Longitudinal melanonychia (LM) is a pigmented linear band—brown, black, or gray—spanning the length of the nail plate due to the presence of excess melanin, which may be attributed to a benign or malignant process and may warrant further investigation.1,2 The majority of patients who present with LM are diagnosed with melanocytic activation of the nail matrix due to their inherent darker skin tone or various triggers including trauma, infection, and medications. Longitudinal melanonychia secondary to melanocytic activation often occurs spontaneously in patients with skin of color.3 Less commonly, LM is caused by a nail matrix nevus or lentigo; however, LM may arise secondary to subungual melanoma, a more dangerous cause.
A thorough clinical history including duration, recent changes in LM manifestation, nail trauma, or infection is helpful in evaluating patients with LM; however, a history of nail trauma can be misleading, as nail changes attributed to the trauma may in fact be melanoma. Irregularly spaced vertical lines of pigmentation ranging from brown to black with variations in spacing and width are characteristic of subungual melanoma.4 Nail dystrophy, granular hyperpigmentation, and Hutchinson sign (extension of pigmentation to the nail folds) also are worrisome features.5 In recent years, dermoscopy has become an important tool in the clinical examination of LM, with the development of criteria based on color and pattern recognition.5,6 Dermoscopy can be useful in screening potential candidates for biopsy. Although clinical examination and dermoscopy are essential to evaluating LM, the gold-standard diagnostic test when malignancy is suspected is a nail matrix biopsy.1,2,6,7
Epidemiology
It is not unusual for patients with darker skin tones to develop LM due to melanocytic activation of multiple nails with age. This finding can be seen in approximately 80% of African American individuals, 30% of Japanese individuals, and 50% of Hispanic individuals.2 It has even been reported that approximately 100% of Black patients older than 50 years will have evidence of LM.3
In a retrospective analysis, children presenting with LM tend to have a higher prevalence of nail matrix nevi compared to adults (56.1% [60/106] vs 34.3% [23/66]; P =.005).8 Involvement of a single digit in children is most likely indicative of a nevus; however, when an adult presents with LM in a single digit, suspicion for subungual melanoma should be raised.2,3,9
Two separate single-center retrospective studies showed the prevalence of subungual melanoma in patients presenting with melanonychia in Asia. Jin et al10 reported subungual melanoma in 6.2% (17/275) of Korean patients presenting with melanonychia at a general dermatology clinic from 2002 to 2014. Lyu et al8 studied LM in 172 Chinese patients in a dermatology clinic from 2018 to 2021 and reported 9% (6/66) of adults (aged ≥ 18 years) with subungual melanoma, with no reported cases in childhood (aged < 18 years).
Although the prevalence of subungual melanoma in patients with LM is low, it is an important diagnosis that should not be missed. In confirmed cases of subungual melanoma, two-thirds of lesions manifested as LM.3,10,11 Thus, LM arising in an adult in a single digit is more concerning for malignancy.2,3,7,9
Individuals of African and Asian descent as well as American Indian individuals are at highest risk for subungual melanoma with a poor prognosis compared to other types of melanoma, largely due to diagnosis at an advanced stage of disease.3,9 In a retrospective study of 25 patients with surgically treated subungual melanoma, the mean recurrence-free survival was 33.6 months. The recurrence-free survival was 66% at 1 year and 40% at 3 years, and the overall survival rate was 37% at 3 years.12