Derm Dx: Scaly rash on the feet - Clinical Advisor

Derm Dx: Scaly rash on the feet

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A patient, aged 46 years, presented complaining of a scaly rash on his feet.

Tinea pedis, commonly called athlete's foot, is an infection of the soles and interdigital web spaces of the feet by a group of fungus called dermatophytes. Tinea pedis, most commonly caused by fungal species Trichophyton rubrum, is the most common...

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Tinea pedis, commonly called athlete’s foot, is an infection of the soles and interdigital web spaces of the feet by a group of fungus called dermatophytes. Tinea pedis, most commonly caused by fungal species Trichophyton rubrum, is the most common fungal disease.

Dermatophytes include three genera of fungi that have the ability to invade keratinized tissue such as hair, skin, and nails. These three genera are Microsporum, Trichophyton, and Epidermophyton.

Tinea pedis occurs more frequently in men than in women. Most of the United States adult population has experienced tinea pedis at some point in their lives.

Tinea pedis is due to a moist environment created by occlusive shoes or from fungus acquired from going barefoot in public facilities such as locker rooms or gyms.

The fact that tinea pedis is uncommon in populations that do not wear shoes provides further evidence for the contribution of occlusive shoes to pathogenesis.

The lack of sebaceous glands on the sole of the feet further predisposes that area to a dermatophyte infection. Dermatophyte infections tend to be more severe and recurrent in HIV-infected patients.

There are four major classifications of tinea pedis:

  1. Moccasin
  2. Interdigital
  3. Inflammatory
  4. Ulcerative

Each category has different morbidities and complications that can affect therapeutic options and prognosis.Some complications include bacterial superinfections, dermatophytid reactions, cellulitis, and in the ulcerative subtype of tinea pedis, osteomyelitis in patients with diabetes.

The most common category of tinea pedis is the interdigital subtype caused by T. rubrum. Clinically, the infection is noninflammatory, and presents as dull erythema with pronounced silvery scaling that involves the entire sole and sides of the foot producing the classic “sandal appearance.”

Sometimes, tinea pedis is localized and limited to a small patch adjacent to an onychomycotic toenail or to a patch between or under the toes. One hand may also be involved.

Interdigital tinea should be distinguished from simple maceration caused by a closed web space because the latter will not respond to antifungal therapy.

Tinea pedis is diagnosed by demonstrating fungus by microscopic examination of scrapings, prepared with KOH solution, taken from the involved site. Because all dermatophytes have hyaline hyphae, they may often look the same in KOH preparations.

Culture on Sabouraud dextrose agar can help further delineate the particular species for further prognostic information. While not normally performed, a biopsy will show hyphae in the stratum corneum. The fungi can be made more apparent by PAS or silver stains.

First-line treatment for uncomplicated and localized tinea pedis includes topical antifungals. The adjunctive use of topical products containing glycolic acid, lactic acid, or urea can help reduce hyperkeratosis and increase the penetration of topical antifungals.

The use of oral antifungals such as griseofulvin or terbinafine may also be considered.

Finally, other dermatophyte infections often occur together with tinea pedis including tinea cruris, onychomycosis, and tinea manuum. The hands, nails, and groin should therefore also be examined.

Christopher Chu, BS, is a medical student at Baylor College of Medicine.
Adam Rees, MD,a graduate of the David Geffen School of Medicine at UCLA, practices dermatology in Los Angeles.

References

  1. Bolognia, Jean, Joseph L. Jorizzo, and Ronald P. Rapini. “Chapter 77 – Fungal Diseases.” Dermatology. [St. Louis, Mo.]: Mosby/Elsevier, 2008. Print.
  2. James, William D., Timothy G. Berger, Dirk M. Elston, and Richard B. Odom. “Chapter 15 – Diseases resulting from fungi and yeast” Andrews’ Diseases of the Skin: Clinical Dermatology. Philadelphia: Saunders Elsevier, 2006. Print.
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