Subungual Nail Erythrasma Presenting as Melanonychia: A Rare Finding

Abstract

Nail pathology may reflect a wide array of localized and systemic dermatological conditions. Certain nail findings such as melanonychia can create diagnostic challenges even to nail experts. We report a case of a 78-year-old man who presented with melanonychia of the great toe. Nail clipping showed focal melanin deposition, and dermoscopy demonstrated a region of localized erythema in the lunula concerning for possible melanocytic neoplasm. Subsequent nail biopsy showed numerous vertically oriented filamentous bacteria and coccobacilli within the nail plate consistent with a diagnosis of subungual nail erythrasma. Nail erythrasma is a rare entity. Additionally, this case highlights a new clinical presentation of nail erythrasma as melanonychia.

Keywords: Cornyebacteria; Erythrasma; Nail matrix; Nail plate.

Fig. 1

a Smudged brown proximal lateral nail plate discoloration. b Residual minimal brown lateral nail plate discoloration, limited to the distal plate, at follow-up visit. c Wood's lamp examination showing coral red fluorescence between the left first and second toe web space. d Wood's lamp examination showing mild fluorescence of the proximal lateral nail plate.

Fig. 2

Nail bed with attached nail plate showing mild hyperkeratosis and parakeratosis with focus of superficial detachment. a H&E, original magnification ×20. High-power photomicrograph showing numerous vertically oriented filamentous bacteria and coccobacilli within the superficial nail plate. b H&E, original magnification ×100. c H&E, original magnification ×400. Gram stain highlighting numerous vertically oriented filamentous bacteria and coccobacilli within the superficial nail plate. d H&E, original magnification ×400.