Conditions that frequently affect a single nail

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.

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.

Work-up

• A thorough history should be completed with a focus on information about any potential trauma, any history of skin disease, and a full medication history (including over-the-counter medications and supplements).

• A review of systems should focus on potential systemic diseases that could cause melanonychia.

• A full-body skin examination, including the genitalia and oral mucosa, should be performed to look for other areas of hyperpigmentation or clues to systemic diseases that can cause nail hyperpigmentation. Examination of all the nails is necessary. If the patient is wearing nail polish, this should be removed.

• If there is thickening of the nail plate or subungual debris, a nail clipping should be sent to the microbiology lab for culture or to the pathology lab for periodic acid–Schiff staining.

• If any of the features previously outlined as concerning for melanoma are present, the patient should be referred to a dermatologist experienced in performing nail matrix biopsies.

Initial steps in management

Management of melanonychia should focus on ruling out malignancy.

The patient should immediately be referred to a dermatologist for evaluation for biopsy in a few instances:

• The pigmented band is new and greater than 3 mm wide.

• The pigmented band is new and changing in color or symmetry.

• There is a personal history of melanoma.

• There is any pigment on the skin around the nail (nail folds).

For solitary pigmented bands less than 3 mm in width and stable, it would be prudent to take a clinical image for the chart and follow up in 3 to 6 months to assess for interval change. Encourage patients to take a photograph of the affected nail with their personal cell phone for reference. This way, they can assess for any changes that may occur before the scheduled follow-up visit.

If there is any sign of skin disease on the finger that could be causing the inflammation and subsequent melanocyte activation, it should be treated with a mid-potency topical steroid, such as triamcinolone 0.1% ointment applied twice daily until resolution.

If there is thickening of the nail plate or subungual debris that has tested positive for nail fungus, treatment with an oral antifungal is warranted. Oral terbinafine 250 mg daily for 6 weeks for fingernails and 12 weeks for toenails is generally effective.

There is no medical reason to treat patients with multiple pigmented bands because there are no treatments available that will lighten the bands. The bands will fade and potentially disappear if they are secondary to melanocyte activation and the source of the activation is treated.

Warning signs/common pitfalls

The ABCDEs of pigmented bands should catch the warning signs of melanoma. Patient history is essential when it comes to changes in size or color of the band.

It also should be noted that melanomas can bleed and present as subungual hematomas. Thus all subungual hematomas should be evaluated and followed up on appropriately until resolution.

Likewise, onychomycosis is extremely common in the general population and can occur in conjunction with nail melanoma. If the onychomycosis is successfully treated and the pigment remains, the patient should be referred for a biopsy.