Description

A healthy woman in her 20s presented to the clinic with a history of rash on her shins for the last 3 months. The rash was painless and non-itchy. It was more prominent on the left leg. She first noticed it during the winter, when she had been using an electric heater which was more close to the left leg. Clinical examination revealed a reddish-brown, non-tender dark coloured reticular pattern of hyperpigmentation prominently on the left lower leg (figure 1). There was no palpable purpura, nodules or gangrene. Based on the history of heat exposure and physical examination findings, a diagnosis of erythema ab igne (EAI) was made. The patient was advised to avoid repeated heat exposure and the rashes disappeared within the next 2 months.

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Figure 1

Reticular, reddish-brown, macular hyperpigmentation predominantly on the left lower leg.

EAI is a reticular, erythematous, hyperpigmented dermatosis that results from long-term heat or infrared radiation exposure.1 It is mainly seen in cases of chronic exposure to heat sources such as heated reclining chairs, hot water bottles, car heaters, heating pads and among laptop users who hold computers on their thighs.2 It is also commonly seen among people who sat near open fires or stoves. Thermal burns occur when the skin is exposed to infrared radiation (800–170 000 nm) from an external heat source with a temperature exceeding 44°C. Long-term exposure to heat below the threshold for thermal burns is the primary aetiological factor in EAI.1 It can occur at any site, with predominantly asymmetrical distribution depending on the location of the external heat source. The clinical pattern in EAI corresponds with the dermal venous plexus. It initially begins as transient macular erythema in a broad reticulated pattern which blanches easily. With recurrent heat exposure, the erythema becomes non-blanchable and evolves into a dusky hyperpigmentation.3 Overlying skin may show focal areas of epidermal atrophy (early stage) or bullae/keratotic plaques (late stage). EAI is characteristically asymptomatic, although some patients may have a slight burning sensation. The hallmark of acute thermal inflammation is the presence of necrosis (epidermal±dermal or subcutaneous tissue depending on the degree on involvement). Histopathological changes in EAI include epidermal atrophy, vasodilatation and dermal pigmentation (melanin and haemosiderin). Focal hyperkeratosis and dyskeratosis along with squamous atypia are noted in later stages.1 3 EAI is a clinical diagnosis. Biopsy is warranted to rule out cutaneous malignancy. After the inciting cause is removed, EAI tends to disappear gradually, but sometimes the pigmentation is permanent.4 Vasculitis is a close mimic of this condition. EAI usually carries a good prognosis. Presence of ulceration should raise the suspicion of secondary malignant transformation (squamous cell carcinoma or Merkel cell carcinoma), which remains a long-term risk.5 Knowledge of this entity may lead the physician to take a proper history and avoid unnecessary investigations and treatment.