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Nerve Conduction Study

Nerve Conduction Study

Nerve conduction study (NCS) is an electrophysiological test that objectively evaluates the functional status of peripheral nerves by applying electrical stimulation to the peripheral nerves and analyzing the speed, amplitude, and form of the evoked response.

AT A GLANCE

At a glance

Nerve conduction testing is an essential test for diagnosing peripheral neuropathy, radiculopathy, and nerve entrapment syndrome (carpal tunnel syndrome, etc.). It consists of a motor nerve conduction test and a sensory nerve conduction test, and can distinguish between demyelination (reduced conduction velocity) and axonal damage (reduced amplitude) [1]. NCS abnormalities are identified in approximately 50% of diabetic neuropathy patients [2], and play a key role in subtype classification of Guillain-Barré syndrome [3].

Definition and Overview

Nerve conduction study (NCS) is an electrophysiological test that objectively measures the electrical conduction function of peripheral nerves. When a surface electrode is attached to a nerve and electrical stimulation is applied, the stimulation propagates along the nerve and a response is recorded in the muscle (motor nerve) or sensory receptor (sensory nerve) at the end.

The functional status of the nerve is evaluated by analyzing the latency, amplitude, conduction velocity, and waveform of this response.

Inspection type

motor nerve conduction test

The motor nerve is stimulated and the compound muscle action potential (CMAP) is recorded from the muscle controlled by the nerve.

The measurement indicators are as follows. - Distal latency: conduction time from the distal stimulation site to the muscle. - CMAP amplitude: reflects the number of activated motor axons - Motor conduction velocity: Calculated by dividing the distance between two stimulation points by the momentary difference. - F wave: evaluates conduction of the proximal part of the motor nerve (up to the anterior horn cells of the spinal cord)

Sensory nerve conduction test

Stimulate sensory nerves and record sensory nerve action potential (SNAP).

  • SNAP amplitude: reflects the number of activated sensory axons
  • Sensory conduction velocity: The speed of conduction of sensory nerves.
  • Antidromic and orthodromic tests

special inspection

  • H-reflex: Assessing the afferent and efferent reflex arcs of the tibia nerve. Useful for S1 radiculopathy.
  • F wave: evaluates motor nerve proximal conduction. Important in the early diagnosis of Guillain-Barre syndrome.
  • Repetitive nerve stimulation test (RNS): Diagnosis of neuromuscular junction disease (myasthenia gravis)

Interpretation of results

Demyelination pattern

This is a finding that appears when myelin sheath damage is the main lesion.

  • Significant decrease in conduction velocity (less than 70% of normal)
  • prolonged distal latency
  • Temporal dispersion: increased waveform width
  • Conduction block: CMAP amplitude decreases by more than 50% compared to distal during proximal stimulation.
  • Representative diseases: Guillain-Barre syndrome (AIDP), chronic inflammatory demyelinating polyneuropathy (CIDP), multifocal motor neuropathy

Axonal loss pattern

This is a finding that appears when the main lesion is damage to the axon itself.

  • Decreased CMAP/SNAP amplitude (reflecting decreased axon number)
  • Conduction velocity is relatively preserved (more than 70% of normal)
  • Representative diseases: Diabetic polyneuropathy, alcoholic neuropathy, toxic neuropathy, axonal Guillain-Barre syndrome (AMAN)

Focal entrapment pattern

This is when a nerve is compressed in a specific anatomical area.

  • Local reduction in conduction velocity through the entrapment site
  • prolonged distal latency (when entrapment is distal)
  • Representative diseases: Carpal tunnel syndrome (median nerve), cubital tunnel syndrome (ulnar nerve)

clinical application

carpal tunnel syndrome

Prolonged distal sensory latency and prolonged distal motor latency of the median nerve are key diagnostic findings. NCS has a sensitivity of about 85-90% and a specificity of about 95% for diagnosing carpal tunnel syndrome.

polyneuropathy

Distal predominant sensory and motor nerve conduction abnormalities are the electrophysiological characteristics of polyneuropathy. Distinguishing between axonal and demyelinating types is important in determining the cause.

Guillain-Barre Syndrome

NCS is essential for subtyping Guillain-Barre syndrome. Distinction between demyelinating type (AIDP) and axonal type (AMAN, AMSAN) is important in predicting prognosis and treatment planning. In the early stages of the disease, F-wave abnormalities may be the only finding, so repeat testing may be necessary.

radiculopathy

A pattern in which denervation is observed on electromyography while the sensory nerve conduction test is normal suggests radiculopathy. Because the dorsal root ganglion is located outside the intervertebral foramen, SNAP is preserved in lesions at the nerve root level.

Limitations of the test

  • Small fibers (C fibers, Aδ fibers) cannot be evaluated. If small fiber neuropathy is suspected, additional tests such as skin biopsy and QSART are required.
  • Normal values vary depending on the skin temperature of the test area (required to be maintained above 32°C), age, height, etc.
  • Since Wallerian degeneration is not complete within 2 to 3 weeks after symptom onset, axonal damage may be underestimated.

QUESTIONS

Frequently asked questions

Q01Does a nerve conduction test hurt?

There is a brief tingling sensation during electrical stimulation, and some people may feel uncomfortable. However, the level of pain is mostly mild, and there is no residual discomfort after the test. The test time is approximately 30 to 60 minutes, and no special anesthesia or pretreatment is required.

Q02What symptoms should I have a nerve conduction test for?

Testing is necessary when tingling, numbness, burning, or muscle weakness in the hands and feet persist. Major indications include cases where carpal tunnel syndrome is suspected (numbness in the hands at night), screening for neuropathy in diabetes, and differentiating the cause of limb paralysis (nerve roots vs. nerve entrapment vs. polyneuropathy) [1].

Q03Are nerve conduction tests and electromyography different?

Although they are different, they are mostly implemented together. Nerve conduction study (NCS) evaluates the electrical conduction function of peripheral nerves, and electromyography (EMG) inserts a needle electrode into the muscle to evaluate denervation findings and abnormalities in the muscle itself. By combining the two tests, the location, type, and severity of the lesion can be accurately determined.

Q04If the test results are normal, does that mean there is no problem with my nerves?

Not necessarily. NCS can only evaluate thick myelinated nerves (Aβ fibers), so small fiber neuropathy (C fiber, Aδ fiber lesions) may be normal on NCS. If the NCS is normal but there is pain or autonomic symptoms, an additional small fiber test (skin biopsy, QSART) should be performed [5].

Q05Are there any precautions to take before testing?

There is no need for any special fasting. To ensure proper electrode attachment, do not apply lotion or cream to the test area. Since a cold environment slows nerve conduction, the temperature in the examination room is kept constant, and if necessary, the extremities are warmed before testing [1].

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This article provides general medical information and does not replace an individual diagnosis or treatment plan. Please seek a medical assessment if symptoms persist.

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