Leukonychia is a common abnormality of the nail with diverse etymologies, such as systemic diseases, hereditary causes, and dermatological conditions. Fungal infections are a possible cause of leukonychia; particularly, superficial white onychomycosis and primary subungual onychomycosis can make the nails white₁. Fungal leukonychia usually affects the toenails, and in cases where fingernails are affected, con- current toenail onychomycosis typically occurs₂. We report two cases of single-fingernail leukonychia without toenail infections and other fingernail changes.

A 59-year-old man with a history of pemphigus vulgaris presented with asymptomatic total whitish discoloration of the fourth right fingernail, the onset of which was several months ago (Fig. 1A, B). Except for the fourth nail on the right hand, no other fingernails or toenails were affected. This patient was receiving daily low-dose systemic steroids (5~10 mg methylprednisolone daily) since 2017 for pemphigus vulgaris. The patient's medical history was unremarkable for any treatment for fingernail abnormalities. To confirm onycho- mycosis, a nail potassium hydroxide (KOH) examination was performed, which revealed several septal mycelia. Fungal culture with subungual debris was performed on a Sabouraud dextrose agar (SDA) with cycloheximide. White fluffy col-onies with a central umbo and a peripheral flat shape were observed from the top view of the SDA plate, and a brownish to wine-red color was observed in the bottom view (Fig. 1C, D). Furthermore, microconidia were observed along the lateral side of the hyphae on lactophenol cotton blue stain (LCBS) (Fig. 1E). Trichophyton rubrum was suspected as the causal organism, and 18S rRNA sequencing revealed Trichophyton rubrum as a significant pathogen (Fig. 2). We grinded the nail surface to allow the drug to permeate well. The nail lesion improved after the application of topical efinaconazole for 3 months (Fig. 1F).

Figure 1. (A) (B) Total whitish discoloration of right 4th fingernail in a 59-year-old male. (C) On Sabourade dextrose agar (SDA) with cycloheximide, white fluffy colonies with central umbo and peripheral flattened appearance were observed from the upper side. (D) In the bottom view, brownish to wine red color was showed. (E) In lactophenol cotton blue stain (LCBS), microconidia along the lateral side of the hyphae (F) After 3 months treatment, improvement of nail lesion was observed.

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Figure 2. As a result of 18s rRNA sequencing, Trichophyton rubrum was identified as the causative fungus.

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A 32-year-old man presented with asymptomatic longi- tudinal ridges and pitting with partial whitish discoloration on the left second fingernail for 5 years (Fig. 3A, B). The patient's medical history was unremarkable. Although he was treated with oral medication at a private clinic, no improve- ment was observed. The fingernail had fallen out several times. KOH examination revealed several short hyphae (Fig. 3C). White cottony colonies were observed from the top and bottom views of the fungal culture on SDA (Fig. 3E, F). Oval-shaped conidia were observed on LCBS (Fig. 3D). Therefore, the Scedosporium species was identified as a causal organism. After 2 months of oral terbinafine combined with topical efinaconazole treatment, the fingernail lesion improved.

Figure 3. (A) (B) Asymptomatic longitudinal ridges and pitting with partial whitish discoloration on left 2nd fingernail in a 32-year-old male (C) Many short hyphae with septum were observed on KOH examination. (D) In LCBS, oval shape of conidia were observed. (E) (F) On Sabourade dextrose agar (SDA), white cottony colonies were observed in the top and bottom view.

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Leukonychia manifests as whitish discoloration of the nail. Its etymology ranges from genetic to acquired, such as fungal infections. Leukonychia can be caused by nail plate or nail bed abnormalities. True leukonychia and pseudo-leukonychia are caused by nail plate abnormalities, and apparent leukonychia, such as Terry's nail, are caused by nail bed abnormalities. Among fungal infections, proximal subungual onychomycosis and white superficial onychomycosis cause leukonychia, both of which originate from nail plate abnormalities₁.

The clinical features of the two patients were white super- ficial onychomycosis, in which they showed a superficial white patch on the nail plate.

According to Piraccini and Tosti, Trichophyton interdigitale is the most common causal organism in white superficial onychomycosis₂. White superficial onychomycosis due to Trichophyton interdigitale always involves the toenails, usually the first one, and presents as 1~5 small whitish opaque friable patches on the nail. Trichophyton rubrum was found to be the cause in four of 79 patients₂. In our cases, Trichophyton rubrum was found to be the causal organism in the first patient as well.

In the report by Piraccini and Tosti, 21% of the cases of white superficial onychomycosis were caused by nonder- matophytes₂. In the second patient, Scedosporium, a non-dermatophyte, was found to be the causal agent. Although rare, Scedosporium has been reported to be the causal fungus in onychomycosis₃.

The involvement of more than one fingernail without a concomitant toenail infection is rare in onychomycosis, unless the patient experienced trauma or is immunosuppressed₄.

White superficial onychomycosis typically affects ≥1 toenails, most commonly the first, second, and third toenails. The involvement of the fingernails in adults has rarely been de- scribed₅.

In our cases, the first patient was on prolonged systemic steroid treatment after the diagnosis of pemphigus. Although the second patient had no history of trauma, he complained of frequent nail loss. Both patients showed the involvement of only one fingernail, and these cases were uncommon, in whom no other nails were affected.

In summary, both patients showed leukonychia on one nail without the involvement of any other nails, although the cause was a fungal infection. Our report suggests that fungal causes should be suspected in leukonychia of the nail, and KOH examination and fungal culture should be performed for an accurate diagnosis.