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Spinal Cord Monitoring

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Topic updated on 08/27/16 10:37am
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)
      • MEP (motor evoked potentials)
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 coricospinal 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 with respect to 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 alone of a surgical instrument to the nerve root will lead to "burst activity" which carries no clinical significance
      • more significant injury or traction to a nerve root will lead to "sustained train" activity, which may carry clinical 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
      • therefore, an electrically stimulated pedicle screw that is confined to bone will not stimulate the nerve root
      • if, however, 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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Qbank (2 Questions)

TAG
(SBQ12.2) Which of the following statements regarding Somatosensory Evoked Potenitals (SSEPs) as a method to detect neurologic injury during spinal deformity surgery is true? Topic Review Topic

1. SSEPs are not reliable with respect to monitoring the integrity of the anterior spinal cord pathways
2. The minimum criteria for determining potentially significant SSEP changes include a 10% decrease in amplitude, and a 50% increase in latency.
3. Signal initiation is performed with transcranial stimulation of the somatosensory cortex
4. SSEPs are more sensitive to the effect of anesthesia than MEPS (motor-evoked potentials)
5. They monitor the integrity of the anterior spinothalamic tract

PREFERRED RESPONSE ▶
TAG
(OBQ06.233) A 75-year-old female with cervical spondylotic myelopathy is undergoing a cervical corpectomy with placement of an interbody cage as seen in Figure A. Immediately following placement of the cage, there is a 75% decrease in the lower extremity somatosensory evoked potentials (SSEPs) and a loss in the motor-evoked potentials (tceMEPs). What is the next most appropriate step in management? Topic Review Topic
FIGURES: A          

1. Proceed with placing an anterior plate as planned
2. Correct the kyphotic deformity by placing a larger interbody strut cage
3. Remove strut cage and monitor for return of motor and sensory signals.
4. Observe for 20 minutes and then repeat motor and sensory neurophysiologic testing
5. Close the wound and proceed with an emergency MRI

PREFERRED RESPONSE ▶




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