Clinical clues for differential diagnosis between verruca plana and verruca plana-like seborrheic keratosis.

Sometimes the clinical differentiation between verruca plana (VP) and VP-like seborrheic keratosis (SK) could be challenged. However, there have been no studies on this issue to date. The aim of this study was to elucidate clinical and dermoscopic differences between these two diseases, and also to suggest a diagnostic algorithm of VP and VP-like SK without skin biopsy. The patients who had lesions clinically considered as VP or VP-like SK were the target of our study. We took clinical and dermoscopic photos with informed consent and conducted a questionnaire. All patients had their diagnoses confirmed by biopsy. Thirty-three patients were enrolled in our study. Seventeen patients were finally diagnosed with VP (51.5%) and 16 patients with VP-like SK (48.5%). In clinical findings, VP-like SK showed significantly more scattered distribution than VP (P = 0.039), which exhibited more clustered or grouped distribution (P = 0.039). In dermoscopic findings, brain-like appearance was more commonly observed in VP-like SK (P = 0.003) whereas VP showed more red dots or globular vessels (P = 0.017) and even-colored light brown to yellow patch (P < 0.001). Sex, onset age, the size of each lesion, location, color and shape showed no significant differences between them (P > 0.05). Based on our results, we suggest a diagnostic algorithm using Koebner's phenomenon, dermoscopic findings, distribution of each lesion and biopsy for multiple VP-like lesions in adults, and we think it will be a very useful diagnostic tool in daily clinical dermatological practice.