. (Minimal Erythema Dose or MED concept)

• You can also undergo a photopatch test. A photopatch test is an adhesive patch containing known photosensitizing materials which is then applied to the upper back, removed after two days, and light is shone on the area. The reaction is observed two days later.

So, how can you prevent photodermatosis if you have photosensitivity?

• You should confine summer excursions outdoors to early in the morning or late in the evening. You also need sun protection even if you are on shaded areas or inside the car because UVA can pass through the window glass.
• You should also block out all light with an opaque material such as sun protective clothing such as dark colored and densely woven fabric. Wear shirts with high collars and long sleeves, trousers or a long skirt, socks and shoes, a wide-brimmed hat and even gloves. Some clothes are now labeled with UPF (Ultraviolet Protection Factor), the sun protection factor for fabrics. Choose those with a UPF of 40+.
• You can also use topical sunscreen (Chemical or Physical) agents. Physical blockers (reflectant sunscreens or more simply called a Sunblock) are very effective sunscreens as they block out UVA and UVB by reflecting the ultraviolet radiation. Their only drawback is they can be messy to use and cosmetically unappealing. You should also select a sunscreen with a very high Sun Protection Factor (SPF 30+), which is water resistant.
• There are also additional things to try. You can also use UV-absorbing film which can be applied to your windows at home or in the car. Masks can be made to cover the face for trips outside too, however they are physically distracting.