Variable Sized Cellular Remnants in the Nail Plate of Longitudinal Melanonychia: Evidence of Subungual Melanoma

Dear Editor:

Subungual melanoma (SUM) is a rare variant of melanoma that occurs in the nail unit. However, in Asia, SUM accounts for approximately 20% of cutaneous melanoma1. In its early stage SUM is often misdiagnosed histopathologically2. The delay in diagnosis results in progression of the disease, which may be associated with poor prognosis.

Previously, it was reported that the attached nail plate epithelium is helpful for the diagnosis of SUM3. Here we describe the need for the evaluation of the nail plate in the diagnosis of SUM showing longitudinal melanonychia.

A 65-year-old woman presented with color change of the 1st fingernail (Fig. 1). Brown pigmentation was observed around the proximal nail fold. Under the impression of SUM, the nail was extracted. Histopathology from the nail plate showed numerous variable sized cellular remnants (Fig. 2A, B). The attached nail plate epithelium had a considerable number of atypical melanocytes. On immunohistochemical analysis, the cellular remnants within the nail plate were positive for HMB45 (Fig. 2C).

Fig. 1
Brown colored longitudinal melanonychia and periungual hyperpigmentation around the 1st fingernail are seen.

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Fig. 2
(A) Histopathology shows an extracted nail plate (H&E, ×10). The rectangle indicates B. (B) Numerous variable sized cellular remnants in the nail plate and atypical cells in the attached epithelium are found (H&E, ×200). (C) HMB45 is positive in the cellular remnants within the nail plate (immunoperoxidase staning, ×200).

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SUM usually derives from nail matrix melanocytes and spreads to the surrounding areas including the nail bed, hyponychium, and proximal nail fold4. When SUM originates from the nail matrix, it presents clinically as longitudinal melanonychia. In the nail unit, the nail plate is formed mainly from the nail matrix. Thus, melanoma cells in the nail matrix may influence the formation of, and be incorporated into, the nail plate. Based on our case, the numerous variable sized cellular remnants of the nail plate seem to be possible evidence for the diagnosis of SUM. In cases presenting with longitudinal melanonychia, evaluation of the extracted nail plate alone may be useful for the diagnosis of SUM.

Cellular remnants in the nail plate may be found in a variety of pigmented lesions that occur within the nail unit. However, in benign pigmented lesions, melanin pigments are mainly observed in the nail plate rather than in the cellular remnants (unpublished data).

For the evaluation of longitudinal melanonychia several biopsy methods have been used5. Nail biopsy is performed with or without nail extraction. Whether the nail is extracted or not in the biopsy procedure, careful examination about the nail plate is necessary for the evaluation of longitudinal melanonychia.