Onychomycosis: Putting Our Best Foot Forward

By Robert Law, MD

The prevalence of onychomycosis (OM) in the United States has been estimated at nearly 10% of the general population. With the pervasiveness of popular press advertisements for newer antifungal agents, you are met with an increasing number of patients presenting to your office for treatment.

Onychomycosis, simply defined, is a fungal infection of the nail. It has been further subdivided into groups based on clinical appearance (Table 1). A patient presenting with nail dystrophy may well have onychomycosis. However, only 50% of cases of nail dystrophy are due to fungal infections. Therefore non-infectious, inflammatory, or neoplastic processes must first be excluded. Entities such as psoriasis, traumatic onycholysis, lichen planus, alopecia areata, and spongiotic disorders may mimic onychomycosis. As described in previous THE FOCUS newsletters, nail biopsy with a periodic acid-schiff (PAS) stain is vital in arriving at the correct diagnosis.

Dermatophytes are by far the most common cause of onychomycosis. This group comprises the keratinophilic genera Trichophyton, Epidermophyton, and Microsporum. However, recent review of epidemiologic data reveals that non-dermatophytes continue to cause approximately 10% of cases of onychomycosis (Table 2). As dermatopathologists have gained experience with the interpretation of fungal forms utilizing nail biopsies and PAS staining, correlation with cultures have made it clear that distinguishing dermatophytes from non-dermatophytes is not feasible utilizing this technique. However, the superiority of establishing a diagnosis of onychomycosis using this technique is well established. Dr. Boni Elewski, an expert in dermatologic mycology points out the limitations of microscopy: “The test serves only as a screening test for the presence or absence of fungi but cannot differentiate among the pathogens”1. Our own experience at PROPATH in comparing patient nail biopsies with subsequent culture results confirms this. Often the PAS stain will allow distinction of Candida from non-Candida fungal elements. However, this is not the case in attempting to separate dermatophytes from non-dermatophytic molds.

Figure 1 is from a patient who failed 3 months of terbinafine therapy. While the morphologic features on the PAS stain are consistent with a dermatophyte, fungal culture revealed the non-dermatophyte mold Fusarium spp.

Management of onychomycosis has been quite limited until the advent of the newer antifungal agents. With the introduction of the imidazoles (ketoconazole, itraconazole), the allylamines (terbinafine), and topical ciclopirox, mycologic cures have become a reality. Given the prevalence of onychomycosis, clinicians need a reliable, sensitive technique on which to base treatment.

Terry L. Barrett, M.D., dermatologist and director of Dermatopathology at PROPATH, suggests the following protocol as a practical approach for patients with nail dystrophy. First, a nail biopsy is performed for histologic examination and PAS staining. If fungal elements are present, he uses a topical agent such as ciclopirox and combines it with a systemic drug (assuming there are no contraindications) such as terbinafine. If this treatment fails, then a culture is taken to determine if the fungus is resistant to the systemic drug. If so, switching to another agent such as itraconazole may be useful.

While nail biopsy with PAS staining is the current stateof- the-art, PROPATH remains committed to ongoing test development to continue to provide you with the latest diagnostic methods.

Table 1. Presentations of onychomycosis

Table 2. Implicated pathogens in onychomycosis

References:

1. Elewski B, Onychomycosis: Pathogenesis, diagnosis, and Management. Clinical Microbiology Reviews 1998 11(3):415-429.

2. Mahoney JM, Bennet J, and Olsen B, The diagnosis of onychomycosis. Dermatol Clin 2003 21:463-467.

3. Weinberg JM, Koestenblatt E, Tutrone WD, Tishler H, Najarian L, Comparison of diagnostic methods in the evaluation of onychomycosis. J Am Acad Dermatol 2003; 49:193-197.

4. Foster KW, Ghannoum MA, Elewski B, Epidemiologic surveillance of cutaneous fungal infection in the United States form 1999 to 2002. J Am Acad Dermatol 2004; 50:748-752.

Date of last revision: Winter 2005.