Peripheral Nerves Injury & Repair

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Topic updated on 05/11/13 8:11pm
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
        • fibrillation potentials
        • positive sharp waves (PSW)
        • high amplitude - long duration MUPS
      • 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
    • neurotmesis
      • complete nerve division with disruption of endoneurium
      • no recovery unless surgical repair performed 
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 neurapraxia
    • 2nd degree
      • same as axonotmesis
    • 3rd degree 
      • injury with endoneurial scarring
      • most variable degree of ultimate recovery
    • 4th degree
      • nerve in continuity but at the level of injury there is complete scarring across the nerve)
    • 5th degree
      • same as neurotmesis
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
      • remains the gold standard of repair for segmental defects > 5cm is autologous nerve grafting 
    • allograft
      • the only synthetic graft which shows equal results to autologous nerve grafting is a collagen conduit
      • collagen conduits allow for nutrient exchange and accessibility of neurotrophic factors to the axonal growth zone during regeneration

 

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Qbank (3 Questions)

TAG
(OBQ12.46) The patient sustains the injury seen in Figure A from a gunshot injury. The physical exam is notable for lack of sensation in his fourth and fifth digits as well as a positive Froment's sign. Which of the following factors has not been shown to be a significant prognostic indicator of functional recovery following nerve repair? Topic Review Topic
FIGURES: A          

1. Duration to time of repair
2. Repair level
3. Length of repair
4. Postoperative physical rehabilitation
5. Type of autograft used

PREFERRED RESPONSE ▶
TAG
(OBQ09.268) You are seeing a 24-year-old male in the emergency room after he was involved in a knife fight. He has severed the common digital nerve to the index finger on his dominant hand, with an 8mm gap between nerve ends. In counselling him about repair, which of the following options is as good as autologous nerve grafting? Topic Review Topic

1. Glycolide trimethylene carbonate conduit
2. Collagen conduit
3. Silicone sleeve
4. Primary end-to-end repair
5. Polyglycolic acid conduit

PREFERRED RESPONSE ▶
TAG
(OBQ04.257) Which of the following peripheral nerve structures functions to cushion the nerve against external pressure? Topic Review Topic

1. Endoneurium
2. Fibronectin
3. N-cadherin
4. Epineurium
5. Perineurium

PREFERRED RESPONSE ▶



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