Chapter 201: Onychomycosis

PATIENT STORY

A 29-year-old woman presents with thickened and discolored toenails for 1 year (Figure 201-1). She is embarrassed to wear sandals and wants treatment. The entire nail plates are involved and there is subungual keratosis. She did not realize that she had tinea pedis, but a fine scale was seen on the soles and sides of the feet indicative of tinea pedis in a moccasin distribution. A KOH scraping from the subungual debris was positive for hyphae. She has no history of liver disease or risk factors for liver disease. An oral antifungal was prescribed for 3 months.

INTRODUCTION

Onychomycosis is a term used to denote nail infections caused by any fungus, including dermatophytes, yeasts, and nondermatophyte molds, which are the predominant organism in patients with human immunodeficiency virus (HIV) infection.¹ 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 mean prevalence of onychomycosis in Europe and North America in population based studies is 4.3% (1.9% to 6.8%). In hospital-based studies the mean prevalence is 8.9% (4.3% to 13.6%).² Most patients (7.6%) have only toenail involvement, and only 0.15% have fingernail involvement alone.³

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

ETIOLOGY AND PATHOPHYSIOLOGY

• Dermatophytes are responsible for most finger- and toenail infections.

• Nonpathogenic fungi and Candida (in the rare syndrome of chronic mucocutaneous candidiasis) also can infect the nail plate.

• Dermatophytic onychomycosis (tinea unguium) occurs in five distinct forms: distal lateral subungual (Figure 201-2), proximal white subungual (Figure 201-3), distal superficial white (Figure 201-4), endonyx, and total dystrophic (Figure 201-5).⁴

• The vast majority of distal and proximal subungual onychomycosis results from Trichophyton rubrum (Figure 201-6).

• Distal white superficial onychomycosis is usually caused by Trichophyton mentagrophytes, although cases caused by T. rubrum have also been reported⁴ (Figure 201-7).

• Yeast onychomycosis is most common in the fingers and is caused by Candida albicans.