Nerve Conduction Studies and Electromyography Flashcards
What is electromyography?
Clinical examination where impulses from
- large myelinated axons
- muscle fibres can be measured
Detects LOSS of function in PNS
Localise problem and severity
Cannot tell you the cause
What pathologies can you detect with EMG
On contraction
- myopathies - conditions that affect somatic muscles
- neuropathies - denervation, demyelination
- NMJ issues - single fibre EMG
At rest - normally electrically silent
- fibrillations - abnormal spontaneous activity from denervation or myopathy
- fasciculations
Myotonia - difficulty relaxing muscle
Nerve stimulation
- CMAP, patterns it can show
- RNS, patterns it can show
- SNAP, patterns it can show
CMAP - compound motor action potential
- denervation - small CMAP
- presynaptic NMJ (LEMS) - small CMAP that increases with repetitive nerve stimulation
- inexcitable muscle fibres - small CMAP
- demyelination - prolonged CMAP
RNS - repetitive nerve stimulation
- post synaptic (MG) - decreases with stimulation
- presynaptic (LEMS) - increases with stimulation
SNAP - sensory nerve action potential
- median, ulnar, radial, sural, superficial peroneal nerve
- detect lesions at DRG onwards => Wallerian degeneration after 10days detetctable
3 types of nerve injury and their severity
Neuropraxia - focal demyelination, axon intact
- compression
- recovery in months => good prognosis
Axonotmesis - Wallerian degeneration of axons distal to lesion (lost SNAP)
- severe/sustained compression, laceration
- axons regrow 1mm/day
Neurotmesis - nerve transected => all axons degenerate
-no recovery unless nerve repaired
The rate of CMAP and SNAP amplitude loss after a complete transection
Abnormalities in nerves take time to appear
Wallerian degeneration - 10-12 days to complete
-SNAP recorded from nerve => drops to 0 over this timeframe
Axonal transport stops immediately - 5-7 days for NMJs to fail
-CMAP recorded from muscle => drops to 0 over this timeframe
