Onychomycosis refers to nail infections caused by any fungus, including yeasts and nondermatophyte molds. Dermatophytic onychomycosis (tinea unguium) occurs in three main forms: distal subungual, proximal subungual, and white superficial. One or several of the toenails or fingernails may be involved, seldom all. Most affected individuals have or have had tinea pedis as well. Most toenail onychomycosis is due to dermatophytes; however, many cases of fingernail onychomycosis are due to yeast.
This topic will review the diagnosis and management of onychomycosis. Skin infections due to dermatophytes and yeast are discussed separately. (See "Dermatophyte (tinea) infections" and "Candidal intertrigo".)
EPIDEMIOLOGY AND MICROBIOLOGY
There are few large community-based studies looking at the epidemiology of onychomycosis. A study of 15,000 patients in Canada presenting to the offices of three dermatologists and one family physician found that 8 percent had onychomycosis . Most patients had only toenail involvement (7.6 percent); few had fingernail involvement alone (0.15 percent) or both toenail and fingernail involvement (0.27 percent).
In small studies, the prevalence of onychomycosis varies from 4 to 18 percent depending upon the age and population studied [2,3].
Toenail onychomycosis is most commonly due to dermatophytes [1,4,5]; fingernail onychomycosis is more likely than toenail onychomycosis to be due to yeast , and some studies have found that yeast are the most common cause of fingernail onychomycosis [4,5]. Nondermatophyte molds appear to cause less than 10 percent of toenail onychomycosis and to be a very rare cause of fingernail onychomycosis .