Spinal Cord Monitoring

Risk Factors

• that make neurologic injury more likely
  1. surgical factors
     --sublaminar wiring, spinal distraction, spinal osteotomy, skeletal traction
  2. pathophysiologic factors
     --severe and rigid curves, congenital scoliosis, kyphosis, postradiation curves, pre-existing neurologic
     deficits, neurofibromatosis
  3. anesthetic factors
     -- hemodilution and induced hypotension
     - no decrease in spinal cord flow with nitroprusside-induced hypotension

Mechanisms of spinal cord damage

1. compession to blunt trauma
2. stretching or disruption of neural elements
3. ischemia

• believed that the latter is the most common explanation
  • Dolan and colleagues --> experimental spinal distraction caused cord ischemia, which was already severe at the point that the SEP became significantly delayed and attenuated
  • Further distraction abolished the SEP as well as cord blood flow at the distraction site
• early detection and reversal of ischemia ---> crucial in diminishing or preventing neurologic injury

Spinal cord monitoring

• divided into two categories - clinical and electrophysiologic

Clinical testing

• physical examination
  • the most important form of monitoring pre- and postoperatively
• the wake-up test
• the ankle clonus test

Electrophysiologic testing

• measures the nervous system response to and ability to convey stimuli (typically electrical)
• described in terms of the stimulus site
  • somatosensory evoked potentials (SEP)
  • spinal evoked potentials (SpEP)
  • motor evoked potentials (MEP)
  • cerebellar evoked potentials (CEP)

The Wake-up Test ( Stagnara )

• lightening the anesthetic state to the point that the patient can respond to a command
  • asked to squeeze her hand --> demonstrating that she responding --> then to move her feet and toes
  • if unable to move feet --> distraction reduced and the test repeated until a safe level of distraction demonstrated
  • during scoliosis surgery stretch of neural & neurovascular structures leads to ischaemia
• gold standard of intraoperative spinal cord monitoring for scoliosis surgery
  • simple to perform & requires no additional equipment or personnel --> inexpensive and widely applicable
  • most importantly --> reliable test of motor function
• potential hazards
  
  • accidental extubation, dislodgment of orthopedic instrumentation, self injury, broncho-spasm, recall of intraoperative events and psychologic trauma, air embolism, and cardiac ischemia
    • use midazolam (Dormicum) as premed --> amnaesia
    • contraindicated in IHD
  • false negative tests
    
    • rare
      • isolated posterior cord injury
      • isolated nerve root injury
  • not applicable in young children, deaf, retarded

Somatosensory Evoked Potentials ( SEP )

• electrophysiologic response of the nervous system to sensory stimulation
  • from electrical stimulation at a peripheral nerve the signal travels via peripheral nerve through plexuses to nerve root to ipsilateral dorsal column and also spinothalamic and spinocerebellar pathways
  • the signal crosses over at the level of the brain stem and progresses rostrally to thalamus and on to thalamocortical projections on
    primary sensory cortex
  • stimulation --> median or post.tibial nerve
  • recording --> needle electrode in interspinous ligament, epior subdurally
    • "significant change from baseline"