Interdigital Erythrasma
A 58-year-old man presented with a 1-year history of itchy and painful erosions on the interdigital spaces of the right foot, previously diagnosed as interdigital tinea pedis and treated with oral and topical antifungals without improvement. He complained of bromhidrosis and malodour. During the physical examination we observed scaling, maceration, and exudative erosions on the interdigital spaces.

The lesion exhibits coral-red fluorescence under Wood's lamp. Corynebacterium minutissimum was detected by Gram staining of skin scrapings from the infected site, showing clusters of long, large bacillus such as are common in epithelial cells. No isolation of dermatophytes was detected in Sabouraud agar. We started treatment with topical fusidic acid twice a day, a 1:10000 potassium permanganate solution, and 500 mg of oral erythromycin every 6 hours with an improvement of the clinical condition.

Interdigital foot infections are mostly caused initially by dermatophytes, yeasts, and less frequently by bacteria. Interdigital erythrasma can be easily confused with interdigital tinea pedis. Quick diagnosis is allowed with examination under Wood's lamp.

In interdigital foot maceration, the combined use of direct microscopy under potassium hydroxide 20%, Gram staining, and culture increases the possibility of dermatophytes and C. minutissimum identification.

Source: https://www.mayoclinicproceedings.org/article/S0025-6196(19)30404-5/pdf
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