Perplexing pigments: from melanoma to pigmented onychomatricoma

Summary: A 70-year-old female presented with 10 years of melanonychia of her 2nd right toenail. On exam, she had hyperkeratosis, a thickened pigmented nail plate with proximal nail fold pigmentation, as well as an increase in transverse curvature of the nail plate. Differential diagnosis included subungual melanoma, onychomatricoma, onychomycosis, onycholemmal horn, subungual keratoacanthoma, subungual SCC, subungual verruca, and psoriasis. A punch biopsy included nail fold epithelium, nail plate, nail matrix, and onychodermis. The nail matrix included finger-like papillations with variable pigmented epithelium and compact parakeratosis with serum and melanin in the cornified layer. Pigmented areas were negative for SOX-10, MART-1, and PRAME. Fontana-Masson showed no increased density of melanocytes. PAS showed no fungal elements. A matrical melanocytic proliferation was not revealed. CD34 highlighted spindle cells in the underlying onychodermis. (1) Taken together these findings support the diagnosis of a pigmented onychomatricoma. Onychomatricomas are rare benign fibroepithelial digitating neoplasms that classically demonstrate a yellow band pattern, increased transverse curvature, splinter hemorrhages and finger-like projections due to abnormal nail matrix differentiation.(2) Pigmentation is an uncommon feature. Genome-wide analyses of onychomatricomas have noted many genomic alterations including deletions on chromosome 11 at STIM-1 and Cathepsin C.(3) More studies are needed to understand onychomatricoma development.