9 Neurophysiologic Monitoring in Spine Surgery
I. Key Points
– Somatosensory evoked potentials (SSEPs). Monitor the electrophysiologic integrity of the dorsal column–medial lemniscus pathway. The vascular supply of this tract in the spinal cord is predominantly from the paired posterior spinal arteries. Taken as a single modality, SSEPs monitor only the dorsal aspect of the spinal cord.
– Motor evoked potentials (MEPs). Monitor the electrophysiologic integrity of the corticospinal tract. The vascular supply of this tract in the spinal cord is from the single anterior spinal artery. MEPs monitor tracts in the ventral aspect of the spinal cord.
– Spontaneous electromyography (sEMG). The measurement of spontaneous electrical activity within a specific monitored muscle. Reflects neurotonic discharges within the muscle caused by mechanical, thermal, or metabolic irritation of the nerve or nerve root. Useful for monitoring nerve roots and peripheral nerves.
– Triggered electromyography (tEMG). The measurement of electrical activity within a specific muscle caused by electrical stimulation of the nerve, usually by stimulation of a structure in proximity to the nerve, such as the pedicle, by way of pedicle screw stimulation, to evaluate for disruption of the pedicle wall. Also allows discrimination of nervous tissue from non–nervous tissue during surgery for spinal cord tumors or the release of tethered spinal cord.
II. Essentials of Neuromonitoring
– SSEPs
• The stimulation of mixed sensory and motor fibers caudal to the region of the spinal cord at risk, together with the recording of signals rostral to the region of spinal cord at risk. The most commonly stimulated nerves are the median and ulnar nerves for the upper extremity and the posterior tibial and peroneal nerves for the lower extremity. Responses are then monitored over the dorsal neck and scalp.
Technique
Electrodes are placed over the ulnar nerve at the wrist and the posterior tibial nerve at the level of the medial malleolus.
One method involves a constant current of 15 to 25 mA for the ulnar nerve and 25 to 35 mA for the posterior tibial nerve, provided via a square wave pulse of around 4.7 times per second. The duration of the pulse is between 10 ms and 2 ms.1 The intensity of stimulation is based on the maximal-amplitude response for a given patient.
Averaging of signals continues until a clear, reproducible waveform is identified. Supramaximal stimulation results in the activation of axons of both the dorsal column and spinothalamic pathways.
The largest contribution to the signal is from the dorsal column due to the presence of the A-α and A-β fibers, the largest and fastest-conducting of the sensory fibers.
Baseline recordings are obtained and evaluated immediately after the induction of anesthesia but prior to positioning. SSEPs are assessed again after positioning and every few minutes thereafter until completion of the surgery.
Alarm criteria are a 50% or greater decrease in amplitude and/or a 10% increase in latency.2
Advantage
Allows continuous monitoring of spinal cord integrity
Disadvantages
Delay in assessment caused by signal averaging
Assesses only sensory pathways (predominantly dorsal column–medial lemniscus)
Not sensitive to indicators of motor pathway or nerve root injury
Susceptible to signal degradation due to halogenated anesthetics, nitrous oxide, hypotension, and hypothermia
Signals undergo central amplification and can retain amplitude despite nerve root injury
Technical pearls
SSEPs can be recorded from cortical or subcortical sources. Cortical sources have larger amplitude and may supply the only response when there is previous root damage; however, they are more susceptible to the effects of inhaled anesthetics.
Recordings at the level of the medulla reflect the nucleus gracilis and cuneatus with no intervening synapses between the sites of stimulation and recording. These recordings are more resistant to the effects of anesthetic agents.
Detailed examination of the dorsal column pathway prior to surgery should be performed because prior deficits can affect the ability to record accurate signals. This should include assessment of two-point discrimination, vibration, and position sense.
Alterations in anesthetic depth can affect the ability to obtain useful signals. This problem is minimized by the use of raw electroencephalography (EEG) by the anesthesia team to monitor anesthetic depth.
– MEPs
• Involve the transcranial stimulation of the corticospinal pathway with assessment of compound motor action potential at the level of the innervated muscle
Technique
Subdermal needle electrodes or electrode discs in contact with the skin are used for transcranial electrical stimulation.
Multipulse electrical current of 200 to 500 V is delivered using five to nine pulses 1.1 to 4.1 ms apart with pulse trains lasting around 50 μs.3 The resulting compound motor action potential (CMAP) at the level of the muscle is of high enough amplitude that signal averaging is not required. The resultant descending excitation of the corticospinal pathway and its generation of a CMAP is then detected via surface electrodes on the skin over the selected muscle groups or via the subdermal needle electrodes.
There are three types of monitoring options: recording at the muscle (CMAP), nerve (neurogenic MEP, CNAP), or direct spinal cord recording (D wave and I wave). The most frequently used monitoring utilizes electrodes at the level of specific muscle groups.
