Tough Decisions for Longitudinal Melanonychia: Approach to Evaluation, Sampling, and Laboratory Processing

Dr. Adam Rubin presented information about the clinical presentation and biopsy collection of longitudinal melanonychia. Most cases of nail unit melanoma (NUM) present as longitudinal melanonychia. NUM has a higher mortality rate compared with other cutaneous melanomas due to delayed diagnosis and more advanced disease on presentation.

Adam Rubin, MD, professor of dermatology at the New York University Grossman School of Medicine and director of the Section of Dermatopathology, Ronald O. Perelman Department of Dermatology at NYU Langone Health

First, Rubin described the clinical presentation of longitudinal melanonychia. Concerning features for longitudinal melanonychia include advanced age, band width >3 mm, lesion size > 40% of the nail, irregular or blurred band border, thumb or big toe affected, and Hutchinson sign (pigmentation of the periungual skin). Melanonychias that occur in infancy are almost always benign; those that appear in children and young adults < 25 years old are usually benign. Lesions that begin after age 25 are more suspicious; those with onset after age 40 are often malignant. 

Non-melanocytic causes of longitudinal melanonychia include pigmented onychopapilloma, onychocytic matricoma​, subungual longitudinal acanthoma, onychocytic carcinoma in situ, squamous cell carcinoma in situ, pigmented onychomatricoma, subungual hematoma, fungal melanonychia, and pigmented Bowen disease. Fungal melanonychia may appear yellow or scaly. 

Second, Rubin discussed when and how to collect and section biopsies for longitudinal melanonychia. Dermatologists should biopsy suspicious lesions without delay. Any lesion that appears concerning with dermoscopy should be biopsied. A triangular shape of the band of melanonychia indicates rapid growth over time; this is a red flag in adults but less concerning in growing children. 

Dermatologists should consider the clinical features of a melanonychia lesion to select the most appropriate biopsy technique. The biopsy needs to include the origin of the band and the entire lesion because a smaller portion may not be representative. Rubin urged dermatologists not to biopsy only the Hutchinson’s sign, as it can look benign with histopathology. Melanonychias that originate in the distal matrix and are < 3 mm wide can be sampled with a punch biopsy; melanonychias that originate in the matrix or have a band width > 3 mm should be sampled with the matrix shave technique. 

After the biopsy has been collected, appropriate grossing and sectioning techniques are needed to achieve an accurate, reliable diagnosis. Rubin advocates for longitudinal sectioning of the matrix. The specimens should be oriented on an inked map that indicates the precise location on the nail unit from which the specimen originated. The map and specimens should be placed directly into a formalin bottle. Clinical photos can aid the dermatopathologist and laboratory staff in employing appropriate grossing techniques.

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