AMELIA ISLAND, FLA. — Every case of melanonychia striata must be evaluated individually, Dr. Richard K. Scher said at a symposium sponsored by the Dermatology Foundation.
Longitudinal bands of pigmentation in nails are among the trickiest of dermatologic phenomena to diagnose. Unlike the approach to cutaneous lesions that may or may not be melanoma, there are no reliable clinical or histologic criteria to help the clinician determine the melanoma probability of any given pigmented nail band. Some general prognostic information is available, but exceptions come along far too often for a physician to feel secure in a diagnosis.
“You can't trust the nail. It just doesn't follow the rules you try to use when evaluating skin lesions,” said Dr. Scher, professor of clinical dermatology at Columbia University, New York.
Melanonychia striata affect about 1% of whites and 11% of Japanese individuals. One study found that among blacks, the prevalence rises dramatically with age, from 2.5% of children aged 0–3 years to 77% of adults older than 20 years, to 96% of those older than 50 years. But the risk is very pigment dependent, with darker-skinned blacks having higher rates than those with lighter complexions.
Melanonychia and subungual melanoma are most common in the thumb, great toe, and index finger, so it's particularly important to examine all the finger and toe nails of patients who have pigmented bands in any of those three areas. But, keep in mind that 20% of subungual melanoma are amelanotic, Dr. Scher warned.
Clinical features of the pigmentation can provide clues, but not reliable answers. In general, the lighter and more narrow the band, the less likely it is to be melanoma. However, “I've seen 1- to 2-mm pigmented bands which were melanoma in situ, light bands that were melanoma, and dark bands that were not melanoma.” And of course, a fungal infection also can present as a dark black nail band.
Hyperpigmentation that extends into the proximal nail fold, known as “Hutchinson's sign,” is melanoma until proved otherwise. Sometimes it is something other than melanoma, in which case it's called “pseudo-Hutchinson's sign.”
Uniformity of color is a good sign, whereas bands that are darker in some areas than others are more likely to be melanoma. Pigmentation that covers the entire nail also increases the melanoma probability. And, as with cutaneous lesions, a nail band that changes in color or size over time requires urgent evaluation. Involvement of multiple digits makes melanoma less likely, but any one that looks distinctly different from the others “should be regarded with some degree of suspicion,” Dr. Scher said.
Because the nail matrix is the source of pigmentation (about 90% of melanocytic bands arise from the distal matrix and 10% from the proximal matrix) biopsies must be taken from the nail matrix and not the nail bed. A recent article has described the use of dermoscopy of the free edge of the nail to determine the level of nail plate pigmentation and the location of its probable origin in the proximal or distal matrix (J. Am. Acad. Dermatol. 2006;55:512–3). But, there are no standardized criteria for the use of dermoscopy in melanonychia, and the procedure requires training and expertise, but “dermoscopy can help distinguish [subungual hematoma] from melanoma.”
The role of trauma in subungual melan- oma is controversial. Some people believe it is a contributing factor, others say evidence does not support that idea. About 25% of subungual melanomas have a history of trauma to the nail. This can prove to be a diagnostic nightmare, given that even the confirmed presence of a subungual hematoma does not exclude the possibility of a coexisting cancerous lesion.
The probability of melanonychia striata in children is far lower than it is in adults, comprising just 1%–4% of all melanomas in individuals less than 20 years of age. The new thinking is that, because most melanonychia striata in children are nevi and not melanoma, observation during childhood is an option as long as the lesions are stable and not atypical in appearance. In general it's still a good idea to biopsy any lesion you're uncomfortable with. “When in doubt, biopsy,” Dr. Scher said.
From left to right: malignant melanoma that arose from longitudinal melanonychia; longitudinal melanonychia that proved to be melanoma in situ; biopsy from melanoma in situ
From left to right: longitudinal melanonychia caused by a benign melanotic macule/lentigo; another case resulting from a fungal melanonychia; and another resulting from a nevus. Photos courtesy Dr. Richard K. Scher
