A 42-year-old woman presents with discoloration and discomfort involving her back. The discomfort, which started less than 24 hours ago, is not severe, but she is worried about the changes in color she can see in the mirror.

Her history is significant for metastatic melanoma. She has been undergoing treatment with chemotherapy and localized radiation and for several months has been taking systemic steroids for intracerebral edema.

Additional history taking reveals that the night before the appearance of the skin changes, she fell asleep lying on a heating pad because she was cold. She denies doing this habitually.

Examination reveals a large patch of reticular erythema covering the entire central back, sharply sparing the area under the bra strap. The erythema is bounded laterally by sharply demarcated linear margins that mimic the exact shape and size of the heating pad in question. Focal blisters can be seen within the erythema, but the overall effect is macular (flat). Neither tenderness nor increased warmth is noted on palpation.

ANSWER
The correct answer is erythema ab igne (choice “a”) caused, of course, by the effects of the heating pad. The reticular pattern and acute onset are quite characteristic of this unusual condition.

Contact dermatitis (choice “b”) would not have presented in this reticular pattern and would typically have itched intensely.

Cellulitis (choice “c”) represents a superficial infection usually caused by strep and/or staph. It would not have been reticular, would have been painful, and would have required a break in the skin for the offending organism to gain entrance.

Poikiloderma vasculare atrophicans (PVA; choice “d”) describes vascular changes seen focally in patches that evolve slowly over months or even years. It is significant because of its reported potential to evolve into cutaneous T-cell lymphoma; however, PVA does not present with a reticular pattern, nor does it appear acutely.

DISCUSSION
The superficial vascular plexus, configured in a reticular pattern and normally invisible, is sensitive to repeated exposure to the infrared portion of the magnetic spectrum (wavelength 700 to 1,100 nm). This exposure initially produces erythema, which over time turns livid, then permanently hyperpigmented.

Erythema ab igne (EAI) was classically seen in those sitting close to an open fire or stove (producing temperatures of 43°C to 47°C) for extended periods, often for hours each day. With the advent of central heating, other triggers of EAI evolved, including prolonged use of laptop computers, heating fans, and as in this case, heating pads. It can even be due to occupational exposure to intense heat, as with glassblowers, bakers, and steelworkers.

Most of the skin changes seen with EAI resemble those seen with chronic sun damage, including vasodilatation and melanin incontinence with melanophages in the upper dermis.

This particular patient had only one exposure to the heating pad, but had turned it on high and lain on it all night. This produced the changes seen, which will likely become permanently etched in her skin in the exact pattern of the causative heating pad. Months of dexamethasone therapy may also have contributed to the problem, by thinning the patient’s skin enough to render her susceptible to this single exposure to heat.

With EAI patients in general, thought needs to be given to possible underlying issues, such as the source of pain being treated with the heating pad—most commonly, on the low back—or the possible reason for constantly feeling cold, such as anemia or hypothyroidism.

Treatment choices include the application of tretinoin cream or 5-fluorouracil cream, or ablation with laser (YAG, ruby, alexandrite).