Derm Dx: Thickened toenails

Onychomycosis is a term that encompasses all fungal infections of the nail plate by fungal dermatophytes ­­and non-dermatophytes, whereas tinea unguium refers specifically to an infection of the nail unit by dermatophytes. Tinea unguium occurs worldwide. The most common pathogen is...

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Onychomycosis is a term that encompasses all fungal infections of the nail plate by fungal dermatophytes ­­and non-dermatophytes, whereas tinea unguium refers specifically to an infection of the nail unit by dermatophytes.

Tinea unguium occurs worldwide. The most common pathogen is T. rubrum. The other two most common causes are T. mentagrophytes and E. floccosum.

There are three classic patterns of onychomycosis based upon the point of fungal entry into the nail.

The first type, distal/lateral subungual, is due to invasion of the fungus through the hyponychium and is most commonly caused by T. rubrum.

The second type, white superficial onychomycosis (leukonychia trichophytica), is due to invasion through the dorsal surface of the nail plate and is most commonly due to T. mentagrophytes.

The third type, proximal subungual onychomycosis, is due to invasion under the proximal nail fold and is most commonly caused by T. rubrum and may be an indication of immunosuppression such as from HIV/AIDS.

Onychomycosis can have a variety of presentations. Most classically, T. rubrum induced onychomycosis starts at the distal corner of the nail and involves the junction of the nail and its bed. A yellowish discoloration may occur at the junction, which then spreads proximally at a streak in the nail.

As the infection progresses, subungual hyperkeratosis (scaling under the hyponychium) occurs and spreads until the entire nail is affected. The entire nail may eventually become brittle and separate from the bed because of the build-up of subungual keratin.

Toenail infections are more common than fingernail infections, and fingernail infections are rarely present without concomitant toenail infections. In most cases, multiple nails are involved in both hands and feet. Both fingernails and toenails present similarly, and the skin of the soles are usually afflicted as well, with characteristic scaling and erythema.

While some patients consider onychomycosis as only a mild cosmetic concern, some patients complain of discomfort and pain associated with nail trimming, running, and other activities. In patients diagnosed with diabetes or immunocompromised diseases, serious complications such as cellulitis can result.  

The diagnosis of onychomycosis can be made by demonstrating fungus in clippings or curetting of dystrophic subungual debris by microscopic exam or by culture. Rapid in-office diagnostics can be performed on very thin shavings or curetting samples examined with potassium hydroxide (KOH) solution.

A variety of stains such as chlorazol black E can be used to improve diagnostic yield. Histopathologic examination of formalin-fixed PAS-stained nail plates demonstrating hyphae is a quick and reliable method for diagnosing onychomycosis and has proved more sensitive than either KOH or culture. While histologic examination is quicker than cultures, identification of the specific pathogen can only be performed with culture.

The current standard of treatment still recommends oral systemic therapy with oral terbinafine being the most effective medication. Recurrent disease after successful treatment, especially in the toenails, is common.

Clinicians should always recommend preventative measures such as breathable footwear and cotton socks, antifungal or absorbent powders, and frequent nail clippings.  Patients with asymptomatic onychomycosis may not seek treatment.

Christopher Chu, BS, is a medical student at Baylor College of Medicine.
Adam Rees, MD, a graduate of the David Geffen School of Medicine at UCLA, practices dermatology in Los Angeles.