SANTA BARBARA, Calif. - The only way to definitively rule out melanoma in a case of melanonychia striata is to perform a biopsy of the nail matrix, according to Dr. Richard K. Scher.
"The source of pigmentation is in the nail matrix, so the biopsy specimen must be taken from there," Dr. Scher said at the annual meeting of the California Society of Dermatology and Dermatologic Surgery.
Photo courtesy: Dr. Richard K. Scher and Dr. Kenneth Kulp
This 4-year-old girl presented with an 8-month history of melanonychia and a 3-month history of possible nail fold pigmentation. Dermoscopy was consistent with a nevus diagnosis but a pathology report noted the presence of atypical junctional melanocytic proliferation with increased numbers of single melanocytes. Seven pathologists and clinicians at a melanoma conference all agreed that complete excision of the nail unit be performed.
"It's of no value to do a biopsy of the nail bed. There are almost never melanocytes there, but it's amazing to me how many dermatologists are still doing nail bed biopsies," Dr. Scher, professor of dermatology at the University of North Carolina, Chapel Hill.
While some clinicians prefer to use dermoscopy to inspect cases of melanonychia striata, it is not as accurate or as reliable as microscopic examination.
"At most, dermoscopy should be an aide," he commented.
There are probably too many biopsies being performed, but "I really don't know what the alternative is," Dr. Scher noted. "We need reliable clinical criteria to determine melanoma probability, but we don't have it yet. ... We also need reliable histologic criteria to determine melanoma probability, but we don't always have that, either."
Dr. Scher estimated that 90% of melanocytic bands arise from the distal matrix, which contains more melanocytes than the proximal matrix does. Melanonychia melanoma most commonly occurs in the thumb and the big toe, followed by the index finger. "About 20% of nail melanomas are amelanotic," he said. "That's a number that keeps me awake at night."
Dr. Richard K. Scher
Two risk factors for developing melanonychia include older age and being African American. "In general, African Americans do not get melanomas to a great extent, but they do have a lot of benign melanonychia," Dr. Scher noted. "However, acral melanoma is more common in African Americans, compared with other populations."
Other causes of melanonychia include trauma to the nail, infection, and certain medications including antibiotics and chemotherapeutics agents.
Wider, darker nail lesions are more likely to be melanoma, compared with thinner, lighter lesions. "That's true, but I've seen very narrow bands that were melanomas, and I’ve seen very wide bands that were not," Dr. Scher said. "Uniformity of pigment is relatively more reliable than some of the other criteria. So if you see lack of uniformity, think of things like atypical nevi, dysplastic nevi, or melanoma."
In addition, if a patient presents with pigmented nails in four or five digits, "it's not likely that patient will have four or five melanomas," he said. "But if you see a patient particularly a dark-skinned individual who has normal ethnic melanonychia in many digits, but one of them is very different, then you have to become suspicious."
Evolution of the lesion is another worrisome sign. "If the band is changing if it’s getting wider or darker or more streaky then you have to think that you’re dealing with a melanoma."
He emphasized the importance of retrieving the nail plate during the nail biopsy procedure and sending it to the pathologist with the nail matrix specimen, "because sometimes it's hard for the pathologist to find the pigment," he explained. "If the pathologist sends you back a report that there’s no pigment here, the implication is that you missed the lesion. But if you have nail plate there, which has pigment in it, you didn’t miss the lesion. The pathologist did."
Some recent articles in the medical literature suggest that if cases of melanonychia striata in childhood are stable and not atypical in appearance, observation may be an option. However, Dr. Scher urged caution with this approach, saying that "every case must be evaluated individually."
To illustrate his point he discussed the case of a 4-year-old healthy girl who presented to his office with an 8-month history of melanonychia and a 3-month history of possible nail fold pigmentation (see image above). Family history revealed a maternal second cousin with acral melanoma. Dermoscopy was consistent with a nevus diagnosis.
The parents agreed to proceed with a nail biopsy, Dr. Scher said. The pathology report read (in part): "atypical junctional melanocytic proliferation with increased numbers of single melanocytes ...; although this lesion is most probably a nevus, it is recommended that it be completely but conservatively excised."