What is melanonychia?
Melanonychia is brown or black discolouration of a nail. It may be diffuse or take the form of a longitudinal band.
Benign longitudinal melanonychia
Who is at risk of melanonychia?
Melanonychia can present in individuals of all ages, including children, and affects both sexes equally. It is more prevalent in people with skin of colour, especially Fitzpatrick skin type V and VI.
- Nearly all Afro-Caribbean people will develop black-brown pigmentation of the nails by the age of 50.
- Melanonychia affects up to 20% of Japanese people.
- White-skinned people are less commonly affected.
Melanonychia can also be associated with genetic disorders, injury, medications, nutritional deficiency, endocrine disease, connective tissue disease, inflammatory skin disease, a local tumour, or nail infection.
What causes melanonychia?
The nail plate is a hard, translucent structure made of keratin. It is not normally pigmented. Melanocytes typically lie dormant in the proximal nail matrix where the nail originates. Melanin is deposited into the growing nail when melanocytes are activated, resulting in a pigmented band — this is longitudinal melanonychia.
The deposition of melanin in the nail plate can result from 2 processes:
- Melanocytic hyperplasia
- Melanocytic activation.
Melanocytic hyperplasia refers to an increased number of melanocytes within the nail matrix. This can represent a benign or malignant process.
Melanocytic naevi arise more commonly in children. Histologically there are nests of naevus cells.
Lentigines are seen more commonly in adults. Nests are absent.
Melanoma of the nail unit most commonly affects the thumbs, index fingers and big toes.
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. The causes of melanocytic activation are listed in the table below.
Nail biting, chewing, breaking and picking
Friction due to foot deformity or footwear
|Inflammatory skin disease|
|Nutritional deficiency||Vitamin B12 or folate deficiency|
|Another systemic disease||
Pathogens can cause an irregular melanonychia as they stimulate inflammation activating melanocytes. For example:
- Gram-negative bacteria (Proteus mirabilis, Klebsiella, Pseudomonas)
- Onychomycosis due to a dermatophyte (Trichophyton rubrum) or yeast (Candida albicans).
External agents can stain the nails.
- Hair dye
What are the possible complications of melanonychia?
Complications depend on the cause of melanonychia.
- Subungual melanoma can result in metastasis and ultimately, the death of the patient. The prognosis for nail matrix melanoma is as a rule worse than melanoma in other sites.
- Trauma, infection, and inflammatory disease can result in fissuring and splitting of the nails and unsightly or painful nail dystrophy.
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 and may require amputation 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 its withdrawal.