SpineSpecial Considerations/Special Considerations

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.

Assoc. Prof. Recep DINCER· Suleyman Demirel University Orthopaedics and Trauma Department
Apr 20, 2026

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% 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.

← All TopicsMore in Spine →