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Melanonychia

What is melanonychia?

Melanonychia is a common condition characterised by brown to black discolouration of the nail. It most frequently arises from benign conditions, such as a pigmented melanocytic naevus (a mole). However, a band of brown pigment in a single nail must be examined and investigated with caution, as melanonychia may be the presenting sign of melanoma of the nail unit.

Benign longitudinal melanonychia

See more images of benign melanonychia ...

Who is at risk of melanonychia?

Melanonychia occurs more commonly in dark-skinned individuals. Studies suggest that nearly all Afro-Caribbeans will develop black-brown pigmentation of the nails by the age of 50. It may also be present in up to 20% of Japanese patients. White-skinned people are less commonly affected.

Melanonychia can present in individuals of all ages, including children, and affects both genders equally.

What causes melanonychia?

The nail (nail plate) is a hard and translucent structure that is not normally pigmented and is made of the skin protein, keratin.

Pigmentation results from the deposition of melanin by the pigment cells, or melanocytes. These typically lie dormant in the nail matrix where the nail originates. As the melanin is continuously deposited in the keratinocytic cells of the growing nail, a longitudinal streak arises. This is termed longitudinal melanonychia.

This deposition of melanin can result from 2 broad processes; melanocytic activation or melanocytic hyperplasia.

Melanocytic activation

Melanocytic activation is an increase in the production and deposition of melanin into the nail cells (onychocytes), without an increase in the number of melanocytes. This benign "activation" has been ascribed to a number of causes.

Melanocytic hyperplasia

Melanocytic hyperplasia refers to an increased number of the pigment cells (melanocytes) within the nail matrix. This can represent either a benign or a malignant process.

Pigmentation due to external sources

Pathogens can also cause melanonychia.

• Some gram-negative bacteria (Proteus mirabilis, Klebsiella, Pseudomonas) and dermatophytes (Trichophyton rubrum) can produce melanin that stains the nail.
• Other pathogens involved in infections of the nail (onychomycosis, paronychia) can stimulate inflammation that activates the melanocytes.

The pigment may be exogenous and deposited on the top of the nail plate. Possible sources include:

• Tobacco
• Potassium permanganate
• Silver nitrate
• Dirt

Stained nails due to pseudomonas infection

What are the clinical features of melanonychia?

Melanonychia typically manifests as a single brown-black pigmented streak. It may affect a single nail, or be observed in multiple nails.

• Longitudinal melanonychia extends from the nail fold (cuticle) to the free edge of the nail.
• Complete melanonychia involves the entire nail plate.
• Transverse melanonychia runs across the nail. Transverse melanonychia is rare, and is often associated with iatrogenic causes such as irradiation and medications.

How is the diagnosis made?

Physical examination must involve inspection of all 20 nails, nail folds and mucosal membranes to detect associated signs that may suggest the underlying cause. In particular, pigmentation of mucosal membranes is associated with Laugier-Hunziker syndrome and Addison disease.

Pathogen-induced pigmentation should be suspected when pigmentation appears at the nail edge or can be removed by scraping. Exogenous pigmentation grows-out with the nail plate, or may similarly be scraped off. Ascorbic acid at 10% concentration may be used to remove potassium permanganate staining.

Dermoscopy

Dermatoscopic examination of benign longitudinal melanonychia should reveal light to dark brown lines or bands that are parallel, regular in colour, and regular in width as the band extends from the nail fold to the free edge. The borders should be clearly defined and usually of a width of less than 3 mm.

Dermatoscopic examination from the free edge of the nail may help reveal the origin of the implicated melanocytes. The proximal nail matrix produces the superficial (dorsal) nail plate, whereas the distal nail matrix and nail bed produces the deep (ventral) nail plate.

An International Dermoscopy Society study concluded that acquired longitudinal melanonychias in adults should give rise to suspicion of melanoma if any of the following are present.

• Involvement of more than two-thirds of the nail plate 
• Grey or black colour
• Irregular brown pigmentation
• Granular pigmentation
• And/or nail dystrophy.

Nail biopsy

Definitive exclusion of melanoma of the nail unit is obtained with a nail matrix biopsy. There should be a low threshold for biopsy especially in elderly patients where melanonychia has appeared in a single digit.

What are the differential diagnoses of melanonychia?

The most important differential to exclude is melanoma of the nail unit. Features that should raise suspicion are given by the ABCDEF mnemonic:

Another common differential is subungual haematoma (bleeding under the nail). This is typically preceded by trauma to the affected digit, although this may be unnoticed trauma. It can often be due to tight shoes. On dermatoscopy, the pigment is red to purple in colour and presents as dots or globules with a well-circumscribed edge closest to the cuticle. Within a few weeks, the purple patch may begin moving outwards towards the free edge of the nail.

Subungual haematoma

What is the management of melanonychia?

Where melanonychia is attributed to a benign cause, no further treatment is necessary.

The management of melanoma of the nail unit requires complete excision of the tumour, which may require amputation of part of the digit.

What is the outlook for patients with melanonychia?

Melanonychia tends to persist, except when it is related to medication – in which case it fades following withdrawal of the medication.

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