Nerve Conduction Studies (NCS)

• Nerve conduction studies (NCS) of peripheral nerves are indicated when clinical assessment suggests nerve cell body, root, plexus, peripheral nerve axon, myelin sheath, neuromuscular junction, or muscle abnormality.

Learning Outcomes

• Observe the methods for nerve conduction studies (NCS) and electromyography (EMG).
• Outline the physiological basis of routine NCS and EMG.
• List the parameters assessed during routine NCS.
• State the clinical application and relevance of parameters assessed in routine NCS.

Nerve Conduction Study (NCS)

• Neuronal function is assessed electrophysiologically using nerve conduction studies, evaluating a nerve's ability to conduct electrical impulses.
• The action potential from an electrical impulse is recorded using special equipment.
• Waveform parameters are assessed to evaluate nerve functions.

Material

• NCS and EMG machines have components, including:
  • Stimulating electrodes (cathode and anode)
  • Ground electrodes
  • Surface recording electrodes (active and reference)
  • Electrode gel
  • Connecting cables
  • Digital box (main head box)
  • Computer with a monitor
  • Printer

Assessment of Motor Nerve Conduction

• Conduction of electric impulses along motor nerves is assessed by examining compound motor action potentials (CMAP).

Motor Nerve Conduction of the Median Nerve

• Motor nerve conduction of the median nerve is assessed frequently due to the high prevalence of carpal tunnel syndrome, which is a compressive neuropathy of the median nerve in the carpal tunnel at the wrist.

Procedure

• The recording electrode is placed on the abductor pollicis brevis (APB) muscle.
• The reference electrode is placed on the first metacarpophalangeal joint.
• Stimulation occurs about 7cm proximal to the recording electrode, between the flexor carpi radialis and palmaris longus tendons.
• A ground electrode is placed between recording and stimulating electrodes to prevent power-line noise.
• Stimulating current is gradually increased until the CMAP amplitude doesn't change.
• The latency and amplitude of the response are obtained.
• For conduction velocity, stimulate a proximal site (antecubital fossa over the brachial artery pulse).
• Proximal stimulation generates a similar CMAP on the APB, but with prolonged latency.
• Measure the distance between proximal and distal stimulating electrodes in mm.

Conduction Velocity Calculation

• Conduction Velocity = (Distance between proximal and distal stimulating electrodes) / (Difference between proximal and distal latencies)

Assessment of Sensory Nerve Conduction

• Sensory nerve action potential (SNAP) is measured directly from the sensory nerve using either antidromic or orthodromic recording.
• Stimulation at one site measures conduction velocity, unaffected by neuromuscular junctions (unlike motor nerve conduction).

Sensory Nerve Conduction of the Median Nerve

• Ring electrodes are placed on the index finger, which has sensory fibers from the median nerve.
  • These act as stimulating or recording electrodes, based on lab technique.
• Orthodromic recording involves stimulating the nerve at the digit and recording responses at the wrist on the median nerve.
• Antidromic recording involves stimulation at the wrist and recording from electrodes on the index finger.

Sensory Nerve Conduction Velocity

• Conduction velocity can me measured by:
  • Conduction Velocity = (Distance between stimulating and recording electrodes) / (Latency)

Additional Notes

• All parameters are calculated by software.
• Waveforms and parameters transfer to printable reports.
• Results' interpretation compares waveforms/parameters with normative data (age, gender, history, examination findings).
• The same technique assesses other long motor nerves.

Electromyography (EMG)

• Electromyography assesses muscle function, done by a neurophysiologist, and is indicated in disorders affecting skeletal muscles.
• Surface and needle electrodes record action potentials in muscle fibres.
• A ground electrode is placed distal to the recording electrode.
• A needle electrode is inserted in a selected muscle allowing observation of activity on screen.
• EMG signals are assessed by visual and auditory inspection.
• Neurophysiologists observe and listen to real-time EMG activity to interpret findings; experienced professionals may detect abnormalities before screen display.
• Signals can be recorded for later inspection.

Observed Activities

• Observed activities include:
  • Insertional activity
  • Resting activity
  • Single motor unit activity
  • Recruitment of motor units with ↑ voluntary contraction