A Reticular Rash on the Leg

Our Diagnosis

Erythema ab igne, also called hot water bottle rash, is a clinical diagnosis based on characteristic cutaneous findings and a clear history of chronic, moderate heat or infrared exposure.¹ Although exposure to space heaters, open fire, radiators, hot water bottles, and heating pads are the classic causes, recently there have been reports of laptop computers, cell phones, infrared food lamps, automobile seat heaters, and heated recliners causing the same type of skin reaction.²

With chronic moderate heat or infrared exposure, the rash usually progresses over days to months. It begins as a mild, transient, reticulated, erythematous rash, which follows the pattern of the cutaneous venous plexus and resolves minutes to hours after removal of the offending source as vasodilation resolves. After months of continued exposure, the dermis around the affected vasculature eventually becomes hyperpigmented due to the deposition of melanin and sometimes hemosiderin.

The rash is usually asymptomatic but has been associated with pain, pruritis, and/or tingling. Once the diagnosis is made, treatment involves removal of the offending source. The discoloration may resolve over months to years, but permanent hyperpigmentation is not uncommon. There are a few case reports on treatment using Nd-Yag laser therapy, topical hydroquinone and tretinoin, 5-fluorouracil, and systemic mesoglycan with topical bioflavonoids.^2-4

While the prognosis of erythema ab igne is excellent if detected early, failure to recognize this condition and remove the offending source can lead to sequalae, such as squamous cell carcinoma, poorly differentiated carcinoma, cutaneous marginal zone lymphoma, and Merkel cell carcinoma.^5-8 Development of malignancy typically has a latency period of > 30 years. Patients should have periodic surveillance of their skin and any suspicious lesion in the involved area should be considered for biopsy.

Rashes may represent systemic or more localized pathology (Table). In contrast to erythema ab igne, the rash associated with a vasculitic process (autoimmune, drug-induced, or infectious) tends to be more generalized and bilateral but still follows the pattern of the cutaneous venous plexus. An example of this would be livedo reticularis. Although this rash is reticular, it is not hyperpigmented.⁹ A variant of livedo reticularis is cutis marmorata, which develops in response to cold exposure, particularly in infants or in the setting of hypothyroidism.Cutis marmorata is erythematous, blanchable, and reversible with rewarming. Unlike erythema ab igne, there is no hyperpigmentation and tends to be more diffuse.¹⁰

When evaluating a reticular rash, consider local and systemic etiologies. If more localized and hyperpigmented, ask about heat or infrared exposure. This may point to a diagnosis of erythema ab igne.