Chapter 163: Onychomycosis

Patient Story

A 12-year-old boy was brought in by his mother with discoloration and some pain in his toenails (Figure 163-1). The clinical diagnosis of onychomycosis was confirmed with microscopic examination of a potassium hydroxide preparation. The onychomycosis resolved after a 3 month course of oral terbinafine.

Introduction

Onychomycosis is a term used to denote nail infections caused by any fungus, including dermatophytes, yeasts, and nondermatophyte molds. One, some, and occasionally all of the toenails and/or fingernails may be involved. Although most toenail onychomycosis is caused by dermatophytes, many cases of fingernail onychomycosis are caused by yeast. Onychomycosis may involve the nail plate and other parts of the nail unit, including the nail matrix.

Synonyms

Toenail fungus, tinea unguium, dermatophytosis of nails.

Epidemiology

• The incidence of onychomycosis has been reported to be 2 to 13 percent in North America.¹

• Most patients (7.6%) only have toenail involvement and only 0.15 percent have fingernail involvement alone.²

• The prevalence of onychomycosis varies from 4 to 18 percent.^3,4

• The disease is very common in adults, but may also occur in children.

Etiology and Pathophysiology

• Dermatophytes are responsible for most toenail (Figures 163-1 to 163-3) and finger infections (Figures 163-4 and 163-5).

• Nonpathogenic fungi and Candida (including the rare syndrome of chronic mucocutaneous candidiasis) also can infect the nail plate (Figure 163-6).

• Dermatophytic onychomycosis (tinea unguium) occurs in three distinct forms: distal subungual, proximal subungual, and white superficial.

• The vast majority of distal and proximal subungual onychomycosis results from Trichophyton rubrum.

• White superficial onychomycosis is usually caused by Trichophyton mentagrophytes, although cases caused by T. rubrum have also been reported.

• Yeast onychomycosis is most common in the fingers caused by Candida albicans (Figure 163-6).