Photopatch testing in seven cases of photosensitive drug eruptions

The most important goal in the management of photosensitive drug eruptions, as in other types of drug eruptions is identification of the causative drugs to prevent reexposure to them.

Seven patients whose lesions were mainly distributed on sun-exposed areas underwent laboratory tests, phototests, and photopatch tests with suspected drugs. Phototests were done with ultraviolet A (UVA), UVB, and visible light. Drugs used in the photopatch tests were usually prepared as 10% concentrations in petroleum base, which did not produce reactions in 10 control subjects, followed by irradiation of suberythema doses of UVA. Systemic provocation by oral administration of small doses of causative drugs with irradiation of suberythema doses of UVA was performed to confirm the results of skin tests in four patients. Two patients were not rechallenged with the causative drugs. None of the patients had systemic lupus erythematosus, porphyria, or pellagra. All showed positive reactions to photopatch testing. Systemic provocation confirmed the results of photopatch tests in four patients. The two patients who were not rechallenged had no recurrence of lesions. One patient ingested only one drug at the time of eruptions, and provocation or avoidance was not attempted. A photoallergic mechanism was considered in five cases.

Although there is no information about the appropriate concentrations or vehicles for suspected drugs, photopatch testing could be reliable for identification of causes of photosensitive drug eruptions. Besides piroxicam (a well-known photosensitizer) and carbamazepine, isoniazid and triflusal were identified as the causes of the reactions.