Dermatophytic onychomycosis

The most common etiological fungi that cause onychomycosis in 90% of toenails and 50% of fingernails are dermatophytes. When dermatophytes also invade the nail plate, this condition is known as tinea unguium. Tinea unguium is most frequently caused by Trichophyton rubrum (T. rubrum) followed by T. mentagrophytes as the second most common fungi. Toenails are most commonly affected whereas fingernails are occasionally affected by non-dermatophyte molds (NDM) also.

Diagnosis

1. Samples for diagnosing tinea pedis, or manuum should be taken separately from fingernails and toenails after the patients has stopped both topical and systemic antifungal drugs for a minimum of 2-4 weeks.
2. The simplest and fastest method for confirming fungal nail infection is by direct microscopy.
3. In order to confirm the diagnosis and to ascertain the exact causative fungus, a culture is necessary. Culturing of the nail specimens can be done by incubating specimens using different media at 25 to 30°C.

• Primary medium contains cycloheximide and can detect most NDM and bacteria, e.g. DTM, mycosel (BBL), and mycobiotic (DIFCO).
• Sabouraud glucose agar (SGA), Littman's Oxgall medium, and potato dextrose agar (PDA) are all secondary media which are free of cycloheximide and that allow isolation of NDM.

Note: To negate bacterial contamination, antibiotics such as chloramphenicol and gentamicin can be added to SGA or PDA.

4. In cases of suspected onychomycosis, histopathology of nail specimens might become compulsory when the KOH and cultures are repetitively negative.
5. Stains which are more selective than PAS include the Grocott Methenamine silver and calcofluor white (CFW) stains.
6. Immunohistochemistry and dual flow cytometry are less commonly used tests to diagnose onychomycosis. These tests are mainly used for the identification of mixed infections and for quantifying of the fungal load in the nail.
7. Recently DNA-based methods such as PCR-RFLP assays have been utilised for checking for the presence of fungi.

Treatment of dermatophytic onychomycosis

Treatment can be done either by topical methods, systemic methods, or a combination of the two. In a recent study, the novel azole antifungal voriconazole had a lower MIC against dermatophytes, Candida and NDM.

1. Ciclopirox 8% and amorolfine 5% lacquers have a broad action towards yeasts, dermatophytes, and NDM and therefore are the commonly used topical antifungals presently.
2. Griseofulvin, azoles such as ketoconazole, itraconazole and fluconazole, and the allylamine terbinafine are the oral drugs used for the treatment of onychomycosis.

Combination therapy

Not only does combination therapy improve efficacy and reduce relapse, but when both oral and topical drugs are used together, taking lower doses of the oral drug may result in better patient tolerance and compliance. In patients in which it is likely that antifungal therapy may fail, antifungals may be given as combination therapy, either in parallel (both oral and topical drugs given simultaneously) or in a sequential form (administration of oral drug alone, followed by topical).

In sequential therapy, the combined use of two oral antifungals that act on two different pathways in ergosterol metabolism is seen. In order to produce sensitive hyphae that are less refractory to antifungal treatment, boosted oral antifungal treatment (BOAT) can be used to target dormant chlamydospores and arthroconidia within the nail plate.

Treatment of onychomycosis in special populations

Close family contacts and children affected with onychomycosis should undergo careful screening for other superficial mycoses.

Children & Teens

• Griseofulvin at a dose of 10 mg/kg/day is the only approved antifungal for children 
• Terbinafine and itraconazole have been used in patients in the age group of 4 to 17 years with no significant children specific side effects
• Fluconazole (3 to 6 mg/kg) is also considered safe in daily doses and in pulsed doses

Elderly patients

• For the elderly, both continuous and pulsed terbinafine and itraconazole are found to be safe
• For dermatophyte onychomycosis, terbinafine is the drug of choice due to its greater mycological cure, lesser side effects, lesser drug interactions, and lower cost when compared to continuous itraconazole therapy

Diabetic patients

Onychomycosis prevalence in diabetic men is 2.5 to 2.8 times more than in the control group population. Yeast is found to be the most common causative agent in this group followed by dermatophytes and NDM.

• Topical therapies in combination with foot care intervention such as nail drilling have been found to be effective
• Due to its low risk of drug interaction, and its good efficacy, terbinafine is regarded as the first line of therapy

This article is part of a series on Fungal Infections. Click on the links below to read the other articles in the series.

Guideline on management of dermatophytosis in India

Managing fungal infections in diabetes: Dr Kiran Godse & Dr. Dakshata Tare