Reticulated erythematous patch on teenager’s foot
At first, there was no obvious cause for the lesion on this patient’s foot, which had slowly grown over 2 years. However, a more detailed history proved revealing.
Diagnosis: Erythema ab igne
Upon further questioning, the patient acknowledged that he occasionally rested his bare feet around a portable heater under his desk while using his computer for a few hours each day (FIGURE 2). He often kept his right foot on the heater while he let his left foot rest on the ground. A punch biopsy was performed; the findings, when combined with the patient’s report of having exposed his foot to heat, supported the diagnosis of erythema ab igne (EAI).
EAI commonly presents as an asymptomatic reticulated erythematous to violaceous patch in an area of the body that has been in contact with heat.1 It originally was described on the bilateral anterior lower extremities after prolonged exposure to burning stoves or open fires.1 With the advent of central heating, these presentations have decreased, but there has been a resurgence of EAI with atypical distributions as a result of evolving technology and new heating sources. Reported causes of EAI include heating pads,1,2 laptop computers3 (FIGURE 3), car seat heaters,4 hot water bottles, popcorn bags, cell phones,5 and space heaters that have resulted in patches on the breast, thighs, arms, and, in our patient, foot.1-5
Blood work, biopsy can help narrow the differential
The differential for EAI includes livedo reticularis, livedo racemosa, cutis marmorata, and cutis marmorata telangiectasia. Livedo reticularis can be associated with autoimmune conditions and coagulopathies. Livedo racemosa is a typical sign of Sneddon’s syndrome and can be seen in up to 70% of patients with antiphospholipid-antibody syndrome and systemic lupus erythematosus. Diagnosis of these conditions is confirmed by elevated coagulation factors, presence of autoimmune antibodies, or history of cerebrovascular accident.6 These tests would be normal in EAI.
Histopathologic changes observed in EAI include an atrophic epidermis with an interface dermatitis, vasodilation, and dermal pigmentation. Necrotic keratinocytes and focal hyperkeratosis can be noted, along with squamous atypia. Although these changes are nonspecific, they can be used to confirm an EAI diagnosis in patients for whom the affected area has been exposed to a heat source.
