Diagnosing melanonychia striata can be challenging due to the need to distinguish between various underlying causes, including serious entities such as subungual melanoma and other nail malignancies [1–3]. While histopathologic analysis remains the gold standard for diagnosing longitudinal melanonychia in adults, clinical clues are invaluable for guiding appropriate sampling methods.
Subungual squamous cell carcinoma is the most common malignant tumor of the nail, although it is relatively rare. Unfortunately, literature describing the clinical and dermoscopic features of subungual pigmented squamous cell carcinoma in situ (SPSCCis) is scarce, reflecting the low incidence of the disease. Clinically, it presents as melanonychia in the pigmented variant, with associated hyperkeratosis, onycholysis, and nail plate destruction.
We have identified a peculiar pattern of longitudinal melanonychia bands in SPSCCis. Melanonychia in SPSCCis often exhibits a curved contour toward the medial part of the nail plate (Fig. 1), unlike melanonychia in melanoma, which typically has straight contours. We hypothesize that this difference arises from the distinct origins of the neoplastic cells. In melanoma, the pigment originates from neoplastic melanocytes in the nail matrix. As these melanocytes and melanin reach the nail plate, the pigment moves distally along with the nail growth, resulting in straight contours and a pyramidal appearance as the tumor progresses. In contrast, SPSCCis may originate from the nail bed, which contains fewer melanocytes compared to the nail matrix, or from the lateral nail fold. This different origin could cause the tumor to grow and expand toward the medial portion of the nail, leading to the characteristic curved contour of SPSCCis melanonychia.
Fig. 1.
a Biopsy-proven SPSCCis showing a band of melanonychia with a curved contour (black arrow). b Histopathologic sections showing atypical keratinocytes throughout the nail bed epithelium.
Acknowledgment
We thank Dr. Sonia Toussaint Caire for generously providing the histopathology images, which allowed for a more comprehensive presentation of our findings.
Conflict of Interest Statement
The authors have no conflicts of interest to declare.
Funding Sources
This study was not supported by any sponsor or funder.
Author Contributions
M.G.-T., S.P.B.-R., and J.G.D.-C. contributed equally to the drafting, revision, and final approval of this letter. All authors approved the final version of the manuscript and agreed to be accountable for all aspects of the work.
Funding Statement
This study was not supported by any sponsor or funder.
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