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More About Onychomycosis

This post provides an abbreviated version of several publications by Antonella Tosti, MD, Professor, Department of Dermatology, University of Bologna, Italy; Department of Dermatology & Cutaneous Surgery, Miller Medical School, University of Miami, Florida.

Onychomycosis (OM) refers to a fungal infection that affects the toenails or the fingernails. Onychomycosis may involve any component of the nail unit, including the nail matrix, nail bed, or nail plate. Onychomycosis is not life threatening, but it can cause pain, discomfort, and disfigurement and may produce serious physical and occupational limitations. Psychosocial and emotional effects resulting from onychomycosis are widespread and may have a significant impact on quality of life.

The main subtypes of onychomycosis are:

  • distal lateral subungual onychomycosis (DLSO),
  • white superficial onychomycosis (WSO),
  • proximal subungual onychomycosis (PSO),
  • endonyx onychomycosis (EO), and
  • candidal onychomycosis.

Patients may have a combination of these subtypes.

Total dystrophic onychomycosis refers to the most advanced form of any subtype.

In distal lateral subungual onychomycosis, the most common form of onychomycosis, the fungus spreads from plantar skin and invades the nail bed via the hyponychium, the epithelium located beneath the nail plate at the junction between the free edge and the skin of the fingertip. It forms a seal that protects the nail bed. Inflammation occurring in these areas of the nail apparatus causes the typical physical signs of distal lateral subungual onychomycosis.

In contrast, white superficial onychomycosis is a rarer presentation caused by direct invasion of the surface of the nail plate. In proximal subungual onychomycosis, the least common subtype, fungi penetrate the nail matrix via the proximal nail fold and colonize the deep portion of proximal nail plate.

Endonyx onychomycosis is a variant of distal lateral subungual onychomycosis in which the fungi infect the nail via the skin and directly invade the nail plate.

Total dystrophic onychomycosis involves the entire nail unit.

Nail invasion by Candida is not common because the yeast needs an altered immune response as a predisposing factor to be able to penetrate the nails. Despite the frequent isolation of Candida from the proximal nail fold or the subungual space of patients with chronic paronychia or onycholysis, in these patients Candida is only a secondary colonizer. In chronic mucocutaneous candidiasis, the yeast infects the nail plate and eventually the proximal and lateral nail folds.

The recent proliferation of fungal infections in the United States can be traced to the large immigration of dermatophytes, especially Trichophyton rubrum, from West Africa and Southeast Asia to North America and Europe. The incidence of onychomycosis has been reported to be:

  • 2-13% in North America;
  • 3-8 in the United Kingdom, Spain, and Finland.

Onychomycosis accounts for half of all nail disorders, and onychomycosis is the most common nail disease in adults. Toenails are much more likely to be infected than fingernails. Thirty percent of patients with a cutaneous fungal infection also have onychomycosis. The incidence of onychomycosis has been increasing, owing to such factors as diabetes, immunosuppression, and increasing age.
Onychomycosis affects persons of all races, more commonly males than females. However, candidal infections are more common in women than in men.

Studies indicate that adults are 30 times more likely to have onychomycosis than children. Onychomycosis has been reported to occur in 2.6% of children younger than 18 years but as many as 90% of elderly people.


Onychomycosis is caused by 3 main classes of fungi:

  • dermatophytes,
  • yeasts, and
  • nondermatophyte molds.

Dermatophytes are by far the most common cause of onychomycosis. Two major pathogens are responsible for approximately 90% of all onychomycosis cases. Trichophyton rubrum accounts for 70% and Trichophyton mentagrophytes accounts for 20% of all cases.

Onychomycosis caused by nondermatophyte molds (Fusarium species, Scopulariopsis brevicaulis, Aspergillus species) is becoming more common worldwide, accounting for up to 15% of cases in some countries.

Onychomycosis due to Candida is rare.

  • T rubrum is the most common pathogen in distal lateral subungual onychomycosis.
  • Proximal subungual onychomycosis due to T rubrum infection is typical of immunosuppressed patients . Additionally, Proximal subungual onychomycosis with periungual inflammation is usually caused by molds
  • White superficial onychomycosis is usually caused by T mentagrophytes; nondermatophyte molds cause deep white superficial onychomycosis.
  • Candida albicans nail infection is observed in premature children, in immunocompromised patients, and in persons with chronic mucocutaneous candidiasis.

Risk factors for onychomycosis include family history, increasing age, poor health, prior trauma, warm climate, participation in fitness activities, immunosuppression (eg, HIV, drug induced), communal bathing, and occlusive footwear.

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