Photographs courtesy of Richard P. Usatine, MD.
THE COMPARISON
A Melanoma in situ manifesting as longitudinal melanonychia (LM) in a single digit in a Black man. Dermoscopy showed irregular dark bands of brown pigmentation and micro-Hutchinson sign on the cuticle (inset).
B Melanoma manifesting as LM with a prominent Hutchinson sign in a Hispanic man, with variable shades of brown covering more than 50% of the nail width.
C Longitudinal melanonychia of at least 2 nails with a pseudo-Hutchinson sign (pigment on the nail folds in a benign case of LM) in a young Black man demonstrating ethnic/racial melanosis. The longitudinal bands, which were caused by benign melanocytic activation, are more gray than brown and are less than 3 mm wide.
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
Key clinical features in individuals with darker skin tones
- In patients with darker skin tones, LM tends to occur on multiple nails as a result of melanocytic activation.2,13
- Several longitudinal bands may be noted on the same nail and the pigmentation of the bands may vary. With age, these longitudinal bands typically increase in number and width.13
- Pseudo-Hutchinson sign may be present due to ethnic melanosis of the proximal nail fold.13,14
- Dermoscopic findings of LM in patients with skin of color include wider bands (P=.0125), lower band brightness (P<.032), and higher frequency of changing appearance of bands (P=.0071).15
Worth noting
When patients present with LM, thorough examination of the nail plate, periungual skin, and distal pulp of all digits on all extremities with adequate lighting is important.2 Dermoscopy is useful, and a gel interface helps for examining the nail plates.7
Clinicians should be encouraged to biopsy or immediately refer patients with concerning nail unit lesions. Cases of LM most likely are benign, but if some doubt exists, the lesions should be biopsied or tracked closely with clinical and dermoscopic images, with a biopsy if changes occur.16 In conjunction with evaluation by a qualified clinician, patients also should be encouraged to take photographs, as the evolution of nail changes is a critical part of clinical decision-making on the need for a biopsy or referral.
Health disparity highlight
Despite the disproportionately high mortality rates from subungual melanoma in Black and Hispanic populations,3,9 studies often do not adequately represent these populations. Although subungual melanoma is rare, a delay in the diagnosis contributes to high morbidity and mortality rates.