Melanonychia is brown or black pigmentation of the nail unit. Melanonychia commonly presents as pigmented band arranged lengthwise along the nail unit, and this presentation is known as longitudinal melanonychia (LM) or melanonychia striata. The most concerning cause of melanonychia is subungual melanoma, although a variety of other causes includes physiologic longitudinal melanonychia, systemic disorders, trauma, inflammatory disorders, fungal infections, drugs, and benign melanocytic hyperplasias.  See the image below.
Melanonychia most often occurs because of increased production of melanin by melanocytes in the nail matrix. A healthy adult has approximately 200 melanocytes per mm2 in the nail matrix, of which the majority remain dormant. When these melanocytes are activated, melanosomes filled with melanin are transferred to differentiating matrix cells, which migrate distally as they become nail plate onychocytes.  This results in a visible band of pigmentation in the nail plate.
Physiologic melanonychia is more common in darker-pigmented individuals. Seventy-seven percent of black individuals older than 20 years and almost 100% older than 50 years have evidence of this condition. [3, 4] LM is present in 10-20% of Japanese individuals.  In the general white population, the prevalence of LM is 1.4%. 
In a study of 68 Hispanic patients with longitudinal melanonychia, melanonychia secondary to skin pigmentation was observed in 48 cases (68.6%).  Of the remaining patients, 6 (8.5%) were secondary to trauma, 5 (7.1%) were secondary to fungal infection, 4 (5.7%) were associated with benign melanocytic hyperplasia, and 4 (5.7%) had a nail apparatus malignancy. The remaining 3 (4.3%) cases were of mixed etiology.
The frequency of melanonychia varies by the degree of skin pigmentation, as described in Frequency, International.
Melanonychia affects males and females equally. 
Typically, melanonychia is more common in older individuals. In children, melanonychia is often caused by melanocytic nevi. Subungual melanoma or melanoma in situ is very rare in children, [9, 10] but has been reported. 
In cases of melanonychia associated with systemic diseases, treatment of the primary condition may improve the nail pigmentation. Similarly, discontinuation of any offending drugs may improve melanonychia.
The prognosis of patients with subungual melanoma is poor. In a 2007 series of 106 subungual melanomas, 48 (45%) patients developed a recurrence or metastasis and, of these, 33 (69%) died from their disease during the follow-up period (median 56 mo). Patients with a melanoma deeper than 2.5 mm had a statistically worse survival rate than those with thin melanomas. Melanonychia secondary to subungual melanoma has the highest morbidity and mortality compared with other body sites, with reported 5- and 10-year survival rates of 30% and 13%, respectively. 
Patients should be instructed to follow lesions of suspected benign causes of longitudinal melanonychia for any change in color, pattern, size of the band, or new onset pain and/or ulceration, as these can be signs of subungual melanoma.
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