PATIENT PRESENTATION

A 39-year-old Caucasian man with type 2 diabetes on oral hypoglycemics had an asymptomatic dark discoloration on both sides of his groin of 1-year duration. Well-demarcated reddish-brown elliptical plaques with sparse scale were present on both medial thighs, yet spared the scrotum.

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Diagnosis: Erythrasma

Erythrasma is a cutaneous bacterial infection caused by Corynebacterium minutissimum. It was named in 1961 and is a lipophilic, Gram-positive, non–spore-forming, aerobic, catalase-positive diphtheroid that makes up to 50% of the normal flora of the skin.1,2,3

Epidemiology

Erythrasma is a common infection present in the toe webs of up to 44% of diabetics;1 it is also seen in patients with advanced age,4 obesity,5 poor hygiene and hyperhidrosis.1 Indeed, if erythrasma is present, a diagnosis of diabetes mellitus should be considered.2 Erythrasma is the most common interdigital infection of the foot, presenting as fissuring, scaling and maceration of the web spaces. It is seen all over the world, but is more common in the tropics than in temperate climates.1

Clinical Presentation

Erythrasma typically affects warm and moist intertriginous areas such as the axilla, groin and inframammary area. It presents as a confluent brownish patch, which later develops into a red-brownish plaque with minimal scale that is usually asymptomatic but can be pruritic.1 In some patients, there may be multiple plaques. In addition to erythrasma, C. minutissimum has less commonly caused abscess formation, bacteremia, catheter-related infection and ophthalmologic infection.6

Erythrasma is often mistaken for tinea corporis. Therefore, a negative KOH examination to exclude dermatophytosis may be helpful. However, as many as 30% of patients with erythrasma may also be co-infected with candida or a dermatophyte.1,2

Erythrasm can be diagnosed by visualizing coral-red fluorescence under Wood’s light examination of the effected area. This color results from the porphyrins produced by C. minutissimum. Gray and Holt, in an earlier study, demonstrated that the bacterial porphyrins consisted of a mixture of at least three porphyrins: coproporphyrin III (98% to 99%), with the remainder being uroporphyrin I and “a hitherto undescribed porphyrin.”1,7

Histology/Pathogenesis

Hematoxylin and eosin staining of a skin biopsy may appear normal. However, the Gram, the periodic acid-Schiff and the methenamine silver stains reveal the rod-like shape of C. minutissimum in the stratum corneum.2 Infection results from an increase in moisture and warmth, which occur in favorable intertriginous environments.

Differential Diagnosis

The distinguishing features of conditions in the differential diagnosis for erythrasma, which include candida intertrigo and tinea corporis, are summarized in Table 1.

Treatment

There is no standard treatment for erythrasma, yet there are a number of available options. For an isolated plaque, topical therapy with either 2% erythromycin or 1% clindamycin in solution, gel or lotion may be used. Alternatively, agents causing keratolysis, such as Whitfield’s ointment (12% benzoic acid and 6% salicylic acid), have been used.1

The treatment of choice for multiple plaques of erythrasma is oral erythromycin: 250 mg four times per day for 14 days.1 Recently three patients with erythrasma in the groin or perineum were treated successfully with a single 1g dose of oral clarithromycin. They all had resolution of their symptoms within 48 hours and a negative Wood’s light examination after 14 days.8

Erythromycin and clarithromycin are macrolides. They are bacteriostatic and inhibit protein synthesis by reversibly binding to the 50S ribosomal subunit of sensitive bacterium. Erythromycin concentrates in Gram-positive bacteria about 100 times more than in Gram-negative bacteria; this results in its greater activity against Gram-positive bacteria like C. minutissimum. Clarithromycin differs from erythromycin by a hydroxy to O-methyl substitution at position 6 on the lactone ring, resulting in greater bioavailability, longer half-life, and broader spectrum of activity. This structural change may account for the successful treatment of erythrasma after only a single dose of clarithromycin.9,10

Resolution

Our patient’s plaques (Figure 1) were a brilliant coral-red fluorescence when examined with a Wood’s light (Figure 2). The plaques were treated with topical erythromycin 2% gel twice daily for 14 days and showed resolution of his erythrasma after 2 weeks of treatment; there was residual macular postinflammatory hyperpigmentation, and the Wood’s light examination was negative.

Conclusion

Erythrasma is a frequent cutaneous infection caused by C. minutissimum most commonly affecting intertriginous areas as a single plaque. The well-defined, usually asymptomatic, red-brown plaques of erythrasma can be easily diagnosed in the office setting by visualizing a coral-red fluorescence under Wood’s light. Isolated plaques are treated effectively with either topical preparations of either 2% erythromycin or 1% clindamycin or keratolytic medications. Multiple plaques can be treated with oral erythromycin or clarithromycin.

Jayantha Thiyanaratnam is a medical student at Baylor College of Medicine, Houston, TX.

Disclosure: The authors have no conflict of interest with any material presented in this column.