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Which of the following statements regarding Somatosensory Evoked Potenitals (SSEPs) as a method to detect neurologic injury during spinal deformity surgery is true?
SSEPs are not reliable with respect to monitoring the integrity of the anterior spinal cord pathways
The minimum criteria for determining potentially significant SSEP changes include a 10% decrease in amplitude, and a 50% increase in latency.
Signal initiation is performed with transcranial stimulation of the somatosensory cortex
SSEPs are more sensitive to the effect of anesthesia than MEPS (motor-evoked potentials)
They monitor the integrity of the anterior spinothalamic tract
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SSEPs are not reliable with respect to monitoring the integrity of the anterior spinal cord pathways. Anterior motor tract damage can occur without concomitant change in SSEPs. Of the listed statements, this is the only statement that is true.
With SSEPs, signal INITIATION comprises stimulation of the posterior tibial nerve behind the ankle or peroneal nerve at the fibular head (lower extremity) or median/ulnar nerve (upper extremity), and signal RECORDING involves transcranial recording of the somatosensory cortex. The SSEP components are mediated ENTIRELY by the dorsal columns (posterior columns) of the spinal cord (fasciculus cuneatus for upper limb SSEPs and fasciculus gracilis for lower limb SSEPs). With MEPs, signal INITIATION comprises transcranial stimulation of the motor cortex, and signal RECORDING is recording of muscle contraction at the extremity (EHL, soleus etc).
Devlin et al summarized that SSEPs provide (1) direct information regarding posterior columns, (2) indirect information regarding anterior motor tracts, (3) no information regarding individual nerve roots. Transcranial electric motor evoked potentials (tceMEPs) allow evaluation of lateral and anterior corticospinal motor tracts, spinal nerve roots, peripheral nerves, and nerve plexuses.
Weiss opined that SSEPs are minimally invasive and easy to record. For nerve root monitoring, EMG is reliable and instantaneous. He advised simultaneous SSEP and nerve root EMG for cervical and lumbosacral procedures. On the other hand, MEPs are technically demanding, controversial, and there is no standardized warning criteria to interpret significant changes in MEPs.
Illustration A shows cortical SSEP recording involving stimulation of the posterior tibial nerve at the ankle with recording of responses from the somatosensory cortex. Illustration B shows tceMEP recording involving transcranial electrical stimulation of the motor cortex, triggering a signal down the corticospinal tract and activating the anterior horn motor neuron.
Answer 2: 50% decrease in amplitude, 10% increase in latency are considered potentially significant changes that require corrective action.
Answer 3: Signal initiation involves stimulating a peripheral nerve. Signal recording is then performed transcranially at the somatosensory cortex (Illustration A).
Answer 4: SSEPs are less sensitive to the effects of anesthesia than MEPs.
Answer 5: SSEPs monitor the integrity of the posterior columns.
Devlin VJ, Schwartz DM
J Am Acad Orthop Surg. 2007 Sep;15(9):549-60. PMID: 17761611 (Link to Abstract)
Devlin, JAAOS 2007
Clin Orthop Relat Res. 2001 Mar;(384):82-100. PMID: 11249183 (Link to Abstract)
Weiss, CORR 2001
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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?
Proceed with placing an anterior plate as planned
Correct the kyphotic deformity by placing a larger interbody strut cage
Remove strut cage and monitor for return of motor and sensory signals.
Observe for 20 minutes and then repeat motor and sensory neurophysiologic testing
Close the wound and proceed with an emergency MRI
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.
Bose B, Sestokas AK, Schwartz DM
Spine J. 2004 Mar-Apr;4(2):202-7.. PMID: 15016399 (Link to Abstract)
Bose, SPINE 2004
Hilibrand AS, Schwartz DM, Sethuraman V, Vaccaro AR, Albert TJ
J Bone Joint Surg Am. 2004 Jun;86-A(6):1248-53. PMID: 15173299 (Link to Abstract)
Hilibrand, JBJS 2004
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