Beyond the Plate: Nail Bed Dermoscopy for Differentiation of Psoriatic, Fungal, and Traumatic Fingernail Onycholysis.

Background: Onycholysis is a common but etiologically heterogeneous nail sign, most frequently due to psoriasis, onychomycosis, and trauma. Since the diagnosis of the underlying cause can be misleading, we investigated whether dermoscopy (onychoscopy) pre- and post-clipping of the onycholytic nail plate can provide reproducible patterns to distinguish these causes without the need for a biopsy.

Materials and methods: In a multicenter retrospective image-based study, 60 onycholytic fingernails were examined (20 patients per diagnosis: psoriasis, onychomycosis, trauma). Onychoscopy of the intact plate was followed by clipping of the detached nail plate and dermoscopic assessment of the exposed nail bed under both dry and wet conditions. Color, amount, thickness, adherence, mobility ("wet test"), and distribution of subungual scales were recorded. Multinomial logistic regression assessed independent associations between dermoscopic features and diagnostic categories.

Results: After clipping, dermoscopy revealed distinct nail-bed patterns. Psoriasis showed a high scale amount (65%), white color (95%), thick (90%), strongly adherent (95%), and immobile scales (95%). Onychomycosis showed a moderate/high scale amount (95%), yellow/yellow-white color (90%), reduced adherence (80%), and mobility (65); when compared with psoriasis it showed higher odds of moderate-to-low scale amount (odds ratio [OR] 7.19), yellowish color (OR 6.02), reduced adherence (OR 75.19), and wet-test mobility (OR for absence 0.026). Trauma showed a low scale amount (75%), thin (85%), loosely adherent (65%), mobile scales (60%), and showed increased odds of reduced amount (OR 13.34), yellowish color (OR 7.15), and reduced adherence (OR 80.95).

Conclusions: Combining pre- and post-clipping nail dermoscopy improves differentiation of psoriatic, onychomycotic, and traumatic onycholysis, supporting post-clipping nail bed dermoscopy as a simple adjunct that enhances diagnostic accuracy and management.