A 50-year-old man self-refers to dermatology for an acute-onset, very itchy rash on the bottom of his foot. The rash appeared a week ago and is so symptomatic that he is unable to sleep. The patient tried applying OTC hydrocortisone 0.5% cream, but it only seemed to worsen his condition.
The patient denies any other significant health concerns. There is no personal or family history of skin problems; no one else in his household has been affected by this outbreak. He has not traveled internationally or domestically in the recent past. The only thing new in his life is a kitten that was rescued from a local animal shelter and given to his teenage daughter by a friend.
EXAMINATION
The rash, confined to the right instep, is composed of tiny, low vesicles surrounded by peripheral scaling. No overt inflammation is noted, although several of the vesicles (which appear to be drying up) have a slightly inflamed appearance. No rash is observed between the toes or elsewhere on the feet.
Scale is carefully collected (with a #10 blade) from the periphery of the site, as well as the dried roofs of some of the old vesicles. KOH examination shows abundant hyphae.
With his condition thus diagnosed as fungal in origin, the patient is given a written prescription for oxiconazole cream. But when he takes the script to his pharmacy, he discovers his insurance will not cover the cost. With advice from the pharmacist, he buys tolnaftate cream and applies it twice a day.
Despite the use of this product, his symptoms continue unabated throughout the intervening weekend. Very distressed, the patient calls in bright and early on Monday morning to complain. He is able to come in and get samples of the originally prescribed oxiconazole cream, which clears the problem within a week.
DISCUSSION
While “athlete’s foot” is a common problem, neither diagnosis nor treatment is always as simple as one might think. First of all, there are three types of tinea pedis—although interdigital is, by far, the most common. Almost always caused by Trichophyton rubrum, it manifests with a white, macerated look between the fourth and fifth toes (occasionally the third and fourth).
Although heredity may play a significant role in terms of susceptibility, sweat and heat are the major factors in the acquisition of tinea pedis. Persistent sweating (and the wearing of shoes, such as boots, that encourage it) often results in a chronic condition that is unlikely to be “cured.”
Almost any imidazole cream (eg econazole, oxiconazole, or clotrimazole) will at least control it, as will allylamine topical preparations (eg, terbinafine and naftin). Patients should understand that, no matter how earnestly the TV pitchman promotes an antifungal cream as “fast actin’,” nothing will change the fact that tolnaftate is ineffective compared to the medications mentioned above.
The second-most common type is moccasin-variety tinea pedis, which is usually asymptomatic and chronic, presenting with a powdery white fine scale that covers the sides and bottoms of both feet. It rarely needs treatment, in part because it’s asymptomatic but also because most affected patients don’t complain about it. This is a good thing, because a cure is virtually impossible.
This brings us to our case patient, who has the most unusual type of tinea pedis: the inflammatory type, which can be anthropophilic or zoophilic (contracted from man or animal). As this case illustrates, it is usually of acute origin, affects the instep, and is highly symptomatic. Unlike the other types, it manifests with inflammatory vesicles, from which peripheral scaling spreads as the lesions resolve. Abundant hyphae are reliably found in the roofs of resolved vesicles, confirming the diagnosis—although this relatively thick skin may take time to be digested by the alkaline KOH.
Trichophyton mentagrophytes, a common pathogen shed in cat hair, can be difficult to treat. Had this case been more severe, oral terbinafine (250 mg/d for two weeks) might have been added to the oxiconazole to clear the problem. The good news is that this type of tinea pedis tends to be episodic and not chronic. The bad news? If the cat is the culprit (by no means is this certain), the condition may recur.
The differential for this condition includes dyshidrotic eczema, contact dermatitis, and scabies.
TAKE-AWAY LEARNING POINTS
• The least common form of tinea pedis is the inflammatory type, which tends to be of acute onset and highly symptomatic (itchy), typically affects the instep, and presents with vesicles.
• KOH prep must be performed to diagnose this condition; it will reveal large numbers of obvious hyphae. The richest source of sample material is the roof of dried vesicles.