focal neuropathy and carpal tunnel syndrome
/ carpal tunnel /
A focal neuropathy is trouble at one specific spot on a single nerve, most often where the nerve passes through a tight tunnel of bone and ligament and gets squeezed, like a garden hose pinched under a heavy foot. The commonest example by far is carpal tunnel syndrome, where the median nerve is compressed at the wrist as it threads through a narrow passage beneath a ligament, causing tingling and numbness in the thumb, index, and middle fingers, often worse at night.
Electrodiagnosis is well suited to this because it can compare the nerve's speed right across the suspected pinch point against its speed elsewhere. In carpal tunnel syndrome the classic finding is that the median nerve conducts normally in the forearm but slows down or is delayed precisely across the wrist segment, while a neighbouring nerve in the same hand stays normal as a control. Sensory fibres are usually affected first, so a slowed or small median SNAP across the wrist is often the earliest sign; in more advanced cases the motor latency lengthens and the thumb-muscle CMAP shrinks, and the needle exam may show denervation in the muscle at the base of the thumb.
This is one of the everyday bread-and-butter referrals in physiatry, and the study earns its place by confirming the diagnosis, grading its severity, and ruling out mimics such as a C6 nerve root problem in the neck that can feel similar. That grading guides whether a wrist splint and activity change might suffice or whether surgery is worth considering. It is worth stressing that the test measures the nerve, not the symptoms: a person can have real, bothersome carpal tunnel symptoms with a near-normal study, especially early on.
A typist with nightly hand tingling has a study showing the median sensory signal crossing the wrist much slower than the ulnar sensory signal in the same hand, while everything in the forearm is normal. The localized wrist slowing, against a normal neighbour as control, confirms carpal tunnel syndrome and grades it as mild, fitting a trial of a night splint before any thought of surgery.
Comparing two nerves in the same hand isolates the median slowing at the wrist.
A normal study does not exclude carpal tunnel syndrome, and an abnormal study does not prove a person's symptoms come from it; the result must be read together with the clinical story, because plenty of people with abnormal numbers have no complaints at all.