anesthetic factors
-- hemodilution and induced hypotension
- no decrease in spinal cord flow with nitroprusside-induced hypotension
Mechanisms of spinal cord damage
compession to blunt trauma
stretching or disruption of neural elements
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