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

Onychomycosis, infection of the nail by fungus, is the most common nail disease.¹ Since the original classification in 1972, onychomycosis has been divided into clinical variants according to the pattern of nail involvement by site and mode of invasion.¹ Subtypes include distal lateral subungual (DLSO), white superficial, proximal subungual (PSO), endonyx, and total dystrophic onychomycosis.¹ PSO, predominantly involving the nail plate from the proximal nail fold,¹ is the rarest form of onychomycosis, occurring in less than one percent of cases.^2,3 Classically associated with an immunocompromised state, PSO is uncommon in individuals without immunodeficiency.⁴

A healthy 51-year-old man presented with a three-month history of discoloration of multiple toenails. He reported white discoloration of his left hallux followed by similar changes of his left third toe and right hallux, second, and fourth toenails. Discoloration began at the proximal edge of the nail plate and extended distally over three months. The patient denied prior trauma and treatment. He reported discomfort while wearing closed-toe shoes, however denied any constitutional symptoms. A review of systems was otherwise negative. His medical history was significant for hypothyroidism treated with levothyroxine.On physical examination, he was a well-appearing Caucasian male with findings notable for white, opaque patches on the proximal third nail plates of the left hallux and third toe and the right hallux, second, and fourth toenails (Figures 1 and 2). The affected digits demonstrated proximal onycholysis and subungual debris. Distal nail plates were uninvolved. There was mild erythema of the periungual skin of the affected digits, with marked edema of the left fifth nail subunit (Figure 1). Both feet featured interdigital scale and maceration (Figure 3), as well as plantar scale and erythema. Laboratory examinations, including routine serologic studies as well as human immunodeficiency virus (HIV) and antinuclear antibodies, were within normal limits. Proximal nail fragments of the left hallux showed sections of dystrophic nail plate with mounds of parakeratosis, collections of neutrophils, and hyphae that highlighted with periodic acid-Schiff (PAS) staining.The patient was diagnosed with PSO and tinea pedis bilaterally. For ease of administration and favorable side effect profile, he was treated with oral fluconazole 200 mg daily for 4 weeks followed by fluconazole 400 mg weekly for 3 months with gradual improvement.

PSO, the rarest form of onychomycosis, is most commonly caused by Trichophyton rubrum, but other dermatophytes as well as non-dermatophytic molds (NDMs) and yeast have also been identified.1,3,5 In the classic description of PSO, fungi initially invade the stratum corneum of the proximal nail fold to establish infection. Subsequently, fungi invade the nail