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Patient Story
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A four-year-old boy presents with a newly pigmented line on his right thumb for 6 months. He already had one pigmented line on that same thumb since age one. His parents want to know if this pigmentation is dangerous. The child is otherwise healthy. On examination there are two longitudinal pigmented lines easily visible on the right thumbnail (Figure 161-1A). The boy is referred to a pediatric dermatologist. Examination with a dermatoscope shows the details of the many lines and confirms his concern for melanoma (Figure 161-1B). The concerns are expressed to the parents and the child is set up for a nail matrix biopsy with sedation. The differential diagnosis also includes a congenital melanocytic nevus that is growing.

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FIGURE 161-1

A. Longitudinal melanonychia on the right thumb of four-year-old boy with two prominent pigmented lines. One of the two lines is new. B. Dermoscopic examination of the nail shows the complex pigment pattern with many lines and melanocytic dots. This is suspicious for melanoma but could also be a congenital nevus that is growing. (Used with permission from Richard P. Usatine, MD.)

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Introduction
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Atypical pigmentation of the nail plate may result from many nonmalignant causes, such as fungal infections, benign melanocytic hyperplasia, nevi and medications. It may also result from development of subungual melanoma. The challenge for the clinician is separating the malignant from the nonmalignant sources.

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Longitudinal melanonychia (LM) is a clinically descriptive term that represents a longitudinal pigmented band in the nail plate (Figures 161-1 to 161-3). It may be caused by any of the conditions listed above but is often due to normal ethnic hyperpigmentation (Figure 161-3). It may involve 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.

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FIGURE 161-2

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. A biopsy showed this to be a benign nevus. (Used with permission from Richard P. Usatine, MD.)

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FIGURE 161-3

During a routine sports physical for a black adolescent boy, a translucent darker stipe was discovered in the nail plate of one of his thumbs. The line was faint and uniform in width. He was reassured it was most likely benign but the lesion was noted in his medical record so it could be rechecked in the future and he was instructed to return to clinic if ...

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