Proximal Subungual Onychomycosis in the Immunocompetent: A Case Report and Review of the Literature

matrix and deeper portions of the ventral nail plate.³ As the nail plate grows, the infection extends distally.¹ Invasion of the proximal nail plate may also occur secondary to paronychia, hematogenous, or lymphatic spread.¹ Regardless of the route of infection, fungi must achieve access to the proximal nail matrix for PSO to manifest.³PSO is considered a sign of immunodeficiency and occurs, almost exclusively, in immunocompromised individuals.^2-4,6 The incidence of PSO in the immunocompetent population is 0.3% while that in the HIV+ population is almost 15 times greater at 4.2–5.0%.2 In immunocompromised individuals, infection with dermatophytes has been found to be the most common cause^2,4, as HIV/AIDS appears to diminish resistance to the traditional pathogens that cause onychomycosis.2 Consequently, PSO may be an indication for an evaluation for primary or secondary immunodeficiency.PSO is most frequently found on the toenails but may involve the fingernails as well.³ Infection presents as leukonychia or opaque, beige-white discoloration of the proximal nail plate.⁴ Discoloration spreads distally from the lunula as the nail plate grows and may be associated with subungual hyperkeratosis, proximal onycholysis, and nail plate destruction.^3,6 Unlike other instances of onychomycosis, concomitant tinea pedis is uncommon.³Diagnosis is confirmed with samples of the proximal ventral nail plate. To obtain a proper sample, the nail plate should be pared down to access the ventral side.⁶ Samples should be prepared with potassium hydroxide, which will reveal hyphae under light microscopy, or examined histopathologically with PAS staining, the gold standard for diagnosing fungal onychomycosis.^3,6-8 In cases in which sufficient sampling is not possible via non-invasive means, punch biopsy of the nail plate may be considered. Mycological culture on agar without cycloheximide can identify the provocative organism.⁶ PSO is challenging to treat and requires systemic therapy following standard guidelines for onychomycosis.¹ A number of oral antifungal agents, including terbinafine^5,8, itraconazole⁵, and fluconazole6, as used in our patient (although not FDA-approved for the treatment of onychomycosis⁶), have been reported to be effective treatments. Occasionally combined therapy with oral and topical treatments with or without nail avulsion/surgery may be necessary.¹ Our case of a 51-year-old man without immunodeficiency, who presented with PSO on multiple toenails, is unusual as PSO is rarely encountered in the immunocompetent.⁴ Immunocompetent patients more frequently present with DLSO, the most common form of onychomycosis.^1,2 To our knowledge, there have been only a few reports of PSO occurring in immunocompetent patients (Table 1).^7-10 But within the limited available literature, NDMs are being increasingly considered as a cause of PSO, particularly in the immunocompetent. In a study of 431 patients with onychomycosis, NDMs were responsible for 13.6% of cases. More than half of those cases presented with PSO with only two associated with immunodeficiency.⁵ Previously, NDMs were thought to be culture contaminants,