Interdigital Macerated Plaques

A 41-year-old man presented with a pruritic rash between his toes that had been present for at least 2 months. Treatment with a nonprescription antifungal cream had not led to significant improvement. He did not have any other skin problems, and a review of systems was unremarkable.

Physical examination revealed moist, macerated, pinkish plaques in the fourth interdigital web spaces bilaterally (Figure 1). Wood lamp examination revealed a distinct coral-red fluorescence.

Based on the patient’s history and physical examination, which one of the following is the most likely diagnosis?

• A. Dyshidrotic eczema.
• B. Erythrasma.
• C. Mixed toe web infection.
• D. Psoriasis.
• E. Tinea pedis.

The answer is B: erythrasma. This superficial bacterial infection of the skin typically occurs in moist areas such as the interdigital web spaces, groin, axillae, and inframammary fold.¹ It is caused by the growth of Corynebacterium minutissimum, gram-positive rods that are part of the natural skin microbiome. Growth is accelerated by heat, moisture, and patient factors, including older age, diabetes, obesity, immunosuppression, hyperhidrosis, and poor hygiene.^1,2 Erythrasma comprises up to 17.6% of bacterial skin infections in older patients. The interdigital variety is encountered often in the clinical setting and is the most common bacterial foot infection. C minutissimum cellulitis and bacteremia are rare and occur primarily in immunocompromised individuals.^1,2

Dyshidrotic eczema presents as firm, deep-seated vesicles of the lateral and medial aspects of the fingers and less commonly the palms, soles, and toes.¹ In some patients, hyperhidrosis is an exacerbating factor. The size of lesions vary. Smaller vesicles can coalesce and create the characteristic look of tapioca pudding.^1,3

Erythrasma tends to present as bilateral red-brown hyperpigmented patches with minimal scale in the intertriginous areas.¹ When the toes are involved, maceration between the interdigital web spaces is common and may be pruritic. Coral-red fluorescence on Wood lamp examination is confirmatory and due to coproporphyrin III produced by the bacteria. However, fluorescence may not occur if the infected site has been recently cleansed.²

Mixed toe web infections begin with dermatophyte growth, but progressive inflammation and skin damage allow for bacteria growth.¹ Patients present with erythematous macerations between digits that can progress to cover the plantar surfaces. Lesions can be purulent, pruritic, and tender to palpation.^1,4 The organisms grown from the interdigital spaces include Pseudomonas aeruginosa, Escherichia coli, Proteus mirabilis, and Morganella morganii.⁴

Psoriasis is an immune system–mediated disorder that presents with inflammatory changes of the epidermis and dermis, resulting in the formation of well-demarcated erythematous plaques with overlying silvery scale on the scalp and extensor surfaces of the elbows, knees, and back.⁵ Psoriasis can appear anywhere on the body, not just intertriginous areas.^1,5

Tinea pedis is a dermatophyte infection of the feet that presents as a pruritic, scaly rash on the soles and may involve the interdigital web spaces.¹ It is common in young adults and teenagers.¹ No fluorescence occurs with Wood lamp examination. Tinea pedis is diagnosed by presence of septate hyphae on microscopic examination of potassium hydroxide preparation of skin scapings.²