DISCUSSION
Fortunately, this patient’s problem is benign and likely to remain so. Termed longitudinal (or linear) melanonychia (LM), these changes are seen in nearly all African-Americans older than 50 (although it is not uncommon for the condition to develop in the third decade of life). Other populations with dark skin are also at risk for LM, albeit at far lower rates. In white populations, the incidence is around 0.5% to 1%.
LM is caused by activation and proliferation of melanocytes in the nail matrix; they are focally incorporated into the nail plate as onychocytes that grow out with the nail. Typically 1 to 3 mm in uniform width, the streaks of LM range from tan to dark brown and can be solitary or multiple in a given nail.
As mentioned, LM is entirely benign, with almost no potential for malignant transformation. However, two notes of caution are in order: First, although African-American persons generally have very low risk for melanoma, the malignancy tends to manifest in this population in areas with the least pigment (eg, palms, soles, oral cavities, nail beds—unusual locations for most other racial groups). Second, the prognosis for these types of melanomas is poor; most patients and providers are unaware of them until an advanced stage that typically includes metastasis.
Therefore, in patients with skin of color, new or changing lesions in the nail bed must be evaluated by a knowledgeable dermatology provider, who may choose to biopsy the proximal aspect of the lesion to rule out cancer. Of course, any such lesion in a white person needs to be monitored carefully as well, since linear melanonychia is relatively uncommon in this group. Changes in the width, color, or border should cause concern, as should extension of the darker color onto the adjacent cuticle.
The differential for linear discoloration in nails or nail beds includes foreign body, warts, benign tumors (eg, nevi), glomus tumors (which are usually painful), and of course, fungal, mold, or yeast infections.
TAKE-HOME LEARNING POINTS
- Longitudinal (or linear) melanonychia (LM) is quite common in African-Americans, approaching a prevalence of 100% in those older than 50.
- Having multiple LMs in more than one finger is common in this population.
- However, a new or changing subungual lesion bears close monitoring, or even biopsy, by an experienced dermatology provider.
- Although African-Americans rarely develop melanoma, when they do, it’s often in the least pigmented areas (eg, palms, soles, mouth, and nails).
- The prognosis for proven melanoma in African-American patients is poor, making close monitoring a necessity.