FRCA Notes

Neurophysiological Monitoring

The intermediate curriculum asks for knowledge of 'monitoring of spinal cord function under general anaesthesia' and 'operative spinal cord monitoring'.

Resources

  • Intra-operative neurophysiological monitoring is employed to minimise the risk of injury to nerve pathways during neurosurgical procedures
  • It helps diagnose nerve injury but also allows salvage of neural tissue before damage is irreversible

  • Mechanisms of such injury include:
    • Direct mechanical disruption of nerves or spinal cord from surgical instrumentation
    • Thermal injury from surgical coagulation
    • Pressure injury e.g. from patient positioning
    • Ischaemic injury from local or global hypoperfusion of the spinal cord

The ideal monitor

  • High sensitivity and specificity
    • False negatives can lead to unnoticed damage
    • False positives can lead to unnecessary intervention
  • Fast response time to changes in patient condition
  • Detects tissue injury early enough to allow therapeutic intervention and reversal of damage
  • Aids clarification of physiological targets
  • Provides prognostic value for subsequent care

Classification

  • Detection of spontaneous activity;
    • EEG
    • EMG

  • Measurement of evoked electrical response of a specific neural pathway
    • Somatosensory evoked potential (SSEP)
    • Motor evoked potential (MEP)
    • Brainstem auditory evoked potential (BAEP)


Type of Surgery Monitoring
Intracranial surgery involving blood supply e.g. AVM resection MEP | SSEP
Vascular surgery involving cranial blood supply MEP | SSEP
Posterior fossa/CPA/brainstem surgery BAEP
Surgery involving spinal cord e.g. decompression, scoliosis, spinal trauma, tAAA repair MEP | SSEP | EMG
Surgery close to peripheral nerves e.g. thyroid, parotid, vestibular schwannoma EMG


  • Transcutaneous stimuli applied to posterior tibial nerve in the popliteal fossa or at the ankle
    • Other sites include ulnar nerve or median nerve at the wrist

  • The impulse is transmitted via ascending sensory tracts (via dorsal columns)
    • Blood supply via paired posterior spinal arteries

  • The impulse is detected using peripheral (Erb's point), epidural or scalp electrodes
  • Typically smaller amplitude than MEPs
  • Not significantly affected by therapeutic levels of volatile agents

  • Injury indicated by either:
    • Decreased amplitude >50%
    • Increased latency >10%

  • Stimuli applied to the motor cortex, either:
    • Transcranially through the scalp
    • Directly through electrical stimulation of the brain

  • The impulse is transmitted via descending corticospinal / corticobulbar tracts
    • Blood supply via the single anterior spinal artery

  • Detected using epidural/intrathecal or muscle electrodes
    • Epidural or IT electrodes measure either D-waves (direct; negative peaks) or I-waves (indirect; positive peaks)

    • Muscles used typically include tibialis anterior, abductor hallucis, vastus medialis or the thenar muscles
    • Electrodes measure effect as compound muscle action potentials; CMAPs

  • Injury indicated by complete loss (absence) of CMAPs
    • Transient loss may not indicate nerve injury

  • Monitor the vestibulocochlear nerve and brainstem function
  • Acoustic stimulus delivered by a device at the ear canal
  • Response recorded by an electrode placed at the mastoid or ear lobe


Drug Effect on SSEP Effect on MEP
Propofol (at common doses) - -
Barbiturates - Suppressed
Benzodiazepines Unchanged at pre-med. dose -
Volatile agents Suppressed at >1 MAC Suppressed at >0.5 MAC
Nitrous oxide Suppressed Suppressed
Opioids - -
Ketamine Enhanced Enhanced
NMBAs - Prevent CMAP recording, but not MEP transmission


  • Any loss of neuromonitoring should be promptly communicated by the neurophysiologist and a multidisciplinary approach to treatment taken
  • The use of a checklist to guide the multidisciplinary response has been advocated

Management

  • Neurophysiologist
    • Investigate technical reasons as to why there may be a loss of signal

  • Surgeon
    • Stop any manipulation
    • Assess surgical field
    • Evaluate ± reverse recent interventions

  • Anaesthetist
    • Ensure adequate perfusion pressure by raising MAP (e.g. >85mmHg)
    • Exclude hypoxia, hypercapnoea, hypothermia or anaemia
    • Ensure no drug cause such as volatile agent >0.5MAC, use of ɑ2-agonists, use of NMBA or propofol accumulation

  • If measures fail to improve the signal loss; urgent imaging, a wake-up test or curtailing the procedure may be required

  • Stimulation of muscles for mastication
    • Bite injury/airway occlusion
    • Bite blocks required

  • Patient movement if high current used, especially during MEP
    • Can cause surgical interference
    • Can interfere with monitoring e.g. saturations, arterial line

  • Bleeding from electrode placement sites