Etiology

• Accident - gunshot wounds, RTA, mechanical blow, fracture, pressure, ischemia.
• Iatrogenic - cutting, crushing, spiking; compression by dressings, casts or splints; injections into or adjacent to nerve.
• Intramedullary femoral pinning procedures have a notably higher incidence of iatrogenic nerve injuries than pinning procedures in other bones.

Predisposing factors

General

• Access to outdoors (increased risk of trauma, eg HBC (RTA)).
• Orthopedic procedure.
• Intramuscular injections.

Pathophysiology

• Mild trauma   →   temporary conduction block   →   physiological disruption of axonal function   →   no structural damage to axon.
• More severe trauma   →   transection of axon (axotomy)   →   degeneration of distal axon divorced from cell body   →   impulses traveling down proximal stump no longer innervate muscle (motor) or receptor (sensory):
  • Motor disruption   →   loss of muscle function (paresis/paralysis), hypotonia, flaccidity.
  • Sensory disruption   →   anesthesia or dysthesia.

First degree injury

• No structural damage, loss of function - neuropraxia.

Second degree injury

• Axotomy, but basal lamina intact - axonotmesis.
• Loss of function to sensory dermatomes and muscle supplied by affected nerve.

Third degree injury

• Axotomy with disruption of basal lamina; perineurium/epineurium intact - neurotmesis I.
• Epineurial and perineurial scarring at site of injury may impede regeneration.

Fourth degree injury

• Axotomy with basal lamina, perineurial and epineurial disruption (ie transection) - neurotmesis II.
• Epineurial and perineurial scarring at site of injury most likely in this form of injury - may impede regeneration.
• Delayed regeneration may fail to fully restore function because of fibrosis/degeneration of target sites.

Timecourse

• Neurological deficits immediately after injury.
• Onset of muscle atrophy and nerve degeneration after 7-10 days.