Neuromonitoring

Types of IONM 

• Somatosensory Sensory Evoked Potentials 
  • Continuous or signalled 
    • INITIATION 
      • UE: median or ulnar 
      • LE: post tib nerve or peroneal nerve 
    • RECORDING  
      • Transcranial somatosensory cortex 
  • Can use with NM blockade (not sensitive to anesthesia) 
  • Advantages 
    • Reliable, unaffected by anesthetics 
  • Disadvantages 
    • Nerve roots not checked 
    • Delay due to summation 
  • Tests 
    • Dorsal Columns 
  • Loss if: 
    • Impacts dorsal column 
• Motor Evoked Potentials 
  • Sensitive 
    • INITIATION 
      • Transcranial stimulation of motor cotex 
    • RECORDING  
      • Gastroc, soleus, EHL muscle contraction 
  • Very sensitive to ischemia 
    • Perfusion (Anterior Spinal Artey – anterior 2/3 cord) 
  • Disadvantages 
    • No NM blockade, strict anesthesia protocol 
  • Tracts Monitored: 
    • Spinothalamic 
    • Lateral Corticospinal 
• Spontaneous EMG 
  • For lumbar/ nerve root 
    • INITIATION 
      • Microtrauma depolarizes root; action potential 
      • BURST activity – contact 
      • SUSTAINED activity – traction 
    • RECORDING 
      • Muscle contraction 
  • Real time 
  • Disadvantages 
    • Sensitive to false positives 
    • No NM blockade allowed 
• Triggered EMG (Screw Testing) 
  • Bone conducts electricity poorly; breach will allow electrical conduction 
  • Testing  
    • INITIATION 
      • Electrical stimulation  
    • RECORDING 
      • Muscle contraction in extremity 
      • Threshold <8 

Events and Responses 

• 50% drop in amplitude or inc latency by 10% 
• Perfusion IONM changes 
  • MEP changes 
  • Fix: perfusion 
• Direct Trauma 
  • Unilateral change to MEP 
  • Fix: remove the offender 
    • If recoverable, MEP recovers in 20 minutes in 2/3 of cases  

Motor (CS) and Dorsal columns cross at pyramids 

• Ipsilateral signal change 

ST crosses 2 in cord 2 levels up