A Subungual Haematoma can occur when a nail is traumatised by an isolated incident like dropping something on the toe.
Repeated minor trauma like long distance running with incorrect footwear can also be a cause.
Faulty biomechanics where there is excessive pronation can lead to toenail damage.
Deep nail plate separation occurs with bleeding under the nail. This can be partial or complete. Should the fluid build up under the nail, it will put a lot of pressure on the nail bed, leading to pain.
It will have a blackened appearance after a few days as the blood coagulates.
The cause has often been blamed on incorrect footwear sizing in sportsmen, this may be the case, but Sport shoe shops have salesmen who are aware of correct sizing, so it would be rare to walk out with poorly fitting shoe.
Runners who are prone to repeated loss of nails after long distance running may have a mechanical problem that leads to the foot moving in such a way that it repeatedly traumatises the nail until it loosens or forms a sub-ungual haematoma.
By correcting or improving foot function, muscle-imbalances and wearing the right shoes for your feet can reduce the trauma to the toes preventing this condition.
Treatment involves puncturing the nail plate to release pressure and drain the fluid. This can give immediate relief from pain. However the nail, due to separation will eventually fall off as the new nail comes through from the nail matrix. The Nail should be clipped gradually until it falls off, to allow maturing of the new nail underneath.
In some cases infection may accompany the condition, a red swollen toe that is exquisitely painful. In this anti-biotics may be necessary and possibly toenail removal.