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.

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.

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.

Acrolentiginous melanoma = acral lentiginous melanoma, subungual melanoma is one type of acral lentiginous melanoma involving the nail unit.

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  LM is more common in more darkly pigmented persons. It occurs in 77 percent of African Americans older than age 20 years and in almost 100 percent of those older than age 50 years.^1,2 It also occurs in 10 to 20 percent 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 percent of the population.¹

  
  
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  Melanoma is the seventh most common cause of cancer in patients in the US. Subungual melanoma is a relatively rare tumor with reported incidences between 0.7 percent and 3.5 percent of all melanoma cases in the general population.³

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  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 (Figure 161-4). 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).

  
  
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  LM may also be caused by chronic trauma, especially in the great toes.

  
  
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  Inflammatory changes accompanying skin diseases located in the nail unit, such as psoriasis, lichen planus, amyloidosis, and localized scleroderma, rarely may result in LM.

  
  
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  Benign melanocytic hyperplasia (lentigo) is observed in 30 percent of the pediatric cases of single-biopsied LM.⁴

  
  
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  Nevi represent almost 50 percent of cases in children. A brown-black coloration is observed in two thirds of the cases and periungual pigmentation (benign pseudo-Hutchinson sign) in 1/3.

  
  
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  Certain drugs may also cause LM, especially chemotherapeutic agents, and antimalarial drugs (mepacrine, amodiaquine, and chloroquine).

  
  
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  Endocrine disorders, such as Addison disease, Cushing syndrome, hyperthyroidism, and acromegaly, can be responsible for LM.

  
  
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  The diagnosis of subungual melanoma must always be considered in patients with LM (Figures 161-5 to 161-7). Separating benign from malignant lesions is often difficult. Both arise most often in the thumb or index fingers, and both are more common in dark-skinned persons.⁵ A biopsy should be performed if the cause of LM is suspicious for melanoma. Table 161-1 lists diagnostic clues for subungual melanomas. Many subungual melanomas have a history of trauma preceding the diagnosis so it is important to not be fooled by this history (Figure 161-7).

  
  
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  Hutchinson sign is the extension of pigmentation to the skin adjacent to the nail plate involving the nail folds or the fingertip. It is an important indicator for nail melanoma (Figures 161-5 to 161-6).⁶

  
  
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  Pseudo-Hutchinson sign is the presence of dark pigment around the proximal nail fold secondary to benign conditions such as racial melanosis and not melanoma (Figure 161-8). Another cause of pseudo-Hutchinson sign is a translucent cuticle below which the pigment of LM is visible. Trauma and drug-induced pigmentation can also produce a pseudo-Hutchinson sign.

  
  
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  Subungual melanoma arises on the hand in 45 to 60 percent of cases, and most of those occur in the thumb (Figures 161-5 to 161-6).⁴ On the foot, subungual melanoma usually occurs in the great toe.⁵ The median age at which subungual melanoma is usually diagnosed is in the sixth and seventh decades. It appears with equal frequency in males and females.⁵