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A healthy 69-year-old man with osteoarthritis of the knees presented with a 2-week history of mottled darkening of the skin on the left thigh. On examination, there was a reticular, reddish-brown, pruritic, nontender, macular, nonblanching discoloration around the medial aspect of his left knee, with a few superficial erosions. He had no fever, chills, or other constitutional symptoms. For several weeks before this event, the patient had applied a heating pad repeatedly to his left knee to relieve the discomfort from the osteoarthritis. The reticular, hyperpigmented erythema is a typical presentation of erythema ab igne, a phenomenon caused by chronic exposure to heat or infrared radiation. The few superficial erosions were thought to be due to a mild burn. Erythema ab igne is reported most frequently in temperate countries where people use a variety of heat sources during the cold weather. In physical medicine and rehabilitation, erythema ab igne develops in some patients after they have received heat therapy for pain and inflammation. This patient stopped using the heating pad, and the discoloration gradually lightened over several months without any other treatment.
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