What can cause a dark discolouration of the nail?
The main reasons a nail can become dark are: blood under the nail (a haematoma); an increase in pigment (melanonychia); and pigmentation of the nail from another cause.
Blood under the nail
A haematoma (blood clot) under the nail is very common and develops from knocks (or other minor trauma) and, for example, walking in shoes that are too tight. The clotted blood usually turns black, but is often somewhat purple at the edges, which the doctor can often see better with a dermatoscope (a special magnifier). A haematoma grows out with the nail and so disappears on its own. The reason to see a doctor anyway is that it is so dark it can resemble a melanoma under the nail; based on the shape of the dark spot and the dermatoscope image, the doctor can usually make the diagnosis of a blood clot.
Increase in pigment (melanonychia)
Pigment-forming cells (melanocytes) lie throughout the skin, including in the nail matrix, the tissue where the nail is formed. In the nail matrix these melanocytes are normally dormant, so a pink colour shines through the transparent nail plate.
From overactive melanocytes. When melanocytes in the nail matrix “awaken” and start producing melanin, this pigment is taken up into the forming nail plate and a dark stripe appears on the nail (melanonychia striata), while the number of melanocytes does not change. Possible causes of this activation include pregnancy; inflammatory nail conditions (such as psoriasis or lichen planus); infections (such as fungal infections); trauma; nail biting; warts; medicines (especially chemotherapy and antimalarials); carpal tunnel syndrome; an overactive thyroid; and Cushing’s syndrome.
From an increase in the number of melanocytes. Instead of overactivity, an increase in the number of melanocytes can also cause (partial) discolouration of the nail plate. This increase can be benign or malignant: a mole (benign cluster of pigment cells), or a melanoma of the nail. A melanoma is a very aggressive skin cancer arising from pigment cells — the sooner the diagnosis is made, the better the outlook — so always see a doctor for pigment changes of the nail to get the right diagnosis.
Other causes of pigmentation
Sometimes the discolouration comes from factors outside the body (exogenous pigmentation), such as dirt, yellow-brown discolouration in smokers, and discolouration from chemicals such as silver nitrate.
Establishing the cause
Through a careful history and examination with the dermatoscope, the dermatologist can usually already establish the cause. In case of strong doubt, further investigation is possible, one option being a biopsy (a small piece of tissue) of the nail matrix for the pathologist to examine.
When to see a doctor
See a doctor for any dark stripe or spot of the nail that is new, widening, irregular, involves the surrounding skin, or that you cannot clearly link to a recent knock — to rule out a melanoma.