An African American medical student presented with a new dark band on her index finger for 1 year (Figure 191-1). The dark color and the lack of melanonychia in other fingers made this concerning. A biopsy of the nail matrix was performed and the result showed a benign nevus.
Longitudinal melanonychia—a single dark band of nail pigment appearing in the matrix region and extended to the tip of the nail. This is concerning for melanoma. The widening of the band in the proximal nail shows that the melanocytic lesion in the matrix is growing. This young woman had a biopsy that showed a benign nevus. (Courtesy of Richard P. Usatine, MD.)
Atypical pigmentation of the nail plate may result from many nonmalignant causes, such as longitudinal melanonychia (LM), inflammatory changes, benign melanocytic hyperplasia, nevi, drugs, and endocrine disorders. It may also result from development of subungual melanoma. The challenge for the clinician is separating the malignant from the nonmalignant sources.
LM is the most common cause and it represents a longitudinal pigmented band in the nail plate (Figures 191-1 and 191-2). It may involve of 1 or several digits, vary in color from light brown to black, vary in width (most range from 2 to 4 mm), and have sharp or blurred borders.
Close up of longitudinal melanonychia in a single finger. Note the color band is translucent. (Courtesy of E.J. Mayeaux, Jr., MD.)
- LM is more common in more darkly pigmented persons. It occurs in 77% of African Americans older than age 20 years and in almost 100% of those older than age 50 years.1,2 It also occurs in 10% to 20% of persons of Japanese descent. LM is common in Hispanic and other dark-skinned groups. LM is unusual in whites, occurring in only approximately 1% of the population.1
- Melanoma is the seventh most common cause of cancer in patients in the United States. Subungual melanoma is a relatively rare tumor with reported incidences between 0.7% and 3.5% of all melanoma cases in the general population.3
- LM originates in the nail matrix and results from increased deposition of melanin within the nail plate. This deposition may result from greater melanin synthesis or from an increase in the total number of melanocytes. Pigment clinically localized within the dorsal half of the nail plate indicates a proximal matrix origin, and pigment localized within the ventral nail plate indicates a distal matrix origin. Look at the distal edge of the nail in a cross-sectional view to see whether the pigment is dorsal or ventral (a dermatoscope may help).
- LM may also be caused by chronic trauma, especially in the great ...