Definition and Overview
A nerve conduction study (NCS) is an electrodiagnostic test that measures the speed and amplitude of electrical signals traveling through peripheral nerves. It is essential for diagnosing neuropathies, entrapment syndromes, and demyelinating conditions [1].
NCS is typically performed alongside electromyography (EMG), which assesses muscle electrical activity. Together, they are called electrodiagnostic studies or nerve conduction velocity (NCV) tests.
Testing Methods
Motor Nerve Conduction Study
Electrical stimulation is applied to a peripheral nerve at two or more points, and the resulting compound muscle action potential (CMAP) is recorded from the innervated muscle. Key parameters include distal latency, conduction velocity, and CMAP amplitude [2].
Sensory Nerve Conduction Study
Records sensory nerve action potential (SNAP) by stimulating and recording along purely sensory nerve segments. Orthodromic (physiological direction) or antidromic (opposite direction) techniques are used [2].
Late Responses
F-wave: assesses proximal nerve segments and anterior horn cells. H-reflex: evaluates the monosynaptic reflex arc, particularly useful for S1 radiculopathy diagnosis.
Interpretation of Results
- Prolonged distal latency + reduced conduction velocity = demyelination
- Reduced amplitude with normal velocity = axonal loss
- Conduction block (amplitude drop >50% between proximal and distal stimulation) = focal demyelination
- Temporal dispersion = acquired demyelinating neuropathy
Clinical Applications
Carpal tunnel syndrome: prolonged median nerve distal sensory latency is the most sensitive finding [3].
Peripheral neuropathy classification: differentiates axonal (diabetic) from demyelinating (CIDP, GBS) types.
Ulnar neuropathy: localizes compression site (elbow vs wrist) by conduction velocity changes across the elbow segment.
GBS subtyping: distinguishes AIDP (demyelinating) from AMAN (axonal) based on conduction patterns [4].
Patient Information
The test involves brief electrical stimulation that causes a tapping or tingling sensation. Each stimulus lasts milliseconds. The test takes 30-60 minutes depending on the number of nerves tested. No special preparation is needed, but patients should avoid applying lotion to the extremities on the test day. Results are available immediately for interpretation by the neurologist.