Sometimes patients think that I am just making up diagnoses. This is a classic example.

I had a patient yesterday who had seen several physicians for evaluation of this strange rash on her back. She had had it for months. It didn’t hurt. It didn’t itch. It hadn’t changed. She and her husband were concerned by its appearance and by the fact that her doctor didn’t know what it was.
I took a look at her and asked one question to make the diagnosis.

“Do you use a heating pad?”

“No,” she replied, wrinkling her brow at my curious question. “Why would a heating pad cause a rash anyway?” She asked.

“It’s not the pad,” I answered, “It’s the heat.”

“Well, I do sit by the fireplace on cold nights,” she added.

“Eureka!” I exclaimed. (Well, actually I didn’t, but I thought it). “Do you sit with one side to the fire?”

“Yes, I sit with my left side to the fire while I read.”

“You have erythema ab igne,” I said.

“You’re just making that up, right?” she asked.

I’m not. Erythema ab igne, also known as toasted skin syndrome, is a red, brown rash that develops as a result of prolonged exposure to heat without an actual burn. The redness develops in a particular pattern, as seen in the photo, called reticulate or net like. Slowing of blood flow in the affected area, called hemostasis, is likely the cause.

The rash can be various colors from red to pink to brown. It improves after stopping exposure to heat, but in some instances, the brown coloring might be permanent.

Any source of prolonged heat like heating pads, water bottles, even laptop computers (if it sits on your lap) can be a cause. It is well documented in old dermatology books on the legs of women who would sit close to the fire or the pot belly stove in the winter to try to stay warm.

Ironically, my patient lives here in San Diego. Cold nights, as it turns out, are a relative thing.

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