Spinal Cord Monitoring

Author: Derek Moore

Topic updated on 03/15/13 7:37pm

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 conduct electrcity poorly
  - therefore a electrically stimulated pedicle screw that is confined to bone will not stimulate the nerve root
  - if however there is a breach in a 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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(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 ▶

DISCUSSION: The clinical situation describes a patient who has neuromonitoring abnormalities following placement of an interbody cage. Loss of sensory and motor signals have been shown to be predictive of an injury to the spinal cord. As there was a direct temporal correlation with the placement of the cage, it should be immediately removed and followed by repeated neuromonitoring signal testing.

In a retrospective, single-surgeon study, Bose et. al reviewed SSEP and tceMEP readings during anterior cervical spine surgery. They concluded that SSEPs and tceMEPs can be used simultaneously to indirectly and directly monitor spinal cord motor function, respectively.

In a prospective cohort study, Hillibrand et al compared SSEPs to tceMEPs during anterior cervical spine surgery and found that tceMEPs are more sensitive and specific than SSEPs in monitoring motor tract injury. In addition, they showed decreases in motor signals by tceMEP were identified earlier than decreases in sensory signals by SSEPs.

A recent review article by Devlin discusses neuromonitoring in detail and includes relevant spinal cord anatomy and neuromonitoring modalities. tceMEPs provide direct monitoring of the corticospinal tracts. SSEPs allow for direct monitoring of the dorsal sensory columns and allow for indirect assessment of the ventral motor tracts, see Illustration A.

Illustrations: A

REFERENCES:

1. Bose B, Sestokas AK, Schwartz DM. Neurophysiological monitoring of spinal cord function during instrumented anterior cervical fusion. Spine J. 2004 Mar-Apr;4(2):202-7. PMID:15016399 (Link to Abstract)

2. Hilibrand AS, Schwartz DM, Sethuraman V, Vaccaro AR, Albert TJ. Comparison of transcranial electric motor and somatosensory evoked potential monitoring during cervical spine surgery. J Bone Joint Surg Am. 2004 Jun;86-A(6):1248-53. PMID:15173299 (Link to Abstract)

3. Devlin VJ, Schwartz DM. Intraoperative neurophysiologic monitoring during spinal surgery. J Am Acad Orthop Surg. 2007 Sep;15(9):549-60. PMID:17761611 (Link to Abstract)

Question Authors:

Terrill Julien MD, Patrick McCulloch MD, Derek Moore,

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Evidence & References Show References

Textbooks

Review of Orthopaedics, 6th Edition, Mark D. Miller MD, Stephen R. Thompson MBBS MEd FRCSC, Jennifer Hart MPAS PA-C ATC, an imprint of Elsevier, Philadelphia, Copyright 2012
AAOS Comprehensive Orthopaedic Review, Jay R. Leiberman. Published by American Academy of Orthopaedic Surgeons, Rosemont IL. Copyright 2009
Orthopaedic Knowledge Update 10, John M Flyn. Published by American Academy of Orthopaedic Surgeons, Rosemont IL. Copyright 2011
Hoppenfeld SP. Surgical Exposures in Orthopaedics: The Anatomic Approach. Lipponcott, Williams, and Wilkins, Philadelphia, PA, Copyright 2009
Orthopaedic In-training Examination (OITE) Questions 2004-2012, American Academy of Orthopaedic Surgeons, Rosemont IL. Copyright 2004-2012
Self-Assessment Examination (SAE) Questions 2004-2012, American Academy of Orthopaedic Surgeons, Rosemont IL. Copyright 2004-2012

Undefined

Bose B, Sestokas AK, Schwartz DM. Neurophysiological monitoring of spinal cord function during instrumented anterior cervical fusion. Spine J. 2004 Mar-Apr;4(2):202-7. PMID:15016399 (Link to Abstract)
Hilibrand AS, Schwartz DM, Sethuraman V, Vaccaro AR, Albert TJ. Comparison of transcranial electric motor and somatosensory evoked potential monitoring during cervical spine surgery. J Bone Joint Surg Am. 2004 Jun;86-A(6):1248-53. PMID:15173299 (Link to Abstract)
Devlin VJ, Schwartz DM. Intraoperative neurophysiologic monitoring during spinal surgery. J Am Acad Orthop Surg. 2007 Sep;15(9):549-60. PMID:17761611 (Link to Abstract)

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Spinal Cord Monitoring

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