Neurophysiological studies - nerve conduction studies (NCS)

What it measures

Measures conduction velocity, amplitude, and latency of electrical signals along peripheral motor and sensory nerves via surface stimulation and recording electrodes
Distinguishes axonal loss (reduced amplitude, relatively preserved velocity) from demyelination (reduced velocity/conduction block, relatively preserved amplitude)
Used for suspected peripheral neuropathy, entrapment neuropathy (e.g. carpal tunnel), radiculopathy (in conjunction with EMG), and demyelinating disease (Guillain-Barre, CIDP)

Pattern	Amplitude	Velocity	Classic cause
Axonal	Reduced	Normal/near-normal	Diabetic, toxic, most length-dependent peripheral neuropathies
Demyelinating	Preserved (early)	Markedly reduced, conduction block, temporal dispersion	Guillain-Barre syndrome, CIDP, hereditary demyelinating neuropathies (CMT1)

Guillain-Barre syndrome - NCS may be normal in the first few days; findings evolve over 1-2 weeks (prolonged F-waves and H-reflexes are often the earliest abnormality, reflecting proximal/root involvement)
Carpal tunnel syndrome - prolonged median sensory and motor distal latency across the wrist, with normal conduction proximal and distal to the compression site
Length-dependent axonal neuropathy (e.g. diabetic) - legs affected before/worse than arms, sensory before motor - the "dying-back" pattern from longest axons first

Confirms/localises a clinical diagnosis and can grade severity (relevant to prognosis in Guillain-Barre and entrapment neuropathies for surgical decision-making)
NCS and EMG together (rather than either alone) are needed to fully characterise most neuromuscular presentations

Diabetic peripheral neuropathy, Guillain-Barre syndrome/CIDP, carpal tunnel syndrome, Charcot-Marie-Tooth disease

Can be repeated to track recovery (e.g. after nerve injury/repair) or progression (e.g. monitoring CIDP treatment response)

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