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A nerve injury can result in weakness or paralysis of a muscle or in numbness of an area of skin. In some people, it also causes pain. To understand nerve injury and recovery, it is important to understand the different types of nerve injury. The type of nerve injury will determine the type of treatment.

Nerve Injury Classification System

Nerve injuries are classified as follows:

  • A first-degree injury, or neurapraxia, will recover within days after the injury, or it may take up to three months. The recovery will be complete with no lasting muscle or sensory problems.
  • A second-degree injury, or axonotmesis, also will recover completely; however, the recovery will take much longer than with a first-degree injury.
  • A third-degree injury also will recover slowly; in addition, only partial recovery will occur.
  • A fourth-degree injury occurs when there is dense scar tissue within the nerve, completely blocking any recovery. Surgery is required for recovery.
  • A fifth-degree injury involves complete separation of a nerve, such as a cut nerve. Surgery is required for recovery.
  • A sixth-degree injury is a combination of other types of nerve injury. Recovery and treatment will vary depending on which types of nerve injury are present.

Diagrammatic Representation And Cross Section


Text and illustrations from Surgery of the Peripheral Nerve by Mackinnon and Dellon, reprinted with permission of Thieme Medical Publishers, Inc.
  • a. Diagrammatic representation of the cross section of a normal peripheral nerve demonstrating the connective tissue and nerve tissue components.
  • b. The cross section of the peripheral nerve demonstrates a mixed, or sixth degree, injury pattern. This fascicle (bundle of nerve fibers) at 11 o’clock is normal.

Moving counterclockwise, the adjacent fascicle demonstrates a first degree injury (neurapraxia) with segmental demyelination (loss of the myelin that covers many nerve fibers).

The next fascicle demonstrates a second degree injury (axonotmesis). This injury involves both the axon and the myelin. The endoneurial tissue (delicate connective tissue network that holds together the individual fibers of a nerve trunk) is not damaged.

The central two fascicles demonstrate a third degree injury, with injury to the axon, myelin and endoneurium. The perineurium (sheath of connective tissue that surrounds a bundle of nerve fibers) is intact and normal.

The fascicles demonstrate at 12 and 1 o’clock a fourth degree injury with marked scarring across the nerve, with only the epineurium being intact.

In a fifth degree injury pattern, the nerve is not in-continuity but is transected. The surgeon will separate the fourth and fifth degree injury patterns, which will require reconstruction from the normal fascicles and the fascicles demonstrating first, second and third degree injury patterns. These latter patterns of injury require, at most, neurolysis (destruction of nervous tissue).

Nerve Recovery and Regeneration

After nerve injury, the nerve will try to repair itself by sprouting regenerating nerve units. These regenerating units will then try to grow down the nerve to reinnervate (restore nervous function to) muscle or skin. If they make a correct connection — motor nerve to muscle or sensory nerve to skin — then recovery of muscle function and skin sensation will occur. However, if the regenerating nerve fibers do not make a correct connection, then no recovery will occur.

Peripheral Nerve Surgery

If surgery is necessary, there are several types of surgery that may be recommended.

For nerve regeneration, the regenerating nerve fibers need the guidance of the nerve for direction to the muscle or sensory unit. If the nerve has been cut, a nerve repair is used to sew the two ends of a nerve together. This usually is possible when the nerve has been cut sharply. However, in cases with more extensive damage, it may not be possible to sew the two ends of the nerve directly together once the damaged nerve has been trimmed away.

In these cases, a nerve graft is used. A small piece of donor nerve is used to bridge the gap between the two nerve ends, and the nerve will regenerate across this “bridge” to provide recovery. The donor nerve is taken from other areas of your body using small, noncritical sensory nerves.

In some cases in which sensory or muscle recovery is not anticipated for a very long time, a nerve transfer may be used. Nerve transfers use functioning nerves that are close to the target muscle or sensory area, and the nerves are transferred to the injured nerve.

A neurolysis (or nerve decompression) refers to the removal of scar or compressive structures (including fascia or tendonous edges of muscles) from the nerve and may be undertaken if external impediments (or tight "tunnels") are pinching the nerve, limiting the ability of the regenerating unit to pass through on its way to its target.

Other techniques to restore muscle function, tendon transfers and free functional muscle transplants, also are covered on our web site.

Patient information on nerve injury