What is melanonychia?
Melanonychia is characterised by brown to black discolouration of a finger or toe nail. Longitudinal melanonychia describes a pigmented band and is due to melanin within the nail plate.
Longitudinal melanonychia is most often benign and arises from a pigmented melanocytic naevus (a mole) or a lentigo (a freckle). 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.
Who is at risk of melanonychia?
Melanonychia occurs more commonly in dark-skinned individuals (Fitzpatrick skin type V and VI). 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 is an increase in the production and deposition of melanin into the nail cells (onychocytes), without an increase in the number of melanocytes. There are a number of causes.
- Racial variation
- Nail biting
- Friction: foot deformity, unsuitable footwear
- Carpal tunnel syndrome
Inflammatory skin disease
Other systemic disease
- Haemosiderosis (iron overload)
- Porphyria cutanea tarda
- Human immunodeficiency virus infection (HIV)
- Systemic lupus erythematosus
- Systemic sclerosis
Iatrogenic (caused by medical treatment)
- Chemotherapy agents (especially hydroxyurea, busulfan, bleomycin, adriamycin, doxorubicin, cyclophosphamide, 5-fluorouracil)
- Anti-malarial therapy
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.
- Lentigines are seen more commonly in adults
- Melanocytic naevi are seen more commonly in children. Histologically these are differentiated by the absence or presence of melanocyte nests
- Melanoma of the nail unit most commonly affects the thumbs, index fingers and big toes
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:
- Potassium permanganate
- Silver nitrate
- Hair dye
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 and is due to pigment in the proximal nail matrix. Partial longitudinal melanonychia can be due to distal tumour under the nail plate.
- Transverse melanonychia runs across the nail. Transverse melanonychia is rare, and is often associated with iatrogenic causes such as irradiation and medications.
- Grey colour often indicates melanocytic activation, whereas a brown band is due to melanocytic hyperplasia.
- Evaluate whether the pigment band has a regular parallel pattern (benign) or an irregular pattern with loss of parallelism and irregular dots (potential melanoma).
- Nail matrix melanoma affects both finger and toenails and most frequent in index, thumb, and the large toe.
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.
Dermatoscopic examination of benign longitudinal melanonychia should reveal light to dark brown lines or bands that are parallel, regular in colour, thickness and spacing 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 nail unit melanoma if any of the following are present.
- Involvement of more than two-thirds of the nail plate
- Grey or black in addition to brown colour
- Irregular brown pigmentation
- Granular pigmentation
- And/or nail dystrophy.
Additional factors of concern include:
- New pigment, especially if there is no preceding trauma or other explanation
- Recurrent spontaneous haemorrhage in the same site
- Variation in colour and thickness of the pigment band.
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:
A: Age >50 years old
B: Brown to black, blurred borders, breadth >3mm
C: Changes of melanonychia or nail plate
D: Digit: single digit, especially thumb, big toe and index finger
E: Extension of pigment into nail fold (Hutchinson sign)
F: Family or personal history of melanoma
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.
What are the potential complications of melanonychia?
Biopsy can result in nail plate deformity.
Melanoma can cause subungual haemorrhage, unsightly nail dystrophy with fissuring and splitting of the nail plate.
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.
Nail matrix melanoma tends to have a poor prognosis.