Diagnosis: Erythema ab igne
Erythema ab igne (EAI) is a common dermatosis caused by repeated exposure to infrared radiation, most commonly in the form of low-grade heat (43–47°C).1 Common heat sources include heating pads, heaters, fire, and battery-charged devices. The distribution of the rash is dependent on the location of the heat source and appears as a hyperpigmented, reticulated rash. The pathophysiology is not well understood, but likely involves changes in dermal elastic fibers as well as the dermal venous plexus.2 Though rare, chronic cases of EAI have been associated with cutaneous dysplasia.3
Diagnosis of EAI is made by a combination of medical history and clinical features. Laboratory tests are not required. Additionally, clinicians should inquire about possible heat sources. In this case, we asked the patient whether she rested anything on her thighs, and she acknowledged that this was where she typically placed her laptop computer.
Differential includes other reticulated conditions
The differential diagnosis of a reticulated patch includes other entities likely sharing vascular pathology. The age, sex, and medical history of the patient offer additional diagnostic clues.
Livedo reticularis presents with reticulated erythema. It is unrelated to heat exposure, but may be associated with cold exposure. It can be physiologic or can be associated with vasculitis or another obstruction of blood flow.
Erythema infectiosum is a parvovirus B19 infection that usually presents in young children. It often results in a lacy reticulated exanthem on the face that resembles a slapped cheek in children. Adolescent and adult contacts often present with a more petechial rash in an acral to periflexural distribution.4
Continue to: Polyarteritis nodosa