Chronic actinic dermatitis (actinic reticuloid)
Some patients with a photoallergic reaction never get over it and go on developing sun-induced eczematous areas long after the drug has been stopped.
This is not clear but some believe minute amounts of the drug persist in the skin indefinitely.
This is the same as a photoallergic reaction to a drug. The patient goes on to develop a chronic dermatitis, with thick plaques on sun-exposed areas.
These patients may be exquisitely sensitive to UVR. They are usually middle-aged or elderly men who react after the slightest exposure, even through window glass or from fluorescent lights. Affected individuals also become allergic to a range of contact allergens, espe-cially oleoresins in some plants (e.g. chrysanthemums).
None, but the persistent severe pruritic eruption can lead to depression and even suicide.
Airborne allergic contact dermatitis may be confused, but does not require sunlight. Sometimes the dia-gnosis is difficult as exposure both to sunlight and to the airborne allergen occurs only out of doors. Air-borne allergic contact dermatitis also affects sites which sunlight is less likely to reach, such as under the chin (Fig. 16.7). A continuing drug photoallergy, a poly-morphic light eruption or eczema as a result of some other cause must also be considered.
Histology shows a dense lymphocytic infiltrate and sometimes atypical lymphocytes suggestive of a lym-phoma, but the disorder seldom becomes malignant.
Persistent light reaction can be confirmed experiment-ally by exposing uninvolved skin to UVA or UVB. Patch tests and photopatch tests help to distinguish between photoallergy and airborne allergic contact dermatitis, and the action spectrum may point to a certain drug. This sort of testing is difficult, and should be carried out only in specialist centres.
Usually cared for by specialists, these patients need extreme measures to protect their skin from UVR. These include protective clothing and frequent applica-tions of combined UVA and UVB blocking agents (Formulary 1). Patients must protect them-selves from UVR coming through windows or from fluorescent lights. Some can only go out at night. As even the most potent topical steroids are often ineffective, systemic steroids or immunosuppressants (e.g. azathioprine) may be needed for long periods.