A 53-year-old woman presents with a 30-year history of an asymptomatic rash on both feet. According to the patient, her mother had a similar condition. Over the years, the patient has tried any number of OTC moisturizers, thinking the rash was just dry skin that always worsened in the summertime. She has mentioned it to several medical providers, none of whom had any useful ideas about what it is or what to do about it.

The patient claims to be otherwise healthy. She is not immunosuppressed and is taking no prescription medications. She further denies a personal or family history of any other skin problems.

Examination reveals extensive marked desquamation of the sides of both feet. The scaling is quite fine and slightly erythematous. The well-demarcated annular proximal borders stop abruptly about halfway up the heel, involving the medial aspects of both big toes and the lateral aspects of both fifth toes. There is no involvement or maceration in the interdigital areas, and both soles are completely spared.

Her skin elsewhere—including elbows, knees, nails, and scalp—is completely free of notable changes.

ANSWER

The correct answer is tinea pedis (choice “a”) of the moccasin variety (see “Discussion”). Contact dermatitis (choice “b”) is highly unlikely, given the lack of symptoms, the absence of vesiculation, and the duration of the problem. Unna-Thost syndrome (choice “c”) manifests with, among other things, plantar hyperkeratosis that is thick and fissured and looks like exaggerated callous formation. Plantar psoriasis (choice “d”) waxes and wanes and is seldom relegated to the sides of the feet. And while it is possible for psoriasis to affect only one area of the body (scalp, nails, or elbows), it is far more common for it to be seen in multiple areas, particularly in cases with a long duration, such as this one.

DISCUSSION
There are three common types of tinea pedis (dermatophytosis of the foot), but the moccasin variety (also called chronic hyperkeratotic type) is by far the most common and probably the least known. The classic interdigital type, with maceration and inflammation between either the fourth and fifth or third and fourth toes, is better known and therefore easier to diagnose. The third type of tinea pedis is inflammatory and typically presents with the sudden appearance of discrete vesicles and pustules on the non–weight-bearing portion of the sole; it is quite pruritic.

Given the appearance and complete lack of symptoms, moccasin-variety tinea pedis often goes unnoticed by the patient, or is written off as mere “dry skin” until a flare (usually in the summer) brings it back to mind. Patients often have a family history of similar problems; in this case, the patient’s mother was affected.

The KOH prep was positive in this case, showing numerous fungal elements that probably represented Trichophyton rubrum. This common dermatophyte also causes jock itch and onychomycosis, among other conditions. Fortunately, it is sensitive to most of the allylamines (eg, terbinafine) and imidazoles (eg, econazole or oxiconazole) in topical form.

Although the use of these medications can lead to marked improvement, a cure is most unlikely. Patients with this type of tinea pedis are probably susceptible to this “infection” and cannot avoid repeated re-exposure to the organism, which will be present in great numbers in their environment (shoes, socks, bedding, carpet).

After a thorough discussion of the diagnosis and prognosis, this patient was initially treated with OTC terbinafine cream, applied twice daily. Within two weeks, the frequency of application can be reduced to once or twice per week, especially during colder months; this should be sufficient to control the condition.

Of note, none of this woman’s immediate family has contracted tinea pedis, despite many years of exposure. This demonstrates the principle that susceptibility is probably more important than exposure.