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Introduction
  • Mechanism  
    • stretching injury
      • 8% elongation will diminish nerve's microcirculation
      • 15% elongation will disrupt axons
      • examples
        • "stingers" refer to neurapraxia from brachial plexus stretch injury
        • suprascapular nerve stretching injuries in volley ball players
        • correction of valgus in TKA leading to peroneal nerve palsy
    • compression/crush
      • fibers are deformed
        • local ischemia
        • increased vascular permeability
      • endoneurial edema leads to poor axonal transport and nerve dysfunction
      • fibroblasts invade if compression persists
        • scar impairs fascicular gliding
      • 30mm Hg can cause paresthesias
        • increased latencies
      • 60 mm Hg can cause complete block of conduction
    • laceration
      • sharp transections have better prognosis than crush injuries
      • continuity of nerve disrupted
        • ends retract
        • nerve stops producing neurotransmitters
        • nerve starts producing proteins for axonal regeneration
  • Pathophysiology
    • regeneration process after transection  
      • distal segment undergoes Wallerian degeneration (axoplasm and myelin are degraded distally by phagocytes)
      • existing Schwann cells proliferate and line up on basement membrane
      • proximal budding (occurs after 1 month delay) leads to sprouting axons that migrate at 1mm/day to connect to the distal tube
    • variables affecting regeneration  
      • contact guidance with attraction to the basal lamina of the Schwann cell
      • neurotropism
      • neurotrophism
      • neurotrophic factors (factors enhancing growth and preferential attraction to other nerves rather than other tissues)
  • Prognosis  
    • factors affecting success of recovery following repair
      • age 
        • is single most important factor influencing success of nerve recovery
      • level of injury 
        • is second most important (the more distal the injury the better the chance of recovery)
      • sharp transections
        • have better prognosis than crush injuries
      • repair delay 
        • worsen prognosis of recovery (time limit for repair is 18 months)
    • return of function
      • pain is first modality to return
Anatomy
  • Highly organized structure consisting of nerve fibers, blood vessels, and connective tissue
  • Functional structures   
    • epineural sheath
      • surrounds peripheral nerve
    • epineurium
      • surrounds a group of fascicles to form peripheral nerve
      • functions to cushion fascicles against external pressure 
    • perineurium
      • connective tissue covering individual fascicles
      • primary source of tensile strength and elasticity of a peripheral nerve 
      • provides extension of the blood-brain barrier
      • provides a connective tissue sheath around each nerve fascicle
    • fascicles
      • a group of axons and surrounding endoneurium
    • endoneurium
      • fibrous tissue covering axons
      • participates in the formation of Schwann cell tube
    • myelin
      • made by Schwann cells
      • functions to increase conduction velocity
    • neuron cell
      • cell body - the metabolic center that makes up < 10% of cell mass
      • axon - primary conducting vehicle
      • dendrites - thin branching processes that receive input from surrounding nerve cells
  • Blood supply
    • extrinsic vessels
      • run in loose connective tissue surrounding nerve trunk
    • intrinsic vessels
      • plexus lies in epineurium, perineurium, and endoneurium 
  • Physiology
    • presynaptic terminal & depolarization
      • electrical impulse transmitted to other neurons or effector organs at presynaptic terminal
      • resting potential established from unequal distribution of ions on either side of the neuron membrane (lipid bilayer)
      • action potential transmitted by depolarization of resting potential
      • caused by influx of Na across membrane through three types of Na channels
        • voltage gate channels
        • mechanical gated channels
        • chemical-transmitter gated channels
    • nerve fiber types
Fiber Type
Diameter (uM)
Myelination
Speed
Example
A
10-20
heavy
fast
touch
B
< 3
moderate
medium
ANS
C
< 1.3
none
slow
pain
 
