Hutchinson's Nail Sign: Key Indicator for Subungual Melanoma and Differential Diagnosis

Hutchinson's nail sign, first described by Sir Jonathan Hutchinson in 1886, refers to the extension of brown-black pigmentation from longitudinal melanonychia onto the periungual skin, most commonly involving the proximal and lateral nail folds. This sign is a critical clinical indication of subungual melanoma. However, it is not pathognomonic and may be observed in various benign conditions, including racial melanonychia, drug-induced pigmentation, Bowen's disease, Peutz-Jeghers syndrome, Laugier-Hunziker syndrome, and traumatic nail changes. Recognized variants include 'micro-Hutchinson's sign,' which is detectable only through dermoscopy, and 'pseudo-Hutchinson's sign,' which results from pigmentation that is visible through translucent nail folds without actual pigment spread. Differentiating these variants is essential to ensure an accurate diagnosis and prevent misinterpretation. The presence of Hutchinson's sign should prompt a thorough clinical evaluation for additional concerns such as increased width of the pigmented band, proximal widening, color variegation, irregular borders, nail plate dystrophy, ulceration, bleeding, or involvement of high-risk digits (thumb, index finger, and great toe). Dermoscopy enhances diagnostic precision by distinguishing malignant patterns, characterized by diffuse, irregular pigmentation with grey to black hues and nail dystrophy, from benign patterns, such as uniform brown pigmentation with a regular, parallel distribution. When clinical and dermoscopic findings remain inconclusive, a nail matrix biopsy is necessary to establish a definitive diagnosis. In conclusion, although the Hutchinson's nail sign is a valuable indicator of potential nail unit melanoma, it must be interpreted cautiously and within a broader clinical context because of the possibility of benign mimickers.