What is nail melanoma?
Melanoma of the nail originates (almost always) at the base of the nail in an area called the nail matrix. The keratinocytes of the matrix produce keratin, and so push the nail (made from keratin) forward from the nail base. The slightly pale curved area found at the base of the nail (like a quarter moon) is called the lanula and melanomas are situated under the lanula. You won’t actually see the melanoma under the lanula. Instead, the melanoma is seen as as pigmented brown to black parallel lines along the length of the nail from the base towards the tip. Melanoma of the nail is one of the causes of Longitudinal melanonychia – lines of parallel pigment found along the length of the nail.
What are the other causes of Longitudinal nail pigmentation?
There’s a fairly long list of possible causes of longitudinal nail pigmentation (longitudinal melanonychia):
- Ethnic hyperpigmentation
- Malignant Melanoma (of the nail matrix)
- A benign mole (nevus)
- drug induced (eg. amiodarone, tetracycline)
- inflammatory causes
- fungal nail infection
- Rare causes eg. Addison’s disease, radiotherapy
The most common causes are benign moles, trauma and ethnicity.
What makes nail melanoma more likely?
- Nail melanoma is very rare in prepubertal children and this minimises the need for nail biopsy in younger children
- Nail Melanoma, though rare, is one of the commoner types of melanomas in people with darker skin (eg. Asian or African skin types)
- Melanoma of the nail is most common in adults from the age of 40 to 70, although they clearly do occur in younger and older adults
- A family history of melanoma
- Nail melanoma most commonly affects the big toe or thumb nails.
What are the signs of a nail melanoma?
The pigmented area is brown to black. The pigmentation of melanoma may spread onto the cuticle (that strip of hardened skin at the base of the nail) – this is called Hutchinsons Sign but may be absent
How is Nail Melanoma Diagnosed?
Concern regarding the nail should lead to a detailed look with dermatoscopy. The ultimate diagnosis can only be made by histology and this involves a biopsy of the nail matrix. Therefore, dermatoscopy is particularly useful to help avoid nail matrix biopsies.
How is dermatoscopy of the nail performed?
A gel is used to interface between the dermatoscope and the nail. The dermatoscope is switched onto the “non polarized” light setting and the nail examined. Under a dermatoscope, the colour of the pigment can be much better distinguished. In simple terms, melanomas or moles are brown or black whereas the other causes are more likely to be grey in colour. Dermatoscopy may also show the lines as being made up of several different colours and separated by varying distances. The lines of a benign nail mole are usually of the same colour and the lines a similar distance apart. Hutchinson’s sign may also be revealed only with dermatoscopy.
It can be very difficult, even with dermatoscopy, to exclude melanoma from a traumatised nail. In these situations, the trauma creates brown lines on the nail (both from bleeding and inflammation that occurs during the healing process) and may look concerning enough to indicate the need for a biopsy.