Diagnosis

The diagnosis of a skin rash caused by the sun should be made by a dermatologist. The dermatologist can determine the cause of the rash by specific questions and other diagnostic means. Important questions are directed at the time the skin was exposed to the sun, the type of rash, accompanying symptoms and the frequency of such symptoms.

However, since different clinical pictures are involved that can cause a rash, the diagnosis must of course be specially adapted. As already discussed, the medical history, i.e. specific questions, is an obligatory part of the diagnosis. This is followed by a detailed examination of the skin so that the rash can be assessed in terms of its quantity and quality.

In combination with the requested information, such as secondary symptoms, time of occurrence, duration of the exposure of the skin to the sun’s rays and the patient’s history of such rashes, it is often possible to narrow down the cause. For example, a patient who has been lying in the sun for 7 hours and now imagines himself with a painful red skin rash is very likely to suffer from sunburn. However, a patient who imagines himself with an itchy, blotchy rash after he has been in the sun for the first time after the winter could be more likely to suffer from polymorphic light dermatosis (“sun allergy”).

In the latter case, diagnostic irradiation with UV-A rays is also possible to provoke a rash and confirm the diagnosis. However, this is usually not necessary as the examination and the patient interview are clear. Another important diagnostic tool is the exposed epicutaneous test, also known as the photopatch test.

This test is performed as follows: By irradiation with UV-A and UV-B radiation, the minimum erythema dose of the skin is first determined. This is a measure of the skin’s tolerance to solar radiation. Test substances are then applied to both halves of the back.

These test substances are possible allergens. One half of the back is then irradiated with UV-A rays. If the irradiated side develops a skin rash, the diagnosis of allergic photodermatitis is very likely. It is therefore a kind of allergy test.