Nerve Integrity Monitoring

Monitoring of nerve integrity:

• Involves applying a known stimulus and measuring the triggered response
  Ischaemia or injury is suggested by:
  • Change in response amplitude
    Usually ~50%.
  • ↑ response latency
    Usually ~10%; though some use up to 50% as threshold for intervention.
  • Total loss of signal
  • Need to ↑ stimulation voltage
• Is generally highly sensitive but non-specific
• May be used:
  • Intraoperatively any procedure where the cord is at risk
  • In ICU to evaluate a spinal cord injury
• Includes:
  • SSEPs
  • MEPs

Somatosensory Evoked Potentials

Method:

• Evaluates entire pathway including subcortical structures
  Deeper monitoring than achieved with EEG.
• Two stimulating electrodes applied to a nerve:
  • Median
  • Posterior tibial nerve
  • Common peroneal
• Monitor may be:
  • Over the sensory cortex
    Scalp divided into sections, with electrodes applied in a standardised way.
  • With epidural electrodes
    Tend to be difficult to place and may move intraoperatively.
  • Along the peripheral nerve
    E.g. at brachial plexus for median nerve.
• Evaluate proprioceptive signals
  Therefore evaluate dorsal column function.

Conduct of Anaesthesia

• Muscle relaxation improves signal:noise ratio
• Volatile anaesthesia and nitrous oxide have a dose-dependent effect on amplitude
  ⩽1 MAC is acceptable, but TIVA is better.
• Opioids have a minimal effect
• Clonidine, and dexmedetomidine have no effect
• Ketamine enhances the effect

Interpretation

• Significantly affected by muscle tremor
• Cord may be at risk if SSEP amplitude ⩽ 50%

Motor Evoked Potentials

Method:

• Short-duration high voltage stimuli applied to the motor cortex
  Corkscrew electrodes applied into scalp.
• Place a bite block
  Voltage stimulation may cause biting and tongue/tube damage.
• Obviously requires avoidance of muscle relaxation
• Measurement via needle electrodes in:
  • Tibialis anterior
  • Abductor hallucis
  • Vastus medialis
• Reference electrodes placed into the small muscles of the hands
• Evaluate motor signals
  Therefore corticospinal tract function.

Conduct of Anaesthesia

Require:

• Minimal muscle relaxation
• Avoidance of volatile anaesthesia
  ⩾0.5 MAC usually incompatible.
• Avoidance of deep anaesthesia
  Burst suppression reduces utility of monitoring.
• Ketamine enhances the effect

Interpretation

• Described as present or absent

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References

1. Nowicki RW. Anaesthesia for major spinal surgery. Contin Educ Anaesth Crit Care Pain. 2014 Aug 1;14(4):147–52.
2. So VC, Poon CCM. Intraoperative neuromonitoring in major vascular surgery. Br J Anaesth. 2016 Sep 1;117(suppl_2):ii13–25.