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Introduction
  • Spinal cord monitoring is a method to detect injury to the spinal cord during operative procedures.
    • most common forms are
      • EMG (electromyography)
      • SEP (somatosensory evoked potentials)
        • 25% sensitive, 100% specific
      • MEP (motor evoked potentials)
        • 100% sensitive, 100% specific
Anatomy
  • Spinal cord pathways
    • sensory (afferent)
      • dorsal column
      • spinothalamic tract
    • motor (efferent)
      • lateral corticospinal tract
      • ventral corticospinal tract
  • Blood supply
    • consists of
      • anterior spinal artery
        • primary blood supply of anterior 2/3 of spinal cord, including both the lateral corticospinal tract and ventral corticospinal tract
      • posterior spinal artery (right and left)
        • primary blood supply to the dorsal sensory columns
Sensory evoked potenitals (SEPs)
  • Function 
    • monitor integrity of dorsal column sensory pathways of the spinal cord
  • Technique
    • signal initiation
      • lower extremity usually involves stimulation of posterior tibial nerve behind ankle
      • upper extremity usually involve stimulation of ulnar nerve
    • signal recording
      • transcranial recording of somatosensory cortex
  • Advantages
    • reliable and unaffected by anesthetics
  • Disadvantages
    • not reliable for monitoring the integrity of the anterior spinal cord pathways 
      • reports exist in literature of an ischemic injury leading to paralysis despite normal SEP monitoring during surgery
  • Intraoperative considerations
    • loss of signals during distraction mandates immediate removal of device and repeated assessment of monitoring signals
Motor Evoked Potential (MEP)
  • Function
    • monitor integrity of lateral and ventral corticospinal tract of the spinal cord
  • Technique
    • signal initiation
      • transcranial stimulation of motor cortex
    • signal recording
      • muscle contraction in extremity (gastroc, soleus, EHL of lower extremity)
  • Advantages
    • effective at detecting a ischemic injury (loss of anterior spinal artery) in anterior 2/3 of spinal cord
  • Disadvantages
    • often unreliable due to effects of anesthesia
  • Intraoperative considerations 
    • loss of signals during distraction mandates immediate removal of device and repeated assessment of monitoring signals
Mechnical Electromyography (spontaneous)
  • Introduction
    • monitor integrity of specific spinal nerve roots
  • Technique
    • concept
      • microtrauma to nerve root during surgery causes deplorization and a resulting action potential in the muscle that can be recorded
      • contact of a surgical instrument with nerve root will lead to "burst activity" and has no clinical significance
      • significant injury or traction to a nerve root will lead to "sustained train" activity, which may be clinically significance
    • signal initiation
      • mechanical stimulation (surgical manipulation) of nerve root
    • signal recording
      • muscle contraction in extremity
  • Advantages
    • allows monitoring of specific nerve roots
  • Disadvantages
    • may be overly sensitive (e.g., sustained train activity does not neccessary reflect nerve root injury)
Electrical Electromyography (triggered)
  • Introduction
    • allows detection of a breached pedicle screw
  • Technique
    • concept
      • bone conducts electricity poorly
      • an electrically stimulated pedicle screw that is confined to bone will not stimulate the nerve root
      • if there is a breach in a pedicle, stimulation of the screw will lead to activity of that specific nerve root
    • signal initiation
      • electrical stimulation of placed pedicle screw
    • signal recording
      • muscle contraction in extremity
  • Advantages
    • allows monitoring of specific nerve roots
  • Disadvantages
    • may be overly sensitive (e.g., sustained train activity does not neccessary reflect nerve root injury)
 

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