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The history of frequent application of heating pads to the area in addition to the examination findings are classic for erythema ab igne.

Erythema ab igne is a well-defined, reticular dermatosis induced by prolonged heat exposure. It may vary from erythematous to violaceous in color initially and becomes hyperpigmented over time. The rash typically takes the shape of the heat source following repeated application or exposure. This was a relatively common disorder observed in many parts of the world where central heat was nonexistent and sitting in front of a fire to stay warm was commonplace.¹ Although the condition is less frequently observed with the advent of central heat, it has more recently been described on the thighs of a child following repeated use of a laptop computer² and in a pediatric patient with a history of frequently applying a hot water bottle to help settle his abdominal pain secondary to Crohn disease.³ Erythema ab igne is frequently associated with cases of chronic pain with attempts to relieve pain by applying heat. It has also been described in patients with cancer, as it can occur at the site of heat application secondary to an underlying malignant neoplasm.⁴ Thus, a detailed history of possible heat exposure is critical in making the diagnosis of erythema ab igne.

The hyperpigmented, reticulated eruption associated with erythema ab igne is typically nontender and does not cause discomfort to the affected patient. Continued heat exposure leads to worsening hyperpigmentation and potential skin atrophy.⁵ The mild tenderness noted on examination in our patient was attributed to gastrojejunostomy tube placement, not erythema ab igne.

Biopsy of the skin reveals mild hyperkeratosis, atrophy of the epidermis, increased pigmentation in the dermis, and elastosis. The pathophysiology is unclear, although histologic changes are similar to those seen in actinic elastosis.⁶ In general, biopsy is unnecessary for diagnosis as the history and skin findings are usually characteristic. The main treatment of erythema ab igne is elimination of heat exposure, although fluorouracil cream and laser therapy have been used in treatment.⁷ Skin pigmentary changes usually take months to resolve and in some cases are permanent. Although rare, epithelia atypia including squamous cell carcinoma and thermal keratoses have been associated with erythema ab igne.⁸ Therefore, erythema ab igne should be monitored to assure that eventual resolution of the skin changes does occur and that more concerning lesions do not arise. Furthermore, evaluation of the initial discomfort that prompted therapeutic application of heat is essential. Erythema ab igne is a complication of direct heat exposure but is not the source of the initial discomfort.

While erythema ab igne is not a common diagnosis, it continues to be seen in cases of recurrent heat application both with more traditional methods such as hot water bottle application and with modern sources such as laptop computer use. Skin findings could be interpreted as a cellulitis or vasculitis, potentially prompting unwarranted therapy. A thorough history and detailed examination are invaluable in diagnosing erythema ab igne.

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