15: Conditions that frequently affect a single nail

Abstract:

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:

melanonychia

erythronychia

habit-tic deformity

acrolentiginous melanoma

Darier disease

subungual hematoma

Melanonychia

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.

A stands for age because the incidence of nail melanoma is higher in people in their 50s, 60s, or 70s.
B signifies borders and breadth. Well-defined, sharp borders and a diameter of less than 3 mm are good signs.
C is for change. It is important to assess whether the lesion has changed over time or been present without change for many years.
D is for digit. The thumb and the great toe are the most common sites of nail unit melanoma.
E is for extension onto the skin around the nail plate. This is called the Hutchinson sign, and it is extremely concerning for melanoma.
F is for family history. As with cutaneous melanoma, a family history of melanoma or dysplastic nevus syndrome is a risk factor for developing nail unit melanoma.

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.

Fig. 15.1 **Acrolentiginous Melanoma.** *Source:* (From the University of Connecticut Department of Dermatology Grand Rounds Collection.)

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

• Trauma from nail biting (onychophagia), nail picking (onychotillomania), ill-fitting shoes, or occupational/recreational hazards can be a cause.
• Physiologic causes are considered a normal variant in people with darkly pigmented skin and usually involve multiple nails.
• There are also numerous common medications that can cause melanocyte activation, including but not limited to the cyclines, psoralens, zidovudine, phenytoin, fluoride, chemotherapeutics, antimalarials, ketoconazole, and ibuprofen.
• Another cause is postinflammatory hyperpigmentation after inflammation of the nail folds from eczema, contact dermatitis, psoriasis, or connective tissue disease.
• Systemic diseases, such as Addison disease, Cushing syndrome, acquired immunodeficiency syndrome, hyperthyroidism, porphyria, graft versus host disease, and malnutrition, can cause melanocyte activation.

Differential diagnosis

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

• Subungual hematomas usually present as a purple or black macule under the nail plate. It migrates distally with the growth of the nail plate.
• Infectious causes are usually fungal organisms or bacteria that produce pigment. There is a growing list of over 20 fungal species that produce subungual debris. If LM is present, it typically is wider distally and tapers proximally. Pseudomonas aeruginosa causes green nail syndrome. It produces pyocyanin and pyoverdine, blue and green pigments that can cause the nail to appear from yellow to green to black in color. It is most commonly seen in people who keep their hands submerged in water for extended periods (e.g., dishwashers, housekeepers, healthcare workers).
• Exogenous pigment found on the nail could include tobacco, dirt, tar, paint, or henna. Often, a supporting history can be elicited and these can be wiped off with an alcohol swab.