Conditions that frequently affect a single nail

15: Conditions that frequently affect a single nail


Nail conditions can be divided into those that affect a single nail and those that affect multiple nails. The nail findings in this chapter are those that predominantly affect single nails. These findings include melanonychia, erythronychia, and habit-tic deformity. This chapter reviews these findings and discusses the different clinical contexts in which they appear.

Key terms:



habit-tic deformity

acrolentiginous melanoma

Darier disease

subungual hematoma


Brett Sloan

Clinical features

Melanonychia is dark brown or black pigmentation on the nail plate. There are numerous causes, the most concerning being melanoma of the nail unit.

Melanonychia can present as diffuse darkening of the nail plate or, more commonly, as a longitudinal line, which is known as “longitudinal melanonychia” (LM). Melanonychia is a misnomer because the nail pigmentation is not necessarily the result of melanin. The discoloration can be because of melanin, exogenous pigment, medications, fungi, or bacteria.

Evaluation of melanonychia is similar to the evaluation of any pigmented lesion. A commonly used acronym is ABCDEF.

Differential diagnosis

The differential diagnosis of true melanonychia includes two broad categories: pigment because of an increased numbers of melanocytes (nail simple lentigines, nail melanocytic nevi, and melanoma) and pigment because of activation of normally quiescent melanocytes (melanocyte activation). The vast majority of melanocytes are found in the nail matrix and do not normally produce melanin. When activated, they produce pigment in the nail plate.

Causes of melanocyte hyperplasia include nail matrix nevi and melanomas (Fig. 15.1). Distinguishing between the two can be difficult and if there are any concerning features, a nail matrix biopsy is warranted. Nail matrix nevi are more common in children and are usually less than 3 mm in size, evenly pigmented, and do not involve any of the nail folds. Subungual melanomas are rare, accounting for between 0.7% to 3.5% of melanomas worldwide. It is the most common form of melanoma in Black people, Asians, and Hispanics.

There are numerous causes of melanocyte activation, all of which can produce melanonychia.

Differential diagnosis

The differential diagnosis for melanonychia includes subungual hematomas, infectious etiologies, and exogenous pigments.

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Jul 22, 2021 | Posted by in Dermatology | Comments Off on Conditions that frequently affect a single nail

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