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 diagnosis, 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 multicolored 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 mechanical therapy to improve outcomes. Training clinicians to identify the canal sign may significantly enhance diagnostic accuracy and therapeutic success.