Classification
  • Seddon Classification
    • neurapraxia 
      • same as Sunderland 1st degree, "focal nerve compression"
      • nerve contusion leading to reversible conduction block without Wallerian degeneration
      • histology
        • histopathology shows focal demyelination of the axon sheath (all structures remain intact)
        • usually caused by local ischemia 
      • electrophysiologic studies
        • nerve conduction velocity slowing or a complete conduction block
        • no fibrillation potentials 
      • prognosis
        • recovery prognosis is excellent
    • axonotmesis 
      • same as Sunderland 2nd degree
      • axon and myelin sheath disruption leads to conduction block with Wallerian degeneration 
      • endoneurium remains intact
      • fibrillations and positive sharp waves on EMG
    • neurotmesis
      • complete nerve division with disruption of endoneurium
      • no recovery unless surgical repair performed 
      • fibrillations and positive sharp waves on EMG
Seddon Type
Degree
Myelin Intact
Axon Intact
Endoneurim Intact
Wallerian Degen.
Reversible
Neurapraxia
1st
No
Yes
Yes
No
reversible
Axonotmesis
2nd
No
No
Yes
Yes
reversible
Neurotmesis
3rd
No
No
No
Yes
irreversible
  • Sunderland Classification
    • 1st degree
      • same as Seddon's neurapraxia
    • 2nd degree
      • same as Seddon's axonotmesis
    • 3rd degree 
      • included within Seddon's neurotmesis
      • injury with endoneurial scarring
      • most variable degree of ultimate recovery
    • 4th degree
      • included within Seddon's neurotmesis
      • nerve in continuity but at the level of injury there is complete scarring across the nerve)
    • 5th degree
      • included within Seddon's neurotmesis

Sunderland Grade
Myelin Sheath
Axon
Endoneurim
Perineurium
Epineurium
I
Disrupted
Intact Intact
Intact
Intact
II
Disrupted
Disrupted
Intact
Intact
Intact
III
Disrupted
Disrupted
Disrupted
Intact
Intact
IV Disrupted
Disrupted Disrupted Disrupted Intact
V Disrupted Disrupted Disrupted Disrupted Disrupted
 
Evaluation
  • EMG
    • often the only objective evidence of a compressive neuropathy (valuable in workcomp patients with secondary gain issues)
    • characteristic findings
      • denervation of muscle
        • fibrillations
        • positive sharp waves (PSW)
        • fasiculations
      • neurogenic lesions
        • fasiculations
        • myokymic potentials
      • myopathies
        • complex repetitive discharges
        • myotonic discharges
  • NCV
    • focal compression / demyelination leads to
      • increase latencies (slowing) of NCV
        • distal sensory latency of > 3.2 ms are abnormal for CTS
        • motor latencies > 4.3 ms are abnormal for CTS
      • decreased conduction velocities less specific that latencies
        • velocity of < 52 m/sec is abnormal
      • motor action potential (MAP) decreases in amplitude
      • sensory nerve action potential (SNAP) decreases in amplitude
Treatment
  • Nonoperative
    • observation with sequential EMG
      • indications
        • neuropraxia (1st degree)
        • axonotmesis (2nd degree)
  • Operative
    • surgical repair
      • indications
        • neurotomesis (3rd degree)
    • nerve grafting
      • indications
        • defects > 2.5 cm
      • type of autograft (sural, saphenous, lateral antebrachial, etc)
        • no effect on functional recovery
Surgical Techniques
  • Direct muscular neurotization
    • insert proximal nerve stump into affected muscle belly
    • results in less than normal function but is indicated in certain cases
  • Epineural Repair
    • primary repair of the epineurium in a tension free fashion
    • first resect proximal neuroma and distal glioma
    • it is critical to properly align nerve ends during repair to maximize potential of recovery
  • Fasicular repair
    • indications
      • three indications exist for grouped fascicular repair
        • median nerve in distal third of forearm
        • ulnar nerve in distal third of forearm
        • sciatic nerve in thigh
    • technique
      • similar to epineural repair, but in addition repair the perineural sheaths (individual fascicles are approximated under a microscope)
    • outcomes
      • no improved results have been demonstrated over epineural repair
  • Nerve grafting
    • autologous graft
      • indications
        • ≥ 3cm gap
      • digital nerve defects
        • at wrist to common digital nerve bifurcation - use sural nerve
        • at MCP to DIP level - use lateral antebrachial cutaneous nerve
        • at DIP level - use AIN, PIN or medial antebrachial cutaneous nerve
      • outcomes
        • gold standard for segmental defects > 5cm    
    • collagen conduit
      • tensioned closures inhibit Schwann cell activation and axon regeneration, compromise perfusion and lead to scarring
      • collagen conduits allow nutrient exchange and accessibility to neurotrophic factors to the axonal growth zone during regeneration
      • indications
        • defects ≤ 2cm
      • outcomes
        • equal results to autologous grafting when gap ≤5mm 
        • quality of nerve recovery drops with gaps >5mm
    • allograft
      • off-the-shelf option for defects up to 5cm
 

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