Photosensitivity

Photosensitivity is the name given to conditions where there are skin changes precipitated by exposure to sunlight. The mechanisms are thought to involve the activation of a chemical by a specific wavelength, which in susceptible people initiates a reaction in the skin. Many people have an abnormal response to sunlight called a polymorphic light eruption when itchy spots, which may form blisters, appear on the skin that has been exposed to the sun within a few hours of exposure, and do not settle for days. The phenomenon of prickly heat may be a mild form of this disorder. The management is to avoid exposure to strong sunlight and use strong sun creams.

Photosensitive dermatitis is a condition where an eczematous reaction occurs particularly in people who have got contact dermatitis when they’re exposed to sunlight, and so particularly affect the face and hands - a common cause being sensitivity to a soap substance. Certain drugs, such as amiodarone, used to treat some irregular heart rhythms, can induce photosensitivity, as can some antibiotics.

There is a group of disorders called porphyria where there is a defect in the metabolism of the breakdown product of haemoglobin. In one of these diseases called porphyria cutanea tarda, which may be the result of the development of liver disease or maybe genetically determined, skin exposed to sunlight becomes blistered because the breakdown product, the porphyrin ring that normally contains the Iron atom in the haemoglobin molecule, is deposited in the skin and responds to sunlight by provoking a vigorous inflammatory reaction which makes the blisters. Attacks maybe precipitated by exposure to alcohol or other drugs. The frequency may be reduced by reducing the amount of circulating blood, in the same way as we treat Haemachromatosis (see the liver in the digestive system section).
 
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