What Is This Rash on an Older Man’s Groin?

Correct answer: C. Erythrasma

Erythrasma is a bacterial skin infection caused by excessive proliferation of Corynebacterium minutissimum in the stratum corneum.¹ The most common sites are the fourth interdigital toe space, inframammary folds, and axillae, but it can also be found in the groin. The organism that causes erythrasma produces porphyrins, which fluoresce under Wood’s lamp examination.² This can help distinguish erythrasma from other etiologies, although fluorescence is not always seen. False negatives are particularly common upon Wood’s lamp examination if the area has been recently cleansed.³

Upon initial examination, erythrasma of the groin may be mistaken for other clinical entities such as tinea cruris, seborrheic dermatitis, inverse psoriasis, or intertrigo.³ It can be readily distinguished if the classic examination finding of coral-red fluorescence on Wood’s lamp examination is present. Erythrasma is important to include on the diagnostic differential diagnosis for groin rash, particularly when it does not improve with various courses of oral and topical antifungal treatments.

Treatment and management. Erythrasma is a superficial skin infection caused by C minutissimum, which is readily treatable with topical antibiotic therapy. It can demonstrate significant clinical overlap with other entities similar in appearance, including dermatophyte infection, candidiasis, inverse psoriasis, seborrheic dermatitis, and eczematous dermatitis, among other possible causes. The appearance of erythrasma can vary, but it is typically more uniformly pink to brown and lacks prominent scale. Dermatophyte infections classically have a scaly red border with central clearing, although this is not always present in moist sites such as the groin.⁴ Dermatophyte infections will typically improve with antifungal treatments. As our patient’s condition did not improve with antifungal treatments, it provided an important clue to an alternative diagnosis. Inverse psoriasis plaques especially may be entirely indistinguishable from erythrasma.⁵

The clinical overlap may lead to delays in diagnosis and effective treatment. Wood’s lamp examination is a low-risk evaluation tool with high potential clinical benefit in initial evaluation of intertriginous lesions, as well as those that have not responded to other therapies. While a simple and effective tool, Wood’s lamp is subject to false negatives in cases where patients have recently cleansed, as porphyrins are water soluble. It is also subject to false positives in the case of clothing fibers or soap residues.⁵

Other clinical clues can also point to a diagnosis of erythrasma in the absence of Wood’s lamp examination. Predisposing factors for erythrasma include diabetes⁶, obesity⁷, humid environment, heavy sweating, poor hygiene, and advanced age.⁸ The appearance of erythrasma can vary but differs slightly from tinea infections and is typically more uniformly pink to brown, whereas tinea infections tend to have a scaly red border with central clearing.⁴

Topical medications such as clindamycin or erythromycin are typically effective for treating erythrasma.² Oral treatments such as a single dose of clarithromycin, 1 g or a 2-week course of erythromycin or tetracycline are also effective and well tolerated.⁹ In settings where a Wood’s lamp is not available, it may be prudent to trial an empiric course of topical clindamycin prior to more expensive specialty visits.