Brown patch on anterior thigh

August 1, 2010

An 18-year-old girl complains of having a peculiar brown patch on her right anterior thigh for 3 weeks. She admits to having a laptop on ther right thigh for a few hours each day.

The Case

Diagnosis: Erythema ab igne

The brown patch on the girl's thigh is typical of erythema ab igne (EAI), which presents as persistent, reticulated, erythematous, telangiectatic patches with hyperpigmentation.

PATHOPHYSIOLOGY

Although the pathogenesis of EAI is poorly understood, it is associated with chronic, repeated exposure to moderate heat from an external source. The exposure doesn't have to be long.1 Women are affected more than men. Infrared sources such as a heating pad or hot water bottle are usual sources. EAI was typically noted in patients with chronic pain conditions requiring use of these therapies.2 A newly recognized cause of EAI is the warm battery of a laptop computer.3

DIAGNOSIS

The diagnosis is largely based on history and physical exam. A thorough workup can include a punch biopsy if the diagnosis is uncertain. Histologic findings are often nonspecific and may include epidermal atrophy, melanin incontinence, collagen degeneration, and, occasionally, keratinocytic and melanocytic atypia.4

Lesions are typically benign. The only morbidity is secondary to the unfavorable cosmetic appearance. Tell patients to discontinue the heat exposure and advise them that discoloration may resolve within several months. Repeated heat exposure may make the hyperpigmentation and atrophy permanent.1 Caution your patient that squamous cell carcinoma can arise in these lesions, especially after many years of infrared exposure.5 We advised our patient to protect her skin from the laptop heat exposure and other sources of heat.

CONCLUSION

This case demonstrates a newly recognized etiology for EAI. It is important for physicians to be able to recognize novel heat sources as potential causative agents for this entity and to instruct their patients to discontinue such exposure.

DR FUSSELL is a first-year resident in pediatrics at Geisinger Medical Center, Danville, Pennsylvania. DR PRIDE is associate professor, department of dermatology and pediatrics, Geisinger Medical Center, Danville. DR COHEN, the section editor for Dermatology: What's Your Dx?, is director, Pediatric Dermatology and Cutaneous Laser Center, and associate professor of pediatrics and dermatology, Johns Hopkins University School of Medicine, Baltimore. The authors and section editor have nothing to disclose regarding affiliation with, or financial interest in, any organization that may have an interest in any part of this article. Vignettes are based on real cases that have been modified to allow the author and editor to focus on key teaching points. Images may also be edited or substituted for teaching purposes.

REFERENCES

1. Page EH, Shear NH. Temperature-dependent skin disorders.J Am Acad Dermatol. 1988;18(5 pt 1):1003-1019.

2. Warycha M, Goldstein J, Hale JJ. What is your diagnosis? Erythema ab igne. Cutis. 2006;78(6):392, 395-396.

3. Bilic M, Adams BB. Erythema ab igne induced by a laptop computer. J Am Acad Dermatol. 2004;50(6):973-974.

4. Page EH, Shear NH. Temperature-dependent skin disorders. J Am Acad Dermatol. 1988;18(5 pt 1):1013-1014.

5. Arrington JH 3rd, Lockman DS. Thermal keratoses and squamous cell carcinoma in situ associated with erythema ab igne. Arch Dermatol. 1979;115(10):1226-1228.