Optimizing Biopsy Decisions in Benign Longitudinal Melanonychia: A Dermoscopic-Pathologic Study of Nail Matrix Nevus vs. Melanocytic Activation.

Abstract

Background: Longitudinal melanonychia (LM) is a common clinical manifestation of nail melanocytic lesions and poses diagnostic challenges in distinguishing benign melanocytic activation (MA) from melanocytic hyperplasia such as nail matrix nevus(NMN).

Objectives: This study aimed to identify clinical, dermoscopic, and histopathologic features that distinguish NMN from MA in LM and facilitate biopsy decision-making.

Methods: We retrospectively reviewed 33 patients with LM (21 MA and 12 NMN) who underwent nail biopsy at our institution between 2019 and 2021. All cases underwent preoperative clinical photography and histopathological examination. Among them, 23 cases underwent dermoscopy, and 16 cases underwent immunohistochemical examination.

Results: No significant differences in age or sex distribution were observed between groups (MA: 39.7 yrs, M:F = 1:2.5; NMN: 35.3 yrs, M:F = 1.4:1). Dermoscopy revealed distinct background colors (MA: gray, NMN: brown; P < 0.05) and higher Hutchinson sign frequency in NMN (45% vs. 8%; P < 0.05). Stripe width differences were non-significant (MA: 3.2 mm, NMN: 2.9 mm; P > 0.05). Histopathologic examination of MA revealed scattered melanin granule deposition in the nail bed or nail matrix, with no obvious nevus cell nests, while NMN showed nevus cell nests. In immunohistochemistry, S100 positivity was detected only in NMN (60% vs. 0%, p = 0.018) CONCLUSIONS: Dermoscopic features of LM (background color, Hutchinson sign), combined with other clinical characteristics such as predilection sites and progressive changes, can provide valuable clues for differentiating MA from melanocytic proliferative LM represented by NMN, thereby reducing unnecessary biopsies.