Department of Physiology

Peripheral Nerve Injury

Because peripheral nerves are long and the cells within them are small and fragile, they are subject to injury. Injuries can result from normal "wear and tear" and can be sufficiently benign that we are unaware of them. Some injuries that we are aware of, such as the loss of sensation that accompanies restricted blood flow to a region of our body, can be short-lived and readily reversed. More traumatic injuries that sever peripheral nerve axons have more serious implications. Even if the injury is eventually fully healed, the regeneration process that is involved can be very long in duration.

This is because when an axon is severed, the portion of the axon distal to the point of injury dies (i.e. the part that is no longer connected to the neuronal cell body). This death precipitates a number of changes that involve other cells, notably the Schwann cells. The overall process is termed Wallerian degeneration. The only way a particular axon can become functional again is through growth of the proximal part out to the periphery following the original route. The success of this regeneration is critically dependent on the cell's ability to navigate over this route. Whether it succeeds is determined most by the degree of integrity of the connective tissue components of the nerve. Thus a crush injury which severs axons but spares the connective tissue is associated with more successful regeneration, whereas a completely severed nerve will almost certainly not regenerate without surgical intervention.

Sunderland's degrees of nerve injury

Sir Sydney Sunderland classified nerve injuries along the lines outlined above into 5 degrees. These are summarised in the diagram below (which was adapted from his book Nerves and Nerve Injuries).

The first degree involves temporary malfunction in a portion of the axon (black) such as can occur with pressure, poor circulation or cold.

The second and subsequent degrees include the severance of the axon, and consequent Wallerian degeneration. The prognosis for a second degree injury is generally for good regeneration once the insult is withdrawn, but very long periods of time can be involved. Regrowth of axons to the periphery rarely exceeds 1mm/day. Consider how long axons would take to extend from an upper arm injury to the hand.

The prognosis is poor for degrees 3 onwards. The loss of the endoneurium (3) and the perineurium (4) restrict axonal growth and navigation.

The fifth degree, complete nerve section, is likely to be permanent unless the cut ends of the nerve can be brought into apposition, usually through microsurgery. If part of the nerve is absent, as can occur following removal of a tumour, or in severe accidents, the gap must be bridged with something through which the axons can grow. In practice this means a piece of nerve from elsewhere must be microsurgically placed into the gap. This requires sacrifice of a length of nerve from elsewhere in the body, so is only done if the cut nerve's function is very important.

Some neurological terminology related to Sunderland's scale.

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