A 40-year-old lady with good past health complained of recurrent rashes over her left forearm
Dr. Adrian KC Cheng

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Clinical history:

A 40-year-old lady with good past health complained of recurrent rashes over her left forearm. She is an active golf-player. The rashes appeared each time after she applied a heat pad for 45 minutes to her left forearm to relieve her pain and tenderness from playing golf. The rashes were not itchy or painful. The erythema resolved in one week.  She had unremarkable dermatological history and no history of autoimmune diseases. She is not on any medication.

What is the diagnosis?

Answer:
C. Erythema ab igne

Erythema ab igne (EAI) is characterised by a reticulated pattern of erythema followed by hyperpigmentation. It is caused by repeated exposure to heat or infrared radiation below the threshold that causes thermal burns. The earlier phase is characterised by a blanching reticulated erythema with the size and shape approximates that of the heat source. In later phase, the erythema evolves into a dusky hyperpigmentation with recurrent heat exposure. Keratotic plaques and bullae may also appear in later phase. The abnormal pigmentation may resolve over months to years, but, permanent hyperpigmentation and scarring may persist. Lesions are mostly asymptomatic, while some patients may experience a slight burning sensation. Chronic heat exposure was thought to cause damage to the superficial blood vessels, such as the venous plexus, that corresponded to a reticulated pattern on the skin. The vascular changes with deposition of melanin and hemosiderin in the dermal layer resulted in reticular hyperpigmentation in later phase. Besides of heat pad, EAI may develop over anterior thighs in those with prolonged use of laptop computers and in forearms of bakers. Historically, EAI may develop from the use of open fire or wood-burning stoves for cooking and keeping warm. EAI carries a risk of malignant transformation into cutaneous squamous cell carcinoma or Merkel cell carcinoma. Regular skin examinations are recommended. The new appearance of ulceration, nodules, hyperkeratosis, or bullae over the reticulated lesions should prompt further evaluation by a dermatologist. The differential diagnosis of a reticulated erythema include livedo reticularis (LR). LR is a pattern of reticulated erythema, usually over the legs, that occurs in response to cold. LR may also occur secondarily to an underlying systemic disease such as autoimmune immune connective disease and antiphospholipid syndrome. Management of our patient is primarily by counselling to avoid using prolonged heat pad. The use of heat for pain relief should prompt an evaluation to determine the aetiology of pain. Luckily, our patient did not develop hyperpigmentation, hyperkeratosis or scarring. The erythema resolved spontaneously within one week of stopping the use of heat pad.

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The slide and the question were prepared by:

Dr. Adrian KC Cheng, ^{MBBS(HK), MRCP(UK), MSc GEOR (CUHK), FHKCP, FHKAM (Medicine)}

Specialist in Dermatology & Venereology;

Associate Consultant,

Division of Dermatology, Department of Medicine, Queen Mary Hospital, Hong Kong SAR