If you stub your toe or drop something heavy on your feet, you may develop a subungual hematoma, which is blood trapped beneath the toenail. This gives the toenail a red, black, or purple discoloration. It is often called black toenail for that reason.
A subungual hematoma is usually caused by blunt trauma from a heavy object or chronic friction from rubbing against the shoe.1
Acute trauma isn't necessarily the cause. People who do a lot of walking or running are more prone to subungual hematomas because of increased shoe friction. Marathon runners and hikers are very familiar with this malady.
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 a subungual hematoma is large and causing pain, medical treatment may be needed to relieve pressure under the nail. If left untreated, the condition could damage the nail matrix, causing the nail to grow incorrectly or not at all.
When to See a Doctor
It's time to go from self-care to seeing a medical professional if any of these six conditions apply in your case.
Blunt trauma to the toenail, such as a heavy object dropped on the toe, which may need to have a fracture or wound treated
Severe swelling, pain, or redness of the toe
Discharge from under the toenail
Toenail becomes loose
If you have neuropathy, diabetes, peripheral arterial disease, or any condition that affects healing or circulation
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.3
If the nail is raised, the doctor may relieve the pressure by drilling a hole through the nail. If it is loose, the nail may be trimmed or even reattached to protect the nail bed while the nail is regrowing.1
You should avoid removing or drilling the injured nail at home. Doing so could lead to infection and slow the healing process. If you are unable to seek immediate care, bandage the nail and avoid trimming or cutting of any sort.
Changes in Nail Structure
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 fungal or bacterial infection. Some of the most common changes are:
Nail thickening: This is caused by damage to the nail's growth center, called the nail matrix. It responds by producing a thicker nail.
Nail dystrophy: You may see permanent nail changes such as splitting or a decrease in size.
Nail avulsion: Sometimes the entire nail or a portion of it becomes loose or falls off. This is often a temporary condition and the nail will regrow over the course of a couple of months.
Permanent nail loss: This can happen due to damage to the nail matrix. The toenail may not regrow.4