When longitudinal melanonychia appears as a sharply demarcated pigment band of even width against normal nail in a child, Hutchinson’s sign with longitudinal brushy pigmentation may be a useful clinical pattern suggesting a diagnosis of nail matrix nevus rather than subungual melanoma, said Jae Ho Lee, MD, of Sungkyunkwan University, Seoul, South Korea, and associates.

In a study of biopsy-proven nail matrix nevi (NMN) in 20 children and 8 adults, average melanonychia width was 3.5 times greater in children than in adults, with 14 children having melanonychia greater than 20% the width of the nail, compared with 2 adults. Total melanonychia occurred just twice, in two children. A total of 12 children had nail dystrophy, while none of the adults did; nail dystrophy was more frequent in wider lesions.

Hutchinson’s sign was seen in seven pediatric patients, but no adult patients. In most cases, Hutchinson’s sign had hyponychial pigmentation, and on dermoscopy showed a pigment pattern presenting longitudinally and resembling a brush mark (longitudinal brushy pigmentation or LBP). LBP of nail matrix nevi is different from the Hutchinson’s sign that occurs in subungual melanoma (SUM), where it is typically a “haphazard pigmentation pattern involving periungual skin.

“We propose that Hutchinson’s sign occurs more commonly in pediatric NMN than in adult NMN, and that the presence of the LBP pattern can help distinguish pediatric NMN from SUM,” the investigators said.

Histologically, the biopsies of the NMN in this study showed some important differences from known SUM histology. All the study biopsies “showed a melanocytic proliferation exhibiting a predominantly nested growth pattern, with the nests mostly located at the dermoepithelial junction and with retraction artifact surrounding the nests. There were variable nuclear hyperchromatism, nuclear sizes, and cytologic atypia within the NMN biopsy specimens,” the researchers said. “In contrast, the histology of SUM demonstrates a predominance of atypical single melanocytes over nests, retraction artifacts around individual melanocytes, and uniform atypia of melanocytes throughout the biopsy specimen.”

SOURCE: Lee JH et al. J Am Acad Dermatol. 2018 Mar;78(3):479-89.