By Gary M. White, MD
Tinea pedis represents a fungal infection of the feet by a dermatophyte.
Fungal infection of the feet usually starts between the toes--often in the web spaces between the 3rd, 4th and 5th toes. From there, the infection can extend onto the toes and sole. At some point, the dermatophyte can extend to the tips of the toes and then invade the undersurface of the nail, causing onychomycosis. Alternatively, it may extend across the sole of the foot and eventually cause a moccasin-type distribution. Ultimately, it may spread to the dorsum of the foot, leg, groin or body. Some patients may have involvement of both feet and one hand, so called "two foot, one hand disease."
The morphology of tinea pedis may be eczematous, with redness and scale, or it may be microvesicular, like pompholyx. The source of the initial infection is usually not known, but public gym or showering facilities where the floor stays wet are usually full of dermatophytes.
The two main considerations for one or two red, scaly feet are tinea pedis and eczema. A KOH prep is usually the easiest way to distinguish the two. Clues that tinea is the cause include i) coexistent onychomycosis, ii) a moccasin distribution (redness and scale along the entire sole and extending up a cm onto the sides), and iii) involvement of the web spaces. Clues that eczema is the cause are i) failure to improve on a topical antifungal medication, ii) involvement of only the instep, and iii) an atopic background (e.g., allergies, hay fever, or asthma).
Rarely, an allergic contact dermatitis to an allergen in shoe wear (shoe dermatitis) may be the cause. In this case, topical antifungals are ineffective and topical steroids help but do not clear. Often, the dorsa of the feet are involved. Patch testing is needed to establish this diagnosis.
If the KOH is negative, but suspicion is high, apply a tape to the side of foot and ask the patient to reapply whenever it falls off. Have the patient return in 3 days for a high-quality sample given the new hot, humid environment [JAAD 2014 May].
The feet and the toe web spaces should be kept as dry as possible. Patients should remove footwear when possible. Zeasorb AF super absorbent powder may be routinely put into the shoes to both kill the fungus and keep things dry.
Topical terbinafine (OTC) has a quicker onset of action compared to clotrimazole and can lead to significant improvement in many patients after one week of BID therapy. Miconazole or clotrimazole are still good medications. Keeping the feet as dry as possible is also important. Patients should remove their shoes when home and may benefit from super absorbent powders (e.g., Zeasorb AF). If the patient wants a prescription, any of a variety of prescription antifungals are effective. Any fungal infection of the nails may lead to recurrence and should be treated (see onychomycosis). Many patients have a propensity to develop tinea pedis. These patients need to use an antifungal agent (e.g., terbinafine, clotrimazole, miconazole) 2-3 times per week long-term. A patient handout with suggestions for preventing onychoycosis is available and may be helpful here. Control, not cure, should be emphasized. One week of oral terbinafine 250 mg/day had the same efficacy as 4 weeks of clotrimazole 1% cream BID in one study [BJD 1998;139;675].
This case was unusual in that only the instep was involved, suggesting eczema. But KOH and culture proved it to be fungal.
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