Fungal nail infections

Other names: Onychomycosis

Diseases of the skin and nails

Classification

Onychomycosis can be classified into several subtypes based on the origin of the infection within the nail plate, the infecting organism, or the appearance of the nail.

• Distal or lateral subungual onychomycosis: The most common form of onychomycosis. Yellowish, brownish, or whitish discoloration begins under the distal edge or sides of the nail and spreads over the entire nail plate. The big toenail is most often affected, but all nails are susceptible.
• Proximal subungual onychomycosis: Infection originates from the proximal nail fold and spreads distally. T. rubrum is the primary causative agent in the United States.
• Superficial onychomycosis: Fungi invade the superficial layers of the nail plate and spread deeper into the nail plate as the infection progresses. Lesions are often white and are most often caused by T. mentagrophytes.
• Endonyx onychomycosis: Nail bed is not involved in the infection; only the interior of the nail plate is infected.
• Totally dystrophic onychomycosis: Often a sign of end-stage distal or proximal subungual onychomycosis. Nail bed is thickened and raised with copious keratotic debris.
• Yeast onychomycosis: Affects fingernails more commonly than toenails, and is often caused by Candida.  May be a sign of underlying immunodeficiency.
• Fungal melanonychia: An uncommon nail infection caused by melanin-producing molds such as  Scytalidium, Alternaria, and Exophiala, causing brownish or blackish discolorations of the nail plate.  May present similar to subungual melanoma.

A patient's left foot - after ten weeks of Terbinafine oral treatment

Diagnosis

Diagnosis of onychomycosis can often be made by visual inspection alone; laboratory tests may not be needed, but include:

• Microscopy: Potassium hydroxide (KOH) stain is a commonly-used method because it is inexpensive and easy to perform. Nail clippings or scrapings are placed in a drop of KOH and examined under a microscope for the presence of fungal elements. 3 Periodic Acid-Schiff (PAS) staining can also be used.
• Culture: Fungal culture on Sabouraud’s medium or dermatophyte test medium (DTM) can be used to identify the infecting organism.

Treatment

• Topical antifungal agents can be used but are often ineffective.
• Oral terbinafine is considered to be the first-line treatment for confirmed onychomycosis; the treatment course is generally 6 weeks for fingernails and 12 weeks for toenails.
• Azoles can also be used.
• Surgical debridement or removal of the affected nail is also a consideration for cases that are resistant to antifungals, and laser treatments for onychomycosis appear to be a promising area for future study.