Black lines in fingernails often result from splinter hemorrhages--bleeding under the nails. Splinter hemorrhages, which occur because of blood vessel swelling or capillary damage, may signal heart disease or other heart conditions.
Lines in fingernails--called splinter or fingernail hemorrhages--can appear in shades of black, red or brown. The term “splinter” refers to the appearance of small vertical lines under the nail bed.
Splinter hemorrhages generally appear in the later stages of endocarditis, a rare heart valve infection prevalent in those with underlying heart disease that causes inflammation of the heart’s inner lining. Another condition, vasculitis, can cause these lines through an extreme allergic reaction that damages skin blood vessels.
Endocarditis results from germs that enter through other areas of the body, such as the mouth and infect the heart. This rare condition emerges most often in people with existing heart disease that undergo heart or dental surgeries. People suffering from vasculitis often develop painful skin lesions, such as sores, blisters or hives in different areas of the body.
If a heart condition exists, other serious symptoms appear that cause concern and likely result in health monitoring before splinter hemorrhages materialize. The National Institutes of Health indicate that physical trauma to the nail as well as drug use through injections can also cause splinter hemorrhages.
If recent nail trauma has not occurred, people with dark lines in fingernails should visit a physician for proper diagnosis and treatment of any underlying conditions and causes.