Interpretation of the response is based on the preliminary baseline; warning signs include a complete loss of response, a decrease in amplitude greater than 80%, an increase in threshold of greater than 100 V to elicit the CMAP response, and changes in the morphology of the response. The “all or nothing” response and the 80% decrease in amplitude are the two most commonly used methods of interpretation.2
Advantages
Allows assessment of corticospinal tracts
Allows for the option of increasing stimulation intensity to increase the size of the current field, increasing the distribution of stimulation to the cortex and subcortical fibers. This correlates with greater axonal recruitment to overcome low signal response in patients with preexisting deficits.
Stimulation trains can be used to increase the temporal summation at the level of the α motoneuron, thereby increasing the likelihood of achieving a response when pathologic conditions exist.
Disadvantages
MEPs do not allow for continuous monitoring.
MEPs cause muscle contraction during surgery, so the surgical team must be informed prior to each round of testing. Tongue laceration may result from forced contraction of facial muscles, requiring the placement of a bite block.
Obtaining MEPs is more technically demanding and has a lower success rate compared with SSEPs. Preexisting motor deficits significantly reduce the likelihood of obtaining useful signals, especially from the lower extremities.
Inhalant anesthetics decrease the pool of α motoneurons available for recruitment. Higher doses of propofol can cause suppression of α motoneurons. MEPs are affected by muscle relaxants, volatile anesthetics, and nitrous oxide. MEPs are also subject to anesthetic fade, which results in the need for increasing stimulation thresholds to achieve the same response in patients with prolonged exposure to anesthetic agents unrelated to dose effects.
MEPs are contraindicated in patients with deep brain stimulators or cochlear implants.
There is a slight risk of seizures with transcranial stimulation, although it has been estimated at less than 0.03%.3
Technical pearls
Minimize the use of inhalant anesthetics by selecting intravenous anesthesia regimens. MEPs are typically obtainable when less than half the minimum alveolar concentration (MAC) of inhaled anesthetic is used.
Alterations in anesthetic depth can affect the ability to obtain useful signals. This can be minimized by the use of bispectral index (BIS) EEG monitoring by the anesthesia team.
– Electromyography (EMG) is the measurement of electrical activity within a specific muscle.
• In sEMG, neurotonic discharges result in electrical activity within the innervated muscle as a result of pulling, stretching, or compression of the nerve or nerve root without any electrical stimulation by the surgeon.
Technique
Electrodes are placed in muscle of interest based on the innervating nerve root.
Manipulation of a nerve root or peripheral nerve results in an action potential that causes depolarization of the muscle at the neuromuscular junction, resulting in a CMAP.
Allows for assessment of electrical discharges within the innervated muscle that indirectly monitors the nerve root at risk
Electrical discharges of interest manifest as spikes, bursts, or trains. Trains are of concern during sEMG because they are a continuous run of neurotonic discharges that represent continued force on the nerve or nerve root. Spikes and bursts are discharges that can alert the surgeon to close proximity to the nerve root or nerve. The surgeon should also be alerted when trains of activity are observed.
Increasing frequency and amplitude of discharges represent increasing recruitment of muscle fibers with an increasing chance of nerve injury. Muscles for monitoring are chosen by the corresponding nerve root to maximize coverage based on the operated spinal level. This utilizes the anatomic redundancy of muscle innervation.
Advantages
Allows for continuous monitoring of the nerve root or peripheral nerve
Can serve as a warning of close proximity to the nerve root during retraction or manipulation when there is no direct visualization
Disadvantages
sEMG is subject to interference from high-speed drills, EEG leads, cautery devices, and other equipment.
Underlying neurologic conditions can affect the ability to obtain useful EMG signals, especially conditions affecting the muscle directly, such as myasthenia gravis, previous botulinum toxin therapy, or muscular dystrophy.
sEMG discharges serve as a warning since many innocent surgical maneuvers can produce discharges of the nerve root or nerve.
The absence of recorded muscle activity does not guarantee nerve integrity since acute nerve transection or avulsion may result in a loss of nerve-derived signals.
Technical pearls
sEMG does not allow the use of muscle relaxants or paralytics during surgery. Patient must show at least three out of four twitches for reliable monitoring.
To increase the ability to detect potential nerve root injury, multiple muscles with overlapping nerve root innervation are monitored for common injury levels. For example, the C5 nerve root in the cervical spine is assessed by concurrent monitoring of the deltoid and the biceps brachii muscles.
• tEMG. The measurement of electrical discharges within a given muscle as a result of electrical stimulation of the nerve root or peripheral nerve within the surgical field. A nerve root or nerve is stimulated, resulting in an action potential that causes depolarization of the muscle at the neuromuscular junction, in turn resulting in a CMAP in the innervated muscle. This is a useful technique to identify the course or location of nerves, demonstrate functional integrity, and identify tissue as nerve or not nerve.
