When asked to demonstrate her typical use of a laptop computer, she held it directly on her bare thighs.
“We then turned her computer on for 30 minutes and then tested the surface heat of its underside with an infrared thermometer,” said Dr. Kimberly Scott of Eastern Virginia Medical School, Norfolk, Va.
The area that corresponded to her right thigh, where the computer's central processing unit was located, measured 125° F, while the area that corresponded to her left thigh was only 95° F.
The diagnosis was erythema ab igne, a condition that is caused by repeated nonburning exposure to infrared radiation. It is characterized by transient erythema that evolves into reticulated hyperpigmentation and possibly telangiectasia.
The exact pathogenesis of erythema ab igne is not known, but it is thought that cutaneous hyperthermia involving temperatures of 43°–47° C (109°–117° F) can result in changes similar to those that occur with exposure to ultraviolet radiation.
Traditionally, the condition was seen among women who sat in front of wood-burning stoves, but since the advent of central heating, most cases involve occupational exposures among patients such as bakers and glassblowers who are exposed to heat for prolonged periods.
Patients with chronic pain also can develop the condition from using heating pads or hot-water bottles excessively. In one novel case, a patient heated a bag of popcorn just to the brink of popping in a microwave oven and applied the bag to her wrist for relief of arthritis pain (J. Dermatol. 2002;29:172–3).
Typical histologic findings include hyperkeratosis of the epidermis, with atypia of the epithelial cells. In the dermis, there may be collagen degeneration that appears similar to solar elastosis.
The course is usually benign if the heat source is removed promptly, but there have been cases of malignant transformation resulting in squamous cell or Merkel cell carcinoma, so follow-up is advisable.
If hyperpigmentation persists, tretinoin or hydroquinone or one of the combination products can be useful, Dr. Scott said at the annual meeting of the National Medical Association.
“And advise patients to not take the term 'laptop' literally,” she said.
Nancy Walsh