Investigations
Nerve conduction studies
- Response recorded from nerve (SNAP) or muscle (CMAP)
- Latency and conduction velocity reflect myelin function
- Amplitude reflects the number of functioning axons
- Demyelination causes prolonged latency, slowed velocity, temporal dispersion and conduction block
- Axonal loss reduces amplitude with relatively preserved velocity
- F waves and the H reflex (S1) assess proximal segments
Electromyography
- Insertional activity increased early in denervation and reduced in fibrosed muscle
- Fibrillation potentials and positive sharp waves at rest indicate denervation
- Motor unit potentials assessed for recruitment and morphology
- Small polyphasic nascent units signal reinnervation before clinical recovery
- Large, long duration polyphasic units follow as collateral sprouting matures
Timing
- Distal segment conducts normally immediately after division
- Motor amplitudes fall over the first week, sensory amplitudes slightly later
- Fibrillation appears at 10 days to 3 weeks, earlier in muscles near the lesion
- Baseline study most informative at 3 to 4 weeks
- Repeat at about 3 months looks for nascent units
- Early study documents preserved voluntary units, proving an incomplete lesion, and gives a medicolegal baseline
Limitations
- Operator dependent and affected by temperature, age, oedema, obesity and tolerance
- Small unmyelinated fibres are not assessed, so small fibre neuropathy can coexist with normal studies
- A normal study does not exclude early compression neuropathy
- Martin-Gruber anastomosis can confuse interpretation
Differential Diagnosis
- Neurapraxia shows conduction block across the lesion, normal distal amplitudes and no fibrillation
- Axonotmesis and neurotmesis show lost distal amplitudes and fibrillation
- Early studies cannot separate Sunderland grades II to V, so serial studies guide management
- Preserved SNAPs in an anaesthetic limb indicate a preganglionic root avulsion
- Paraspinal fibrillation indicates a root level lesion, as dorsal rami supply these muscles
- Short head of biceps separates a sciatic lesion from a common peroneal lesion at the fibular neck
Management
- Intraoperative nerve action potentials across a neuroma in continuity guide surgery
- A recordable response shows regenerating axons crossing the lesion and favours neurolysis
- An absent response favours resection and grafting
Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 10 October 2026.