The function of the human hand relies on peripheral nerves that transmit signals between the brain and tissues. A severe laceration that transects one of these nerves immediately disrupts the communication pathways controlling movement and sensation. Since the hand is highly dependent on this neural signaling, any cut nerve requires urgent medical evaluation to assess the extent of the damage and the resulting loss of feeling and movement.
Immediate Sensory and Motor Loss
When a nerve is cut, it instantly loses all function below the site of injury. This functional deficit manifests as sensory and motor losses depending on which nerve—such as the median, ulnar, or radial—was damaged. Sensory loss means the affected region of the hand or finger becomes numb, losing the ability to feel light touch, temperature, or pain. The loss of protective sensation is concerning because the person can no longer perceive minor injuries like burns or cuts in the denervated area.
Motor function is compromised, resulting in paralysis or weakness in the specific hand muscles innervated by the severed nerve. For example, a complete cut to the ulnar nerve affects the ability to perform fine finger movements and grip strength. The inability to move or feel is due to severed axons being unable to transmit electrical impulses. The pattern of sensory and muscle loss is unique enough to allow a clinician to determine exactly which nerve has been injured through a physical examination.
Understanding Nerve Injury Severity
The long-term outlook for a nerve injury depends on the internal damage sustained by the nerve structure. Nerve injuries are categorized by severity, ranging from a mild block of function to a complete physical division of the nerve trunk. In the mildest scenario, the nerve is only bruised or temporarily stunned, causing a temporary conduction block without structural damage to the axons. Recovery in these cases is often spontaneous and occurs within a few weeks as the nerve’s insulation layer repairs itself.
A more serious injury involves the destruction of the nerve’s internal fibers (axons), while the outer connective tissue sheath remains intact. This scenario requires the axon to regrow from the cell body down the preserved tube, a slower process that holds a better prognosis for recovery.
The most severe injury is when the entire nerve is physically transected. A full severing means there is no continuous pathway left for the axons to follow, and spontaneous recovery cannot occur. Assessment of the connective tissue integrity predicts the potential for functional return and guides the surgeon in determining if surgical repair is necessary.
Acute Medical Treatment Options
Since a completely cut nerve will not heal, medical intervention is necessary to restore the continuity of the neural pathway. Surgical repair is performed, ideally within the first few days, to minimize the time target muscles and sensory receptors are without nerve signaling. The standard procedure for a clean cut with minimal tissue loss is called neurorrhaphy, where a surgeon uses microscopic techniques to stitch the two severed nerve ends together.
This microscopic suturing is performed on the outer layer of the nerve, the epineurium, to ensure the internal fascicles are aligned. When the injury has resulted in tissue loss, creating a gap that prevents tension-free suturing, a different technique is necessary. Repairing a nerve under tension results in poor recovery, as it can pull the repair apart.
In cases of a gap, the surgeon uses a nerve graft. This procedure involves harvesting a nerve segment, often from the leg or forearm, and using it as a scaffold to connect the two damaged ends. The graft provides a living conduit that guides the regenerating axons across the gap toward their original destinations.
The Long Road of Nerve Regeneration
Recovery following nerve repair is a slow process that begins with the degeneration of the nerve segment distal to the injury, known as Wallerian degeneration. Once the surgical repair is stabilized, the healthy portion of the nerve begins to send out new axonal sprouts. These new fibers must cross the repair site and then grow toward the target muscle or skin receptor.
Axons regenerate at a rate of one millimeter per day, or about one inch per month. This slow rate means that an injury high in the forearm may take months for the fibers to reach the fingertips. During this lengthy period, physical and occupational therapy is necessary to prevent the joints and muscles from becoming stiff while they await reinnervation.
Therapy helps maintain muscle flexibility and retrain the brain to interpret the returning sensory signals, which often initially manifest as tingling or shooting pains. Despite surgical repair and rehabilitation, complete recovery is not guaranteed, and permanent dysfunction is common. Long-term complications may include persistent numbness, weakness, or the development of a neuroma at the repair site or on the cut end of a nerve. Functional recovery depends on the axons reaching their targets before the muscle or sensory receptor cells permanently lose their ability to accept the new nerve connection.