10 mark answer on nerve injuries
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Nerve Injuries
1. Introduction
A nerve injury is any disruption to the structural or functional integrity of a peripheral nerve, resulting in impaired motor, sensory, or autonomic function. Injuries range from a transient conduction block to complete nerve transection and are classified by the degree of structural damage to the nerve's components (axon, myelin, endoneurium, perineurium, epineurium).
2. Anatomy of a Peripheral Nerve
Understanding classification requires knowledge of nerve architecture:
- Endoneurium - connective tissue sheath surrounding each individual axon
- Perineurium - surrounds a fascicle (bundle of axons)
- Epineurium - outer sheath enclosing the entire nerve trunk
3. Classification of Nerve Injuries
A. Seddon Classification (1943)
Seddon described three types based on injury severity:
| Type | Structures Damaged | Key Features | Prognosis |
|---|---|---|---|
| Neuropraxia | Myelin only | Focal conduction block; axon intact | Full recovery within days to 6 weeks |
| Axonotmesis | Myelin + axon | Basal lamina intact; Wallerian degeneration occurs distally | Recovery possible but slower; depends on level |
| Neurotmesis | All structures including epineurium | Complete transection or severe disorganization | Poor without surgical repair |
Classic example of neuropraxia: Wrist drop from prolonged compression of the radial nerve at the spiral groove of the humerus ("Saturday night palsy").
B. Sunderland Classification (Further Subdivision)
Sunderland extended Seddon's system into 5 grades to better capture the spectrum of axonotmesis and neurotmesis:
| Grade | Seddon Equivalent | Structures Damaged |
|---|---|---|
| I | Neuropraxia | Myelin only; focal demyelination |
| II | Axonotmesis | Myelin + axon; endoneurium, perineurium, and epineurium intact |
| III | Neurotmesis | Myelin, axon, endoneurium disrupted; perineurium and epineurium intact |
| IV | Neurotmesis | Myelin, axon, endoneurium, perineurium disrupted; epineurium intact |
| V | Neurotmesis | Complete transection of all structures |
| VI (MacKinnon) | Mixed | Combined injury with elements of different grades in the same nerve |
(Sabiston Textbook of Surgery, Table 41.5)
4. Pathophysiology: Wallerian Degeneration and Regeneration
When axonal continuity is lost (Sunderland grades II-V), Wallerian degeneration occurs distal to the injury site:
Figure: (a) Normal nerve; (b) Wallerian degeneration - breakdown of axon and myelin distal to injury; (c) Phagocytosis by macrophages recruited by Schwann cells; (d) Axonal regeneration and remyelination guided by bands of Büngner.
Key steps:
- The axon and myelin distal to the injury break down within 24-72 hours
- Schwann cells dedifferentiate and form bands of Büngner - tubes guiding axon regrowth
- Macrophages recruited by Schwann cells scavenge myelin debris
- Regenerating axons grow at approximately 1 mm/day (or 1 inch/month) from the proximal stump
- Proximal nerve also undergoes retrograde degeneration back to the nearest node of Ranvier
- Neurotropism (guided by growth factors and the extracellular matrix) directs the regenerating axon toward its target (Bailey & Love's Short Practice of Surgery, p.48)
5. Causes of Nerve Injury
| Mechanism | Example |
|---|---|
| Compression | Tourniquet, cast, external pressure |
| Traction/stretch | Brachial plexus during shoulder dislocation |
| Laceration | Knife wound, iatrogenic (scalpel) |
| Crush | Fractures, industrial injuries |
| Ischemia | Prolonged tourniquet, compartment syndrome |
| Injection injury | Intramuscular injection near sciatic nerve |
| Radiation | Post-radiation plexopathy |
6. Clinical Features
- Motor: Weakness or paralysis in the distribution of the nerve
- Sensory: Numbness, paraesthesia, hyperaesthesia in the cutaneous territory
- Autonomic: Loss of sweating, vasomotor changes
- Tinel's sign: Percussion over the site of nerve injury (or regenerating front) produces electric shock/pins-and-needles sensation distally - useful to track recovery
7. Investigations
- Nerve conduction studies (NCS): Identify conduction block (neuropraxia) vs. axonal loss; after transection, conduction persists for up to 72 hours until Wallerian degeneration is complete
- Electromyography (EMG): Fibrillations and fasciculations confirm denervation; begin at 14 days post-injury; voluntary motor units on EMG indicate reinnervation. Optimally ordered 3 weeks post-injury to differentiate neuropraxic from more severe injuries
- High-resolution ultrasound: Assesses nerve continuity and fascicular architecture; useful in the acute setting
- MRI neurography: Best for deep or proximal nerve injuries (brachial/lumbosacral plexus); visualizes edema, neuroma, and nerve-in-continuity lesions (Sabiston Textbook of Surgery)
8. Management
Conservative (Non-operative)
- Indicated for neuropraxia and many axonotmesis injuries
- Expectant management with physiotherapy and splinting
- Recovery of neuropraxia expected within 6 weeks; axonotmesis may take months depending on distance to target
- Serial EMG/NCS at 3 weeks and 3-6 months to monitor recovery
- Radial nerve palsy with humeral shaft fracture: initial non-operative management preferred; spontaneous recovery in majority (Rockwood & Green, p.1440)
Operative
Indicated for:
- Complete transection (neurotmesis), sharp laceration injuries
- No signs of recovery at 3-6 months
- Nerve entrapment, concomitant vascular injury, open fractures, gunshot wounds
Surgical options:
| Technique | Indication |
|---|---|
| Primary neurorrhaphy (direct repair) | Tension-free approximation; sharp wounds; gaps <5 mm |
| Nerve graft (autograft) | Gaps >5 mm; sural nerve most common donor |
| Nerve conduit | Gaps <10 mm; avoids donor site morbidity |
| Nerve transfer | Proximal injuries with long reinnervation distances; reroutes expendable donor nerve |
| Tendon transfer | Late presentation; irreversible muscle fibrosis (>12-18 months) |
Principles of repair: tension-free anastomosis; operating microscope; 8-0 to 10-0 monofilament suture; epineural or grouped fascicular repair. Reinnervation of muscle before 12-18 months is necessary to avoid irreversible atrophy. (Sabiston, p.808)
9. Motor Recovery Grading (MRC Scale)
| Grade | Description |
|---|---|
| M0 | No contraction |
| M1 | Perceptible contraction of proximal muscles |
| M2 | Perceptible contraction of proximal and distal muscles |
| M3 | Contraction against gravity |
| M4 | Contraction against resistance |
| M5 | Full recovery |
The minimum functional goal is M3 (antigravity strength). (Sabiston Textbook of Surgery)
10. Prognosis
- Neuropraxia: Excellent - full and spontaneous recovery within hours to 6 weeks
- Axonotmesis: Good if the level of injury is distal; recovery follows Wallerian degeneration at ~1 mm/day; less predictable with higher-grade injury
- Neurotmesis: Poor without surgery; even with repair, complete functional recovery is rarely achieved. Factors affecting prognosis include patient age, level of injury, mechanism, time to repair, and gap length
Sources: Bradley and Daroff's Neurology in Clinical Practice, Table 64.1 | Sabiston Textbook of Surgery, Table 41.5 and pp.806-809 | Bailey & Love's Short Practice of Surgery 28th Ed., p.48 | Rockwood & Green's Fractures in Adults 10th Ed., p.1440
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