Nerve injury

Anatomy

• Subunit of the peripheral nerve is the axon
• Axons surrounded by endoneurium and grouped into fascicles
• Fascicles encapsulated by perineurium, and groups of fascicles are surrounded by interfascicular epineurium
• Motor endplates within muscle will irreversably degenerate after 12 months, and after this point the muscle cannot be reinnervated

Classification of nerve injuries

• Open - laceration
• Closed - overstretching, blunt, compressive trauma
• Neuropraxia
  • focal or segmental demyelination with preservation of axon continuity
  • Functional recovery expected as the myelin heals
  • Expected to take up to 6 weeks to recover
  • No surgical intervention required
• Axonotmesis
  • Axonal injury with preservation of the endo and epineurial sheath
  • Axon will die back to nearest node of Ranvier, and then regrow at rate of 1mm/day through the distal endoneurial tubes
  • Axons distal to the injury will undergo Wallerian degeneration - this process commences in 72-96 hours, and after this time period the distal axons will irreversably degrade, so repair should be undertaken within this period where it is going to be attempted
  • Range of recovery - can be complete, can be minimal. Depends on degree of damage to surrounding structures and resulting scar tissue.
  • Spontaneous recovery is likely in Sunderland II or III, but not in IV
• Neurotmesis
  • Complete transection of all neural structures
  • Surgical intervention required

Indications for repair

• Early (3 days)
  • Suspected transection with sharp injuries
    • Any nerve deficits are likely to represent Sunderland V or VI
  • Acute nerve compression resulting from vascular or bony injuries, especially in the vicinity of a closed compartment
• Subacute (3 weeks)
  • Blunt or ragged injuries (chainsaw, propeller blades)
    • Some would advocate for still treating those injuries within 3 days
      • Can combine repairs with other procedures
      • Technical challenges of dealing with scarring can be avoided
      • Probably reasonable to explore early, and if nerve is found to be in continuity, can observe it for a few months (intra-op nerve testing would not be helpful because Wallerian degeneration has not yet occurred)
    • These injuries represent neurotmesis (Sunderland V or VI)
    • Delaying repair allows Wallerian degeneration to begin occurring, better defining the zone of injury
    • Nerve endings can be resected back to healthy tissue, which would be difficult to evaluate in early setting
• Delayed (3 months)
  • Lesions-in-continuity (stretch injuries, contusive injuries, gunshot wounds)
    • Hard to predict which path the nerve injury will follow, and whether it will resolve or require repair
    • Continue non-op for nerves showing signs of recovery or partial lesions (90% of nerves that recover will do so within 4 months) and intervene if no evidence of clinical or electrical recovery
    • Delay also allows intra-op nerve testing to distinguish between recovering lesions (Sunderland II or III) and non-recovering (Sunderland IV)
• Late (>1 year)
  • Salvage procedure - delayed presentation or no/incomplete recovery
    • Nerve repair and reconstruction typically does not work well

Operative principles

• Excellent exposure - identify the normal nerve, before carrying dissection forward or back
• Tourniquet up for no longer than 30 mins if intra-operative nerve stimulation is planned
• Really there are two things you are likely to find: neuroma in continuity, or nerve stumps
• Neuroma-in-continuity:
  • Intra-operative nerve action potentials will be helpful, because inspection/palpation does not predict histology or outcomes, and can determine recovery before that recovery is seen either clinically or via electromyogram
  • If NAP is present across a lesion, don't resect - probably better off with neurolysis alone
  • If no NAP, probably needs resection and surgical repair
• Nerve stumps
  • Nerve repair or reconstruction is indicated
• Microsurgery should be used to repair
  • Sharply remove neuroma and scar tissue, until normal fascicular structures are seen (this pattern is called les yeux d'escargot - snail's eyes)
  • Divide protruding fascicles until they lie flush with epineurial sheath - ensure no overlapping of fascicles
  • End to end repair or interpositional grafting - 8-0, 9-0 or 10-0 suture should be used
  • Tension-free
  • Ensure the suture only connects epineurium, and not fascicles - will lead to intraneural neuroma
  • Often only two or three interrupted sutures required
  • Fibrin glue may be used to reinforce the suture line
• Neurolysis
  • Releasing scar tissue surrounding the injured nerve
  • Indicated for neuroma in continuity, with intact NAP conduction
• Nerve graft
  • Most frequent source is sural nerve
  • Suture in reversed orientation, which ensures that axons will not be lost to small side branches in the nerve graft

Timing of repair

• 3+1 rule