Nerve Conduction Studies and Electromyography
Core concepts
Understand the physiology behind nerve conduction studies: compound muscle action potential (CMAP) amplitude reflects the number of excitable motor axons, while conduction velocity reflects the fastest fibers. Sensory nerve action potentials (SNAPs) are small and require averaging. Know the distinction between demyelinating (slowed velocity, prolonged distal latency, conduction block, temporal dispersion) and axonal (reduced amplitude with relatively preserved velocity) patterns. Learn the needle EMG basics: insertional activity, spontaneous activity (fibrillations, positive sharp waves, fasciculations, myotonic discharges), and voluntary motor unit action potential (MUAP) morphology (amplitude, duration, recruitment). Understand late responses (F-waves, H-reflex) and their limits.
Traps
A common trap is attributing reduced CMAP amplitude solely to axonal loss when conduction block or dispersion could explain it. Remember that SNAP amplitude is more sensitive to axonal loss than CMAP in early disease because motor axons have collateral reinnervation. Temperature affects velocity (cooling slows it), so always consider warming. Distinguish myopathic (short, small, polyphasic MUAPs with early recruitment) from neuropathic (large, long, polyphasic MUAPs with reduced recruitment) patterns. H-reflex is a monosynaptic reflex, not a pure motor conduction measure.
Worked example / rule of thumb
If a nerve shows prolonged distal latency, marked slowing across a segment, and conduction block with preserved amplitude distally, the pattern is demyelinating. Rule of thumb: amplitude = axon number; velocity = myelin/fastest fibers. In demyelination, velocity falls disproportionately to amplitude.
Key points
- CMAP amplitude reflects motor axon number; velocity reflects fastest fibers.
- Demyelinating: slowed velocity, prolonged latency, block, dispersion.
- Axonal: reduced amplitude with relatively preserved velocity.
- Temperature and averaging affect NCS results; warm limbs and average SNAPs.
- Myopathic MUAPs are small/short; neuropathic are large/long with reduced recruitment.
- F-waves assess proximal segments; H-reflex is a monosynaptic reflex.