Runner’s nail is a collection of injuries to the toenail that involve mechanical, or pressure, damage to the nail and the cells that grow the nail. It is named runner’s nail because it often involves runners, but can be seen in other people as well. The 2nd toe (the toe next to the big toe) is most commonly involved, but runner’s toe can be seen in other toenails as well.
The most common injury with runner’s toe is bleeding underneath the toenail, which appears as a dark red or blue/purple color under the nail itself. As the foot moves in the shoe during activity, the toes can become damaged as they strike the shoe. If one has an excessively long toe, as often seen in the 2nd toe, the pressure from the toe hitting the shoe before the other toes can cause damage to the skin bed under the nail, resulting in bleeding. If one has a shoe that is too small, this can occur with even normal sized toes. It can even occur in shoes that are too big, as the foot will piston in and out of an oversized shoe, leading to the toe banging against the end of the shoe.
As blood from damaged blood vessels pools under the nail, it causes a separation of the nail from the nail bed, resulting in nail loosening. Sometimes the fluid can drain out, and other times it remains, leading occasionally to bacterial infection and an abscess under the nail.
As a result of this damage, the nail may or may not loosen and fall off, and the nail root cells that grow the nail out can be damaged as well. This can lead to a permanent thickening and deformity of the nail, and can also place the nail at risk for developing a fungal infection.
Prevention revolves around making sure one’s shoes fit properly, especially if one is a runner. The best way to ensure this is to be fitted by a professional shoe clerk in a quality shoe store or runner’s specialty store. Well-padded socks can also help soften the end pressure on the toes. Keeping the toenails short and well trimmed may reduce the effective 'length' of the toe, reducing the pressure to the nail. Orthotic shoe inserts (the prescription kind) can be of some limited benefit if contracted toes are present that compound the pressure on the end of the toe, as orthotics can limit the forces leading to the contractures in the first place. Treatment of an acute runner’s nail may involve removal of the toenail to allow for inspection of the nail bed skin and drainage, especially if the toe is painful or the bruise takes up the majority of the nail surface. The injury usually heals well for the most part, but some deformity and fungus infection risk as described above can be expected, especially if the injury keeps repeating.