Congenital subungual melanocytic nevus with a pseudo-Hutchinson sign

We describe a 29-year-old woman with congenital melanonychia striata and compound nevus of the right first digit. There was extension of the hyperpigmentation onto the proximal nail fold, even beyond the borders established by the band of melanonychia striata. A dermal plaque with irregular borders and variegated pigmentation was also present over the distal digit extending from the pigmented region of the hyponychium. A limited number of biopsy proven congenital subungual melanocytic nevi have been reported in the literature. Interestingly, we found that the majority of these cases present with longitudinal melanonychia in association with periungual hyperpigmentation, constituting a pseudo-Hutchinson sign. Currently studies evaluating the diagnostic test characteristics of Hutchinson sign are lacking. While Hutchinson sign is traditionally considered a worrisome feature it is certainly not pathognomonic and a malignant cause should not be assumed without thorough assessment.