A subungual hematoma, the medical term for blood trapped beneath a fingernail or toenail, typically results from a crushing injury or direct trauma, such as slamming a finger in a door or dropping a heavy object on a toe. The injury damages blood vessels in the nail bed, causing blood to leak into the confined space between the nail plate and the underlying tissue. Because the rigid nail plate covers the injury, the pooling blood has nowhere to escape, rapidly increasing the pressure in the sealed compartment. This pressure buildup causes the intense physical discomfort and dark discoloration characteristic of this injury.
Immediate Effects of Undrained Blood
The most immediate consequence of not draining the blood is intense, throbbing pain. This severe discomfort results from the hydraulic pressure exerted by the collected blood on the highly sensitive nerve endings within the nail bed. The pain is often pulsatile, beating in rhythm with the heart as each beat momentarily increases the pressure. Visually, the trapped blood causes the nail to appear discolored, ranging from deep maroon or purple to almost black. The substantial pressure can also cause the nail plate to begin separating from the nail bed, a process known as onycholysis.
The Body’s Natural Resolution Process
If the subungual hematoma is left undrained, the body begins a process of natural resolution. The trapped blood will eventually coagulate and dry out, forming a solid, dark mass beneath the nail. The discoloration changes over time, typically turning dark brown or black as the blood cells degrade. The dark area remains fixed beneath the nail plate until the nail grows out completely, pushing the dried blood mass forward. This growth process is slow; a fingernail takes approximately six months to regrow entirely, while a toenail may take nine to twelve months or longer.
Risks of Complications and Seeking Care
While many small subungual hematomas resolve without intervention, not draining the blood carries risks, particularly if the trauma was severe. The pooled blood can create an environment conducive to bacterial growth, raising the possibility of a secondary infection. Signs of infection include:
- Increasing pain that returns after initial relief.
- Warmth.
- Spreading redness around the nail.
- The discharge of pus from under the nail fold.
A significant concern is the possibility of an underlying structural injury that was not immediately apparent. Severe trauma capable of causing a large hematoma may result in a fracture of the underlying bone, known as the distal phalanx.
If the hematoma covers more than 50% of the nail surface, the force was likely strong enough to warrant an X-ray to check for a bone break or significant damage to the nail bed. Medical attention is advised if the pain is unrelenting or severe, as drainage (trephination) can provide immediate relief by releasing the pressure. A medical evaluation is also warranted if the injury involves a deep cut to the skin around the nail or bleeding that will not stop. Failure to address a severe nail bed laceration can lead to permanent nail deformity.