Nerve Conduction Studies
Core concepts
Nerve conduction studies (NCS) quantify how well a peripheral nerve conducts: latency (time), amplitude (size of the response), conduction velocity (distance/time), and area/duration. Sensory studies measure the sensory nerve action potential (SNAP) recorded over the nerve or by near-nerve technique; motor studies measure the compound muscle action potential (CMAP) over the muscle. Key parameters: onset latency reflects the fastest fibers; amplitude reflects the number of excitable axons (and, for CMAP, neuromuscular transmission); conduction velocity is calculated from the distance between stimulation sites divided by the latency difference.
Understand the difference between orthodromic (physiologic direction) and antidromic (reverse) recording, and how temperature, age, and height shift normal values. Recognize the pattern of demyelination (prolonged latency, slowed velocity, conduction block/dispersion with preserved distal amplitude) versus axonal loss (reduced amplitude with relatively preserved velocity).
Traps
Questions often mix up latency versus amplitude as the marker of myelin versus axon. Watch for units (ms vs m/s), and remember velocity requires a segment, not a single latency. A common error is attributing a low CMAP only to axonal loss while ignoring conduction block or neuromuscular junction disease.
Worked example / rule of thumb
If stimulation at the wrist gives a CMAP onset of 3.0 ms and at the elbow 7.0 ms over a 200 mm segment, velocity = 200 mm / (7.0−3.0 ms) = 50 m/s. Rule of thumb: use the difference in latencies across a measured distance; never divide a single latency into a distance.