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Onychomycosis is a chronic progressive infection of the nail unit. It is commonly caused by dermatophytes, particularly Trichophyton rubrum and less often Candida and nondermatophyte molds. Therapy is not required in all patients, but is recommended in those who are symptomatic, diabetic, or immunosuppressed or have a history of cellulitis of the lower extremity. Onychomycosis predominantly affects the toenails. Fingernail infection by a dermatophyte is rarely seen in those who do not have underlying toenail disease. Severity is categorized into “mild” (<20% nail involvement), “moderate” (20%-60% nail involvement), and “severe” (>60% and/or involvement of the matrix/lunula). Mycological cure is defined as a negative fungal culture and negative KOH. Clinically cured is a completely clear nail. The goal of treatment is complete mycological and clinical cure. This might not be feasible in all patients because long-standing infections can cause irreparable damage to the nail, such as onycholysis. Treatment decisions should be based on comorbidities, drug-drug interactions, age of patient, and causative pathogen. For example, terbinafine is a potent antidermatophyte drug, while itraconazole and fluconazole have activity against dermatophytes, nondermatophyte molds, and Candida. Topical therapy is recommended for children, adults with limited nail involvement, and those with contraindication to systemic medications. Topical therapy applied under the nail plate, either as monotherapy or in adjunct to systemic therapy, may be helpful when longitudinal spikes are present. Resistance to antifungal therapy has been a continuing challenge; antifungal stewardship is of utmost importance. Diagnosis should be confirmed prior to treatment. See Table 88-1.
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