Nerve injury

Nerve injury includes total or partial transection of the nerve from stretching, cutting (laceration), compression, shearing, or crushing injuries. Nerve injury may complicate injury from accidents, altercations, or other acute trauma, or it may develop more slowly from gradual compression caused by repetitive movements, as in carpal tunnel syndrome, or by sustained positions, as in radial nerve palsy. It may occur during or after surgery from traction or casting putting pressure on a nerve or because of incidental injury to a nerve located in the operative field. Nerve injuries are classified by the degree of pathological change in the nerve. There are three main grades (Burnett, Hyde). The mildest injury is classified as grade I, and the most severe as grade III. Grade I nerve injury is called neurapraxia and is a minor injury from which full recovery occurs. With neurapraxia, there is no loss of nerve continuity, and functional loss is temporary. Grade II nerve injury, called axonotmesis, is a more severe injury, and although some secondary reparative processes occur, recovery can eventually take place. With axonotmesis, there is interruption of the internal nerve fibers (axons), but the external structures covering the nerve remain intact. A situation in which the nerve is severely damaged by crushing, laceration, or complete transection is called neurotmesis. This is a grade III injury, and it results in some degree of permanent impairment. With neurotmesis, there is complete functional loss unless surgery to re-attach the nerve is performed. If a motor nerve is injured, the muscle(s) that the nerve supplies (innervate) loses function. If a sensory nerve is injured, the area innervated by the nerve loses sensation. The closer the nerve is to its origin in the spinal cord, the lower the chance of recovery. Nerve injury also may affect autonomic nerves, which control temperature regulation, blood pressure, heart rate, and digestion.

Diagnosis

History: The individual may complain of pain, tingling sensations (paresthesias), reduced sensation (hypoesthesia), or weakness of an extremity after trauma. Sometimes, no specific trauma is revealed. Physical exam: General physical examination may reveal associated injuries. A standard neurologic exam may show some of the degree of nerve involvement, with confirmation of the complaints noted above. Peripheral nerve testing should include tests of light touch, pain, temperature, and vibration sensation. Other sensory tests include the ability to distinguish two pinpricks applied closely together (two-point discrimination) and the ability to appreciate small movements of the joint (joint position sense, proprioception). Motor testing includes tests of strength of muscles supplied by the affected nerve. In nerve injuries related to entrapment or compression, lightly tapping over the nerve where it is compressed may cause a painful or tingling, electric shock-like sensation (Tinel's sign). Reflexes typically are depressed or absent in the muscle groups supplied by the affected nerve. Ambulation and balance should be assessed in individuals with lower extremity nerve injuries. Tests: Electromyography (EMG) helps to differentiate between grade I and higher grades of injury. A nerve conduction study, done by stimulating the nerve using an electrical stimulus on the skin and measuring the nerve response, indicates which motor and sensory nerves are affected. These tests begin to become abnormal within 10 to 21 days after the injury (Hyde, Sharon). A single test cannot distinguish between injuries that will demonstrate eventual recovery (grades I and II) and those that are unlikely to recover (grade III). Nevertheless, repeated tests over time can show whether improvement is occurring. Somatosensory evoked potentials are abnormal immediately following nerve injury; this a useful test for monitoring nerve function during surgery that might compromise the nerve. If the nerve injury resulted from trauma, x-rays, magnetic resonance imaging (MRI), and computed tomography (CT) scans may be performed to assess associated injury to bones, cartilage, muscles, tendons, and ligaments; this is especially important with nerve injuries originating close to the spine.

Treatment

The timing of treatment of open wound nerve injuries is critical. The wound is explored, and if a clean cut of the nerve is found, the proximal and distal nerve ends are matched and brought together so that microneurosurgical repair (neurorrhaphy) can be carried out. The goal of repair is to mend the ends of the nerve covering that contains the blood supply to the nerve (epineurium) so that new axonal nerve fibers can reconnect inside this covering (“Nerve Injuries”). Repair may be done later; the timing of nerve repair is dictated by the nature of the nerve injury and may be performed after the wound has healed, when some of the swelling and scarring has resolved, and the wound is clean. Delayed repairs usually require nerve grafting using a piece of another nerve to bridge the injured area. Other surgical strategies may involve shortening a nearby bone to create relative length in the injured nerve if the nerve ends have retracted (Sharon). In a closed injury (crush or shear), there is no indication for immediate surgery. However, the degree of nerve injury should be diagnosed as soon as possible. The individual is examined repeatedly over a 3-month period. During this period, gentle active motion of the involved extremity keeps the joints and soft tissue supple. The individual may be given medications (e.g., analgesics, anticonvulsants, corticosteroids) to control pain. If by 3 months there is some clinical or electrical evidence of recovery, the individual continues to be monitored. However, if there is no clinical or electrical (EMG) evidence of return of function at 3 months, the nerve is explored and tested. If the test indicates any activity across the injury, then corrective nerve surgery is performed. If there is no activity, the injured area is cut out (excised) and a graft is performed. If nerve repair is not possible, the surgeon may choose to alter the course of a tendon (tendon transfer) to improve function in the affected area (Sharon).

Prognosis

On average, the rate of axonal regeneration is 1 mm per day (Burnett). With grade I injuries, full recovery eventually will occur, although it may take many weeks (Sharon). During recovery from motor nerve injury, 70% of associated muscle fibers atrophy during the first 8 weeks, causing temporary weakness (Burnett). With grade II injuries, scarring will result in a less than perfect re-innervation with an extremely variable pattern. There is no recovery from grade III nerve injuries without surgical intervention; optimal outcomes are achieved with surgical repair within the first 3 months following injury (Sharon). The outcome also depends on the location and territory supplied by the injured nerve and on whether there is overlap in function with neighboring nerves that are not injured. In some cases, these nerves may be able to take over some functions of the injured nerves. Nerve injuries that occur at or close to the spine have a poor prognosis (Sharon).