Diagnosis: Erythema ab igne
The diagnosis was erythema ab igne, which is characterized by localized areas of reticulated erythema and hyperpigmentation resulting from repeated exposure to moderate levels of infrared radiation or heat. At temperatures below the threshold for thermal burns, heat-induced vasodilatation results initially in a blanchable erythema—often in a size and shape approximating that of the heat source. With chronic continued heat exposure, the erythema is gradually replaced by nonblanchable hyperpigmentation and occasionally epidermal atrophy. Erythema ab igne is generally asymptomatic, although some patients report mild pruritus or burning at the site.
A variety of nonburning heat sources have been implicated in erythema ab igne. Once seen on the shins of those who stood by open fires or coal or wood stoves, the condition occurs now in the lumbosacral area in association with hot-water bottles or heating pads used for pain relief. Other more recently implicated sources include heating blankets, heated reclining chairs, and laptop computers (which cause lesions on the anterior thighs). Ocupational heat exposure, e.g., on the forearms of bakers or face of glassblowers, has also been reported.
The key histopathologic feature of erythema ab igne is squamous atypia, similar to that seen in UV-induced actinic keratoses. Squamous cell carcinoma and, less commonly, Merkel cell carcinoma have arisen from erythema ab igne lesions following a latent period that may be as long as 30 years.
Treatment rests primarily on removing the offending heat source. If the culprit is heat application for pain relief, inquire into the etiology of the pain. Erythema ab igne has been seen with such underlying diagnoses as chronic lumbosacral musculoskeletal back pain, pancreatitis, and metastatic cancer.
Early lesions consisting of erythema and mild hyperpigmentation often fade over weeks to months, while later lesions with prominent hyperpigmentation and atrophy may persist indefinitely. Patients should be informed of the small but reported risk of malignant degeneration and monitored for suspicious changes arising within the lesions.
Our patient had age-related degenerative disk disease. We encouraged him to switch from prolonged use of his heating pad to anti-inflammatory agents and muscle relaxants. His skin changes gradually faded over two to three months.