nerve conduction studies
/ NCS /
If you wanted to check a long electrical cable in a wall, you might zap one end and see what shows up at the other end, how fast it arrived and how strong it was. Nerve conduction studies do exactly that to a person's nerves. A small, brief electric shock is delivered through the skin over a nerve, and a recording electrode somewhere downstream catches the response. It feels like a quick tap or a snap of a rubber band, uncomfortable but brief and safe.
There are two main flavours. A motor study stimulates a nerve and records from a muscle it supplies; the muscle's response is called the compound muscle action potential, or CMAP. A sensory study stimulates a sensory nerve and records the nerve's own response, the sensory nerve action potential, or SNAP. From these waves the examiner reads three numbers: latency, how long the signal took to arrive; amplitude, how big the response was; and conduction velocity, how fast the signal travelled along the nerve. By stimulating at two points and timing the difference, the machine can calculate speed over a defined stretch of nerve.
Nerve conduction studies are the first half of a typical electrodiagnostic consultation, usually paired with needle electromyography. They shine at telling demyelinating problems (slow, with the insulation damaged) apart from axonal problems (small, with wires lost), and at pinning down the exact spot where a nerve is squeezed or injured, which is precisely the question a referring rehabilitation physician needs answered before planning treatment.
To test for carpal tunnel syndrome, the examiner shocks the median nerve at the wrist and again higher up the forearm, recording from a thumb muscle each time. If the signal crawls across the wrist segment but moves normally in the forearm, the slowdown is localized to where the nerve passes through the tunnel, fitting the clinical picture of a pinched median nerve.
Comparing speed across two segments localizes a focal slowdown.
Routine nerve conduction studies test only the largest, fastest, best-insulated nerve fibres. Conditions that damage only small fibres, such as some painful neuropathies, can leave a normal study, which does not mean nothing is wrong.