Longitudinal Melanonychia Biopsy: What Not to Do

The nail matrix biopsy is an important technique in confirming or excluding a diagnosis of melanoma in a patient with longitudinal melanonychia.¹ Due to lack of experience, many dermatologists are reluctant to perform a nail biopsy, viewing the procedure as painful, with a chance of permanent nail dystrophy.² An improper biopsy may result in an inadequate specimen, and therefore, an insufficient or incorrect histopathological diagnosis.² The following case demonstrates how a poor biopsy technique resulted in a misdiagnosis in a patient with melanoma in situ.A 29-year-old male patient with Fitzpatrick skin type III presented with a band of longitudinal melanonychia affecting his right thumb. The lesion had been biopsied 4 weeks before, and he was told that histopathology showed a benign lesion (reported as a benign melanonychia). He was concerned because the proximal nail was still pigmented after the biopsy.Physical examination revealed a 2mm band of longitudinal melanonychia in the central part of the right thumb. The nail plate had a 3mm hole from the prior biopsy, 5 mm distal to the proximal nail fold, with an associated half-moon shaped scar. The scar and the hole resulting prior punch biopsy, which had been taken through the proximal nail fold, nail plate, and directly to the matrix with the same punch, and which did not include the entire width of band (Figure 1a). Dermoscopy showed a triangular-shaped dark brown band with irregular lines, dots, and globules (Figure 1b). The patient had pictures of his thumb immediately before and after the biopsy showing a triangular-shaped band and the subsequent proximal nail fold scar (Figure 2a, b). At this point, the pigmented lesion was completely excised. After reflecting the nail plate, allowing a direct visualization of the band and more precise sampling, a tangential biopsy of the entire pigmented band in the nail matrix was performed. Histopathology in this second biopsy showed definitive melanoma in situ.Different techniques have been described for the biopsy of longitudinal melanonychia, depending upon several factors, such as the degree of suspicion of malignancy, the presence of periungual pigmentation or a Hutchinson’s sign, the exact location of the origin of the band (ie, proximal or distal matrix), and the width of the band.^2,3 A double punch biopsy technique has been recommended in cases where the pigmented band is less than 3 mm in width.² However, with this method, retraction of the proximal nail fold must be performed,³ exposing the origin of the band (direct visualization) and allowing more precise sampling. Zaiac et al⁴ described a technique, termed “submarine hatch,” in which a double punch biopsy is performed, but the nail plate is retained. They reported excellent long-term aesthetic and functional results. In our patient, the clinical findings of triangular-shaped longitudinal melanonychia and dots and globules in the nail plate on dermoscopy increased our clinical suspicion of melanoma. These features have been described by Inoue et al⁵ as suspicious of melanoma.Proper submission of the specimen for histopathological analysis is also important, since a delicate and small nail matrix biopsy may be damaged if it is simply submitted in a bottle of formalin. An easy technique in which the specimen is submitted on paper inside a tissue cassette has been described