Technique
Tissue can be stimulated directly by a probe in attempting to identify nerve versus other tissue or tumor. This technique is useful in spinal cord tumor resection as well as in surgery to relieve tethering of the spinal cord.
Pedicle screw stimulation is the most commonly employed technique during spinal surgery. This involves direct stimulation of a screw to identify breach of the bony cortex. This leads to indirect stimulation of the nerve root and a CMAP recorded in the corresponding muscle. An intact pedicle will have a greater electrical resistance to current, requiring greater levels of stimulation to achieve a response in the specified muscle. If the cortex of the pedicle is violated, the current will take the path of least resistance and lower levels of stimulation will be needed to result in a CMAP.
Lumbar spine stimulus values less than 10 to 11 mA and thoracic spine stimulus values below 6 to 8 mA are associated with pedicle cortical bone violation. Cervical stimulus values below 10 mA are associated with cortical bone violation and screw malposition.2
Advantages
Allows for assessment of screw placement during surgery
Allows for identification of lumbosacral nerve roots during surgery for tethered cord and may alter surgical strategy in up to 50% of cases4
Redirection of screws often closes off the cortical breach with bony fragments, resulting in useful information when the same pedicle screw is stimulated after repositioning.
Disadvantages
Subject to the same interference and signal degradation as sEMG
Technical pearls
This technique does not allow the use of muscle relaxants or paralytics during surgery. The patient must display at least three out of four twitches for reliable monitoring.
During pedicle screw stimulation the screw itself must be stimulated as opposed to the tulip; tulips are often made of different materials, and stimulation of the tulip may lead to false-negative assessment.
It is not typically possible to stimulate percutaneously placed pedicle screws. The metallic screw extensions do not allow for accurate EMG thresholds. Instead, stimulation is performed on a sheathed tap (a metallic tapping instrument housed in a plastic sheath) placed into the pedicle prior to placing of the screw.
III. Practical Issues and Outcomes during Monitoring for Spinal Surgery
The goal of intraoperative monitoring of the nervous system is to prevent injury during surgical treatment of spinal disease. This is often best accomplished by the use of multiple monitoring modalities so that the most complete assessment of neurologic functional integrity of the neural tissues applicable to the specific procedure is obtained. SSEPs are used to monitor the dorsal column–medial lemniscus pathway, MEPS are used to monitor the corticospinal pathway, and free-run sEMG is used for the assessment of the nerve roots and peripheral nerves. tEMG is used to assess screw placement or to guide resection of tumors or the filum terminale.
– Cervical spinal surgery. Spinal cord integrity is of major importance, so the combination of SSEPs and MEPs is used to assess for injury to the cord itself. If nerve root injury is of concern, sEMG can be added for additional safety. Although SSEPs as a single monitoring modality have a relatively low sensitivity, this is due to the fact that they do not assess the ventral cord or corticospinal tracts. Sensitivity and specificity of SSEPs have been reported as 52% and 100%, respectively, and sensitivity and specificity for MEPs were reported as 100% and 96% in the same study,5 suggesting that combined MEP and SSEP monitoring is the most comprehensive method of detecting neurologic injury of the spinal cord.
– Thoracic spinal surgery. Spinal cord integrity is the major concern, especially in light of the vulnerable blood supply to the mid-thoracic region. SSEPs and MEPs, when combined, provide assessment of the functional integrity of the spinal cord at this level with relatively high sensitivity and specificity.
– Lumbar spinal surgery. Below the level of the conus the nerve roots are the primary structures at risk, and sEMG and tEMG in combination with SSEPs provide assessment of the nerve root integrity. There is evidence to support multimodality monitoring with SSEPs and EMG, as this combination leads to an increase in sensitivity and specificity. One study reported a sensitivity and specificity of 28.6% and 94.7%, respectively, for SSEPs, compared with 100% and 23.7% for sEMG, in monitoring for neurologic injury during lumbosacral spinal surgery.6
– Surgery for tethered spinal cord. The success of detethering depends on the accurate identification of the lumbosacral nerve roots. This can be accomplished with multimodality monitoring. SSEPs have a very high specificity with the addition of sEMG and tEMG to compensate for the low sensitivity of SSEPs alone. Together, SSEPs, sEMG, and tEMG can provide near 100% specificity and sensitivity.4 Also, the urethral and anal sphincters can be directly monitored with EMG.
– Surgery for intramedullary spinal cord tumor. As with other surgeries in the cervical or thoracic spine, spinal cord integrity is the major concern here. Multimodality monitoring with SSEPs, MEPs, and both sEMG and tEMG can be used for the highest level of safety. Muscle and D wave MEPs have been shown to have a high degree of correlation with absence versus presence of postoperative motor deficits7 in spinal cord tumor surgery.
Common Clinical Questions
1. What neuromonitoring modalities are necessary for complete coverage of the spinal cord during surgery on thoracic levels?
2. Which neuromonitoring modality does not display a significant delay in obtaining signals?
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