Distal Nail Canal Visible on Onychoscopy as a Diagnostic Sign of Dermatophytoma

Background: Early recognition of dermatophytoma is essential to initiating appropriate therapy and avoiding treatment failure. Dermatophytoma is a variant of distal subungual onychomycosis, characterized by an accumulation of hyphae and scales trapped under the nail plate, making antifungal treatment challenging. Its hallmark feature is a yellow-white mass under the nail with a distal band of onycholysis or “canal”. Objectives: This study aimed to describe the clinical and onychoscopic features of dermatophytoma to improve diagnostic accuracy. Methods: We conducted a retrospective study of 16 dermatophytoma cases diagnosed from 2020 to 2024 in a Nail Disorders Outpatient Clinic. Inclusion criteria were clinical and onychoscopic diag-nosis, microbiological confirmation, and availability of dermoscopic images. Demographic, clinical, microbiological, and dermoscopic data were analyzed descriptively. Results: Fifteen patients (with 16 dermatophytomas) were included (8 males, 7 females; mean age 67.9 ± 14.2 years). The big toe was most frequently affected (87.5%). Trichophyton rubrum was the predominant pathogen (94%). Onychoscopy revealed yellow discoloration in all cases, with multicol-ored lesions in 81.3%. Most lesions were oval (56%) or elongated (25%), with an average transverse diameter of 5.6 ± 2.9 mm. A characteristic distal “canal” of onycholysis was identified in all cases, most commonly linear or wavy. Conclusions: Onychoscopy reveals specific features, particularly the presence of a canal, that can aid early diagnosis. Recognizing these features is essential to initiating combined antifungal and me-chanical therapy to improve outcomes. Training clinicians to identify the canal sign may significantly enhance diagnostic accuracy and therapeutic success.