Updated December 16, 2014.

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

If you stub your toe or drop something heavy on your feet, you may develop subungual hematoma, which is blood trapped beneath the toenail that appears as a red, black or purple discoloration. It is usually caused by blunt trauma from a heavy object or chronic friction from rubbing against the shoe. But acute trauma isn't necessarily a cause -- even people who do a lot of walking or running are more prone to subungual hematomas because of increased shoe friction.

A subungual hematoma can range from a small spot under the nail to a large area of discoloration. Depending on the amount of blood beneath the nail, the nail may come loose. But often the nail does stay intact, and the blood fades as the nail grows out. If the subungual hematoma is large and causing pain, medical treatment will be needed to relieve pressure under the nail.

When to Seek Treatment for subungual hematoma or any other nail trauma:

• If there is blunt trauma to the toenail, for example if a heavy object has been dropped on the toe, it is best to seek treatment to rule out a fracture and treat any wound beneath the nail bed.
• If toenail trauma leads to severe swelling, pain or redness of the toe.
• If there is any discharge from under the toenail.
• If the nail becomes loose. Learn more: Why is My Nail Loose?
• If you have neuropathy or any condition that affects healing or circulation such as diabetes or peripheral arterial disease.
• If the discoloration appears as a linear streak, or a stripe going with the length of the nail. Although a streak of pigment is often normal, in some cases it may be a sign of the skin cancer melanoma.

Nail trauma may not always lead to bleeding beneath the nail but to other predictable changes. These changes may occur due to chronic rubbing against the shoe or in response to irritation from a fungual or bacterial infection. Some of the most common changes are:

• Nail thickening- caused by damage to the nail's growth center, or matrix
• Nail dystrophy- permanent nail changes such as splitting or a decrease in size
• Nail avulsion- entire nail or a portion of it becomes loose or falls off
• Permanaent nail loss- due to damage to the nail matrix

<sub>Source:

Dockery, DPM, Gary L. and Crawford, DPM, Mary Elizabeth (Ed.). Cutaneous Disorders of the Lower Extremity. Philadelphia: W.B. Saunders, 1997. 259-260.</sub>