By Gary M. White, MD
Any pigmented streak of the nail must be carefully evaluated.
Longitudinal melanonychia (LM) may be defined as a longitudinal pigmented streak of the nail caused by melanin in the nail plate.
In one study of 40 children with LM [JAAD 1999;41;17], the cause in 19 was a nevus, in 12 a lentigo, and in 9 functional LM. See pigmented streaks of the nail.
A longitudinal brown streak of the nail is seen. The origin is the base of the nail (proximal nail fold) and the width of the pigmentation is uniform. (Rarely, with rapidly growing lesions, the width may be progressively wider proximally, giving what is called the "triangular sign".) This reliable and distinct pattern helps distinguish LM from subungual hematoma or darkening from onychomycosis. Those patterns of darkening are random, haphazard and often not contiguous with the proximal nail fold.
Pigmentation of the proximal or lateral nail folds, and hyponychium (Hutchinson's sign) can be a sign of melanoma but is not uncommon in benign LM, especially in children [JAAD 2016;75;535]. Complete darkening of the nail is called total melanonychia.
Nail clipping with special stains can be used to confirm that the pigment is melanin.
In one case, a 52-year-old woman presented with multiple longitudinal pigmented bands of melanonychia of the finger and toes. Other nails had a diffuse gray-black pigmentation. The cause turned out to be an ACTH-producing pituitary adenoma [CED 2013;38;689].
See here for a differential.
Biopsy is in order for any new, solitary lesion with suspicious features, especially in an adult. Worrisome features include width greater than 6 mm, multiple colors, or dark black. Pigmentation of the proximal nail fold--Hutchinson's sign--is particularly worrisome.
In one study of pigmented nail bands [JEADV 2017 31;732], key features that suggested melanoma included:
Granular pigmentation, a newly defined dermoscopic criterion, was found in 40% of melanomas and only in 3.51% of benign lesions.
For children--because childhood subungual melanoma is extremely rare--photodocumentation to help monitor the lesion is reasonable, unless there is rapid growth, many fine, dark longitudinal lines within a wider, lightly-pigmented streak or significant variation in pigmentation. In these or other high-risk situations, biopsy should be performed. One paper failed to find a single case report of melanonychia striata that resulted from an invasive melanoma in a child [JAAD 2015;72;773]--although melanoma in situ causing LM has been reported.
One approach is to completely remove all the nail covering the streak and send that for pathology as invariably some of the nail bed will be stuck to the plate. Then do an ellipse longitudinally including the proximal start of the streak. Close as best as you can with a few stitches. Schedule 45 minutes; don't rush.
Another option is outlined in this article, Derm101, with good pictures for biopsy. In summary:
Treatment depends upon the cause.
Multiple streaks in multiple nails tends to be benign.
A rare case of a nevus of the nail bed causing LM. Biopsy was read out as nevus with atypia.
This new, single, very dark black streak was on biopsy a melanoma in situ.
Here, a punch biopsy of the above melanoma in situ of the nail apparatus is shown.
A pigmented streak not caused by a nail matrix nevus, but instead by onychomycosis.
Heavily pigmented subungual lentigo in a child. JAAD 2015;72;773–779
Subungual congenital nevus in a child. JAAD 2015;72;773–77
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