MEMORANDUM DEPARTMENT OF HEALTH AND HUMAN
SERVICES
PUBLIC HEALTH SERVICE
FOOD AND DRUG ADMINISTRATION
CENTER FOR DRUG
EVALUATION AND RESEARCH
DATE:
FROM: Joseph
Porres, M.D., Ph.D., Medical Officer
Division
of Dermatologic and Dental Drug Products,
HFD-540
SUBJECT: NATURAL HISTORY OF TINEA PEDIS
______________________________________________________________________
Introduction
Tinea pedis is a common superficial fungal (dermatophyte)
skin infection of the feet. It may present in several clinical varieties:
intertriginous, moccasin, vesicobullous, or a combination of these. It is often
referred to as “Athlete’s Foot”. Tinea
pedis may be accompanied by dermatophyte infection of other parts of the body
including groin, hands or nails. It is estimated to affect about 15% of the
population at large, being more common in closed communities such army barracks
and boarding schools, in warm weather, among those frequenting swimming pools,
and when the feet are occluded with nonporous shoes. Tinea pedis is estimated to be present in
about 40% of all patients who attend clinics for any medical concern1. Those patients with more severe symptoms
seek medical help and often have concomitant fungal infection of the toenails 2, 3. There are many undiagnosed cases, many of which may be
asymptomatic and unsuspecting of having tinea pedis 2 and
be a possible source of infection for others. Chronic infection is common in
patients with concomitant diabetes, atopy, and immunosuppression. In an
increasingly aging population and with the increasing numbers of
immunocompromised patients, tinea is emerging as an important and a
significantly prevalent infection.
Causative Organism
Tinea pedis is most commonly caused by Trichophyton rubrum (60%), by Trichophyton
mentagrophytes (20%), and by Epidermophyton
floccosum (10%); more rarely by Microsporum
canis, Trichophyton tonsurans and
by other dermatophyte species, with differences by geographic area and by age
of the patient.
Diagnosis
Tineas are first seen by non-dermatologists about 60% of the
time, and 40% by dermatologists. In
clinical practice, diagnosis is often based on clinical presentation and
sometimes also on laboratory studies: direct microscopic examination of skin
scrapings and mycology culture.
The clinical presentation of tinea pedis may include
fissuring, maceration, and scaling in the interdigital or subdigital area of
the feet. Also, the area over the soles and on the side of the foot could be
involved with minimal inflammation but with a diffuse scaling, or there may be
small vesicles or vesicopustules present on the plantar area around the instep
of the foot.
The clinical differential diagnosis of cutaneous eruptions
of the foot includes many look-alike conditions, such as contact dermatitis,
dyshidrosis, eczema, atopic dermatitis, keratoderma, and psoriasis. Among the patients who seek help for skin
problems on the foot, only 32% have produced a positive mycology culture, and
many of the patients with scaling of the foot and tinea pedis may remain
undiagnosed or be misdiagnosed unless a fungus culture is performed 4
.
Mycology
Direct microscopic examination for fungal elements is
considered to be a rather insensitive method, with approximately 15% false
negative5, depending on the particular
technique and the experience of the physician. It has been reported that as many as 41% of specimens with a positive
KOH did not show fungal growth and only 77% of specimens with a positive fungus
culture had been read as a positive KOH6. New and more sensitive techniques are being
investigated, such as confocal* microscopy, but may not be ready for widespread
use for some time7.
Mycology cultures are helpful when positive but it may take
up to 4 weeks for a report to be available.
False negative results are very common, depending on the degree of
inflammation on the skin, how the skin is prepared prior to collection, where
the scraping is taken from, the amount of scraping, and the possible
persistence of a previously used antifungal on the skin.
Treatment
Dermatology
textbooks often recommend treatment with both oral and topical medications.
However, recommendations for treatment are often given without sufficient
detail to properly guide the prescribing physician. Some references recommend that tinea pedis
“moccasin type” be treated with systemic antifungals. Others recommend concomitant use of oral and topical
agents. A similar recommendation has
been made for intertriginous tinea pedis. Some reported treatment success rates
for topical antifungal treatment are: terbinafine cream (95% for 1-week, 76%
for 1-week, 86% for 4-weeks), clotrimazole cream (35% for 1-week, 70% for 4-weeks),
miconazole cream (95%, no duration specified).
Unfortunately some of these references are not accompanied by sufficient
details to effectively guide the prescriber.
__________________
* Confocal microscopy is a non-invasive
technique that provides high resolution images of intact skin comparable to
routine histology, without requiring specimen preparation.
1. Burzykowski, T., Molenberghs, G., Abeck, D.
et al. 2003. High prevalence of foot
diseases in
2. Maruyama, R., Hiruma, M.,
Yamauchi, K. et al. 2003. An
epidemiological and clinical study of untreated patients with tinea pedis
within a company in
3. Ogasawara,
Y., Hhiruma, M., Muto, M., and Ogawa, H.
2003. Clinical and mycological study of occult tinea
pedis and tinea unguium in dermatological patients from
4. Fuchs,
A., Fiedler, J., Lebwhol, M., Sapadin, A., et al. 2004. Frequency of
Culture-Proven Dermatophyte Infection in Patients with Suspected Tinea Pedis. Am. J. Med. Sci. 327(2):77-78.
5. Liu, D., Coloe, S., Baird, R., and Pedersen,
J. 2000.
Application of PCR to the identification of dermatophyte fungi. J. Med. Microbiol. 49:493-497.
6. Miller, M., and Hodgson, Y. 1993. Sensitivity
and Specificity of Potassium Hydroxide Smears of Skin Scrapings for the
Diagnosis of Tinea Pedis. Arch. Dermatol. 129:510-511.
7. Markus, R., Huzaira, M., Anderson, R. R., and
Gonzales, S. 2001. A Better Potassium Hydroxide Prepartion? In
vivo diagnosis of tinea with confocal microscopy. Arch. Dermatol. 137(8):1076-8.