A black splint or splinter that appears under the fingernail or toenail is most likely a splinter or fingernail haemorrhage. It is a thin, vertical line that is formed by blood beneath the nails and with time it turns from reddish-brown to black. In some cases, splinter haemorrhages can be indicative of a systemic illness. If they appear without obvious physical trauma and are combined with other symptoms, you need to see a doctor.
The blood vessels of the nail bed are arranged in parallel ridges that run vertically. When blood vessels in the ridges are broken, blood fills in the space between the ridges. The splinter haemorrhage runs in the same direction that the nail is growing and it gets its name because it looks as if a splinter is lodged under the nail. The bleeding is often the result of extremely small clots that damage the tiny blood vessels beneath the nails. Splinter haemorrhages can be important in diagnosing a variety of ailments.
Splinter haemorrhages can be caused by physical trauma to the nail, much like bruising. They can also be caused by heart disease known as endocarditis. Endocarditis is an infection of the valves of the heart and the splinter haemorrhages can be the result of damage to the blood vessels caused by swelling. When splinter haemorrhages are caused by heart disease, other symptoms of the disease are most likely to be present and it's likely you'll already be under the care of a doctor for the underlying illness. The other symptoms of endocarditis include shortness of breath, fever, general feeling of ill health and sore muscles. Other ailments that produce splinter haemorrhages include scurvy, cirrhosis and vasculitis. If all the nails are affected at the same time, trichinosis or round worm infection may be responsible.
The location of a splinter haemorrhage can give you clues as to the cause and the underlying pathology. If it is located in the middle or distal third of the fingernail, which is the part furthest away from the nail bed, then it is most likely caused by physical trauma. However, splinter haemorrhages that occur in the lower part of the nail bed are more likely to be caused by systemic illnesses like endocarditis, or connective tissue disorders. Trauma-related splinter haemorrhages are more common in the elderly while those that occur closer to the bottom of the nail are most often found in young adults with systemic illnesses.
If you have a splinter haemorrhage, you may be concerned that it is a subungual melanoma. This is a cancerous growth beneath the nail that may be the same dark colour as a splinter haemorrhage. Subungual melanomas are far less common than splinter haemorrhages and generally start at the nail bed whereas splinter haemorrhages can appear almost spontaneously in any part of the nail, depending on its cause. Nail apparatus melanomas most commonly affect people of Japanese or African American descent.