Anatomy of the peripheral nerve
Endoneurium
- loose collagenous matrix
- surrounds the individual nerve fibers within the fascicle
Perineurium
- thin, dense connective tissue sheath that surrounds each fascicle
Epineurium
- a loose meshwork of collagen and elastin fibers
- provides a supportive and protective framework for the fascicles
- collagen fibers in the epineurium are thicker than those in the endoneurium / perineurium
Definition
Neuropraxia
- focal demyelination 2° ischaemia
- full recovery by 3 months
Axonotmesis
- axon disrupted
Neurotmesis
- nerve division
Seddon & Sunderland Grading
1st Degree / Neuropraxia
- localised conduction block leading to segmental demyelination
- axons are not injured
- remyelination and recovery < 3 months, no Tinel's
2nd Degree / Axonotemesis
- axonal injury with distal segment Wallerian degeneration
- full recovery
- nerve fibres regenerate 1 mm per day
- but > 3 months, advancing Tinel's
3rd Degree / Endoneurium disrupted
- incomplete recovery due to fibrosis
- advancing Tinel's
4th Degree / Perineurium divided / Epineurium intact
Nerve is in continuity but complete block due to scar
Poor prognosis
- perineurium disrupted
- becomes filled with scar
- no recovery, no Tinel's
- no SNAP, denervation potential
- no rennervation at 3/12 (polyphasic AP)
Often needs repair
5th Degree / Epineurium divided
- 100% divided / neurotmesis
6th Deg (added by Susan McKinnon)
- neuroma in continuity
- mixed recovery
Injury Response
Wallerian Degeneration
Axon dies distally
- remains intact for 3/24 (until stores depleted)
Schwann cell proliferation and macrophage ingrowth
- clear distal stump of axoplasm & myelin
- prepares way for new axon
Axon Regeneration
Axon sprouts enter distal endoneural tubes
- 1-2 mm/day
- survive if contact an end organ
- sprouts often enter wrong tube & wrong end organs
- some axons fail to cross repair site & form neuroma
Axon Guidance
1. Directional Neurotropic Cues
- target releases tropic factor
2. Survival Neurotropic Cues
- tube supports correct axon
3. Mechanical Alignment
- worst
4. Contact Recognition
- path selection based on surface
Sensory Fibres
Survive years of degeneration
- sensory return order
- pinprick / moving touch / constant touch / vibration
Muscle Fibres
Myofibrils atrophy without nerve
- 50% in 2/12
- fibrotic by 12/12
- need to repair < 9/12
Neuromuscular Junction
Receptor dispersal over 12 months
- needs to be re-innervated prior to 12 months
Receptor becomes more sensitive to ACh
- spontaneous fibrillation start Day 10
- significant EMG changes > 2/52
Neuronal Response
Cell body dies if very proximal axon injury
- 2° ion leakage
- if neurone survives body & nucleolus enlarges
- regenerative proteins produced
- responsible for delay in nerve recovery / regeneration
Mechanism Injury
1. Open wounds
2. Compression
- pressures > 30 mmHg impair venular epineural flow
- retards axonal transport
- alteration in intraneural BV permeability
- leads to nerve function deterioration
- relative to length of compression & absolute pressure
Tourniquet
- UL 200 mmHg / Max 90 min
- LL 300 mmHg/ Max 120 min
3. Traction
4. Thermal
5. Irradiation Neuritis
6. Injection
Recovery Assessment
EMG
Denervation
- 3 weeks
- fibrillation potentials
- positive sharp waves
- spontaneous AP
Reinnervation
- polyphasic AP
Tinel's Sign
Percuss along the nerve
- transient tingling in nerve distribution not at injury site
- indicates axonal sprouts progressing along tube that haven't remyelinised
- response fades proximally secondary to progressive myelinization
Sweat Test
Sympathetic fibres very resistant to injury
- sweat preservation
- 20+ magnification lens
Management
Outcome Factors
General
- age of patient / most important / < 30 best prognosis
- level of injury (proximal worse than distal)
- health of patient
- time delay to repair
- pure sensory nerves do better than mixed nerves
Local
- cut or crush
- single or double level
- surgeon factors
- nil gap, no tension on repair
Contraindications to Repair
Noncompliant patient
Elderly
Hopeless outcome
Insignificant nerve eg SRN -> surgery to avoid neuroma
Insufficient skills
Types of Repair
1. Direct
Epineurium repair without tension
Primary repair
- best chance of fascicular matching / best fit
- minimal retraction & gap formation
- historically best results
- 8/0 or 9/0 nylon
Fascicular Repair
Not usually indicated except
- distal 1/3 forearm median nerve
- distal 1/3 forearm ulna nerve
- sciatic nerve in thigh
Approach
Median nerve
- release PT and FDS radial insertion
- can transpose anterior to pronator and FDS
Results
Rujis et al Plastic Recon Surg 2005
- meta-analysis
- age > 40 / proximal lesions / delay to repair poor prognostic indicators
2. Nerve Grafting
Indications
- gaps > 2.5 cm
Options
- cable graft
- vascularised graft
Cable graft
Graft options
- Sural / MCNF / LCFN / Saphenous
Vascularised graft
Technique
- mobilise on BV pedicle
Advantage
- faster recovery
3. Neural tubes
Description
- absorbable synthetic tubes
- epineurium sutured to each end of tube
- nerve fibrils grow into and along tubue
Advantage
- tension free repair
Results
Aberg et al J Reconst Plast Aesthet Surg 2009
- RCT of epineural repair v tube in sharp distal median and ulna nerve injures
- no difference between two groups
3. Nerve Transfer
4. Neuroma
Resect, diathermy & bury nerve end deeply in good tissue not bone