Spinal Cord Monitoring
Spinal cord monitoring is an essential intraoperative tool used to prevent neurological injury during spinal surgery. The main modalities include somatosensory evoked potentials (SEP) for dorsal column function, motor evoked potentials (MEP) for corticospinal tracts, and electromyography (EMG) for nerve root integrity. SEPs are reliable and anesthetic-resistant but limited to sensory pathways, while MEPs are highly sensitive to anterior spinal ischemia yet affected by anesthesia.
EMG, both spontaneous and triggered, helps identify nerve irritation or pedicle screw breaches in real time. A >50% reduction in signal amplitude or latency prolongation indicates potential cord compromise requiring immediate correction. Combined multimodal monitoring significantly improves intraoperative safety and postoperative neurological outcomes.
SPINAL CORD MONITORING
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
anterior spinal artery
primary blood supply to 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 POTENTIALS (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 involves stimulation of ulnar nerve
signal recording
transcranial recording of somatosensory cortex
Advantages
reliable and unaffected by anesthetics
administering propofol with ketamine intravenously is recommended
neuromuscular blocking agents do not affect the SEP
Disadvantages
not reliable for monitoring the integrity of the anterior spinal cord pathways
reports exist of an ischemic injury leading to paralysis despite normal SEP monitoring during surgery
changes in body temperature, blood pressure, circulating blood volume, arterial blood oxygen saturation, and intracranial pressure influence the SEP
Intraoperative considerations
loss of signals during distraction mandates immediate removal of device and repeated assessment of signals
decrease in amplitude of 50% and/or 10% prolongation in latency is considered a significant change
changes should be confirmed by at least three recordings.
When the wave pattern suddenly changes, the following factors should be checked:
· The surgical procedure, accidental lesion to the spinal cord, aggressive distraction, derotation, etc.
· Hardware-related issues, electrode dislodgment, cable lesion, and amplifier and stimulator problems. If these issues occur, the artifact pattern is affected.
· Changes in the volume of the anesthetic agent and neuromuscular blocking agent.
MOTOR EVOKED POTENTIALS (MEP)
Function
monitor integrity of lateral and ventral corticospinal tracts 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
>100 V increase in threshold is suggestive of an early injury
>50% decrease in MEP amplitude is considered significant
ELECTROMYOGRAPHY (SPONTANEOUS)
Introduction
monitor integrity of specific spinal nerve roots
Technique
concept
microtrauma to nerve root during surgery causes depolarization 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 significant
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 (i.e. sustained train activity does not necessarily reflect a 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 a 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
thresholds <8 mA may be indicative of a breach
Advantages
allows monitoring of specific nerve roots
Disadvantages
may be overly sensitive (i.e. sustained train activity does not necessarily reflect a nerve root injury)
References:
Banoub M, Tetzlaff JE, Schubert A. Pharmacologic and physiologic influences affecting sensory evoked potentials: implications for perioperative monitoring. Anesthesiology. 2003;99(3):716–737.
Lall RR, Hauptman JS, Munoz C, Cybulski GR, Koski T, Ganju A, Fessler RG, Smith ZA. Intraoperative neurophysiological monitoring in spine surgery: indications, efficacy, and role of the preoperative checklist. Neurosurgical Focus (FOCUS). 2012;33(5):E10. doi:10.3171/2012.9.FOCUS12235.
Abbasi H, Moore DJ, Rajaeirad M, Zhan J. Screw stimulation thresholds for neuromonitoring in minimally invasive oblique lateral lumbar interbody fusion (OLLIF): a correlational study. Cureus. 2024;16(6):e62859. doi:10.7759/cureus.62859.