Longitudinal Melanonychia

Authors:
Xing Wang, BS
McGovern Medical School at UT Health, Houston, Texas

Alissa Chen, MPH
McGovern Medical School at UT Health, Houston, Texas

Miriam Matthews
Sugarland, Texas

Lynnette Mazur, MD, MPH
McGovern Medical School at UT Health, Houston, Texas

Citation:
Wang X, Chen A, Matthews M, Mazur L. Longitudinal melanonychia. Consultant. 2019;59(2):61-63. doi:10.25270/con.2019.02.00001

Two pediatric patients presented to clinic for well child visits and had blackened toenails on physical examination.

The first patient, a 5-year-old Asian American girl, was noted to have a dark line on her left great toe beginning at 2 years of age (Figure 1). Over the next 2 to 3 years, the darkness had widened and now covered half of the nail.

Figure 1. Longitudinal melanonychia on the great toe of a 5-year-old girl.

The second patient, a 5-year-old African American boy, had a dark line on his left great toe at birth (Figure 2). However, unlike with the first patient, the discoloration had remained the same size.

Figure 2. Longitudinal melanonychia on the great toe of a 5-year-old boy

Both children received a diagnosis of longitudinal melanonychia (LM).

Discussion. Melanonychia is a general term for melanin-derived brown or black pigmentation of the nail. Also termed melanonychia striata, LM is characterized by pigmentation that is oriented lengthwise along the nail. Melanonychia most commonly occurs in the longitudinal orientation, with total or transverse melanonychia occurring rarely.

The prevalence of LM is up to 77% in the African American population over age 20, 1.4% in the white population, and 20% in the Japanese population.^1,2 The differential diagnosis of LM in our patients included nail matrix nevus, subungual lentigo simplex, and functional longitudinal melanonychia (Table 1).

The approach to LM in children includes combining history with clinical and dermoscopic features. A handheld dermoscope helps differentiate among types of LM and nonmelanocytic lesions such as subungual hematomas and fungal infections.³ Melanocytic lesions have continuous long bands, whereas nonmelanocytic lesions do not.⁴ Additionally, melanocytic bands have melanin in cellular inclusions or granules with less than 0.1 mm of pigment inside.³ Melanocytic lesions due to hyperplasia (increase in number of melanocytes) have brown bands on dermoscopy, while lesions due to activation (melanin overproduction) have gray bands. While biopsy remains the gold standard for diagnosing nail pigmentation, dermoscopy can prevent unnecessary and painful procedures.⁵

Nail matrix nevi are benign, congenital, or acquired proliferations of melanocytes in the nail matrix. Junctional nevi are the most common, and they most frequently occur in the fingernails and toenails; the thumb is most commonly affected digit.⁶ Up to 50% of reported LM cases in children are nevi.^4,7 A feature strongly suggestive of a nevus is an age of onset of less than 13 years.⁶ They present as dark brown bands, often containing linear lines that overlie the proximal nail fold, or light bands that may spontaneous fade with time.⁶ Histopathology findings of polygonal or rounded melanocytes in nests at the dermoepidermal junction are diagnostic.⁶

Subungual lentigo simplex is a benign, uniform, brown, dark brown, or tan pigmented macule that may be congenital or acquired.⁸ It is caused by increased proliferation of melanocytes in the basal and suprabasal layers of the epidermis, but unlike nevi, it does not feature melanocytes that form nests.⁹

Functional LM is hyperpigmentation caused by melanocytic activation leading to melanin overproduction with no increase in the number of melanocytes.⁷ Melanocytic activation displays racial variation, and certain racial groups (African, Chinese, Japanese, and Native American) are more susceptible to activation and more likely to have benign longitudinal pigmented bands.

While the majority of cases of LM in children are benign, they may be an early sign of melanoma. Malignant melanoma is extremely rare in prepubescent children and accounts for less than 0.5% of melanoma cases in all ages.¹⁰ Of reported cases of pediatric LM, less than 6% have been melanoma in situ.¹¹ Only 12 cases of childhood nail melanoma have been reported.¹² Worrisome features are described by the ABCDEF mnemonic (Table 2).¹³ The majority of melanomas arise de novo and are not associated with preexisting LM. The lifetime risk of a nevus transforming into melanoma ranges from 0.009% to 0.03%.¹⁴

Dermoscopy can be a useful tool to differentiate between benign and malignant lesions; however, histopathologic diagnosis remains the gold standard.⁵ Dermoscopy findings of subungual melanoma can include varying pigmentation width, multicolored lines, positive Hutchinson sign (pigmentation that extends into the proximal or lateral nail fold), nail dystrophy, and lines with irregular coloration, spacing, or thickness.^5,15 While these signs suggest melanoma in adults, the specificity of dermoscopy is low in children, and false positives can occur. In children, benign lesions can have an irregular pattern, darker color, multicolor, Hutchinson sign, evolution, and nail dystrophy.^4,15 There are no universally accepted clinical or dermoscopic criteria to diagnose nail pigmentation in children.¹⁶ Biopsy is indicated when the band rapidly enlarges and involves the whole nail, or when the color is dark black such that the lines are difficult to see.¹⁶

In conclusion, childhood LM of a single digit is most commonly a nevus, followed by subungual lentigo and functional longitudinal melanonychia. Conservative management with serial photographs, dermoscopy, and close follow-up (every 6 months) is usually all that is needed. Biopsy is indicated if the band rapidly changes, enlarges, or the band is dark black.

References:

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