Tiny, red lines under the toenail
It could be: A heart infection. Red streaks underneath the toenails or fingernails could be broken blood vessels known as splinter hemorrhages. These occur when small blood clots damage the tiny capillaries under the nails. They can signal endocarditis, an infection of the heart’s inner lining.
People who have an existing heart condition, have received a pacemaker, or who have chronically suppressed immune systems are at higher risk of developing endocarditis. The infection can result in heart failure if left untreated. If you notice splinter hemorrhages on your toenails or fingernails, and haven’t experienced any recent trauma to the nail, see your doctor to check your heart and blood circulation.