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Electromyographic monitoring for pedicle screw placements
13.1
Introduction
Through the ages, various treatments for spinal deformity have evolved. In 1962, Harrington ushered in the revolutionary utilization of metallic, internal fixation devices for spinal deformity when he reported on the utilization of a distraction rod construct for the treatment of scoliosis. In 1982, Luque demonstrated how spinal deformity could be corrected by the utilization of segmental fixation and the application of transverse forces. In the thoracolumbar region of the spine, spinal instrumentation constructs consisting of hooks and rods eventually became the standard of care for the surgical management of degenerative spinal disease and traumatic insults. It later became very popular to use pedicle screws rather than hooks to hold the rods in place for the purpose of segmental transpedicular fixation in the lumbosacral region. The use of a transpedicular screw was first reported in 1959 by Boucher and was subsequently popularized by Roy-Camille and colleagues in the 1970s. At present, transpedicular screw fixation is widely and routinely used throughout the vertebral column including the lumbosacral and more recently the thoracic and cervical regions of the spine because of strong evidence for its effectiveness in providing rigid stabilization and the ability to achieve better correction . However the accurate placement of pedicle screws remains technically demanding, particularly for the thoracic region because of the smaller size and more complex morphology of thoracic pedicles. Multiple methods have been developed to facilitate accurate screw placement including fluoroscopy and stereotactic-guided techniques . Although these techniques slightly increase placement accuracy, they are also associated with increased radiation exposure to the patient and surgeon as well as increased operative time .
Proper placement of pedicle screws requires that a surgeon be very experienced and knowledgeable about the anatomical characteristics of all aspects of the spine. Despite continuing improvements in surgical and image-guided techniques, transpedicular screw insertion remains a demanding procedure due to the complex morphologic features of the pedicle. It is of utmost importance that screws be properly positioned because violation of a pedicle wall or vertebral body may result in fixation failure or injury to neural, vascular, or visceral structures. From a monitoring perspective, it is important to remember that the spinal cord ends in the conus medullaris at about the T12–L1 level of the spine. Therefore the placement of pedicle screws below these levels potentially places nerve root(s) rather than spinal cord function at risk. Ideally, pedicle screws should be placed such that they pass through the pedicle with about 1 mm to spare on both the medial and lateral walls without any breaches of either the pedicle or vertebral body walls. However when significant deformity is present, even a skilled surgeon can misplace screws. Because nerve roots tend to position themselves near the medial and inferior aspects of the pedicles as they exit the spinal canal through the spinal foramen, screws that are placed such that they protrude through the medial or inferior pedicle walls can cause nerve root irritation or injury. However at the thoracic and cervical levels of the spine, different neural elements are placed at risk by screw insertion . For thoracic and lower cervical vertebrae, the pedicles are smaller with more varied angles. As a result medially placed screws threaten the spinal cord rather than nerve roots. For lateral mass screws placed at the C3–C6 levels, the screws are directed away from the spinal cord but excessive screw length or medial angulation may threaten nerve roots and vertebral arteries.
In preparation for the placement of pedicle screws, markers are generally placed into the pedicles in order to visualize, via radiographs, the trajectories that the pedicle screws will take. These trajectories may place nerve roots at risk for injury because both lateral and anterior–posterior radiographs are subject to reading errors that can result in misidentification of undesirable medial placements of pedicle markers and screws . The acquisition of radiographs is then followed by removal of the markers and tapping of the marker holes. The resulting holes can then be palpated in order to detect breaches in the pedicle walls. This is followed by placement of pedicle screws. Placement of the screws can result in fractures of the pedicle, breakthroughs of the pedicle walls, and/or extrusion of pedicle fragments. Studies investigating the accuracy of transpedicular screw instrumentation, using various imaging-guided techniques, have reported a wide range of misplaced screws; many of these studies have been reported by experienced surgeons . The literature reports pedicle cortical perforation rates of 5.4%–40% even with experienced surgeons . Such events may go undetected unless the pedicle walls are visualized. However most surgeons are reluctant to routinely visualize screw placements unless this action is warranted. To visualize each pedicle, the surgeon must do multiple laminotomies, which is time consuming and these additional procedures by themselves could also affect postoperative outcomes. New imaging technologies, including intraoperative CAT scans and stereotactic imaging technology, aimed at reducing the incidence of misplaced screws continue to evolve. Despite the use of surgical inspection and such imaging techniques, misplaced screws have still been frequently associated with neurologic functional impairment. It is reported that the incidence of neurological deficits associated with misplaced screws ranged from 1% to more than 11% .
13.2
Monitoring pedicle screw placements
When pedicle screws were first introduced, existing electrophysiological techniques that had been used to monitor the functional integrity of other nervous system tissues during surgery were used to assess pedicle screw placements. These techniques included mixed nerve somatosensory evoked potentials (SEPs) and dermatomal somatosensory evoked potentials (DSEPs) . The measurement of electrical tissue impedance was also suggested as a means for assessing whether a breach in a pedicle wall occurred prior to the placement of pedicle screws . Later, other techniques that relied upon both spontaneous and triggered electromyogenic activity were introduced .
There are certain criteria that monitoring techniques should meet if they are to be widely and effectively utilized to assess pedicle screw placements and preserve nerve root function. First of all, implementation of the techniques should be practical (e.g., the techniques should not require special equipment or expertise). Otherwise, these factors will be a deterrent to their utilization. For economic and practical reasons, the techniques should utilize standard equipment that may already be used for monitoring purposes, the techniques should be easy to perform, and the anesthetic requirements for their utilization should not be exceptional. Second, the techniques must be effective. They should provide an instantaneous indication of nerve root irritation in order to prevent injury or further damage of a nerve root that is already irritated. They should also be able to detect the presence of a misplaced screw that is not causing nerve root irritation, but may have the potential to do so. Finally, utilization of the techniques should be cost-effective and should produce accurate results that make a difference in patient outcomes. Only techniques that have relied on spontaneous and/or triggered myogenic activity to assess pedicle screw placements satisfy these requirements. As a result these monitoring techniques have become widely utilized.
13.2.1
Electromyographic techniques
A myotome is a group of muscles that receive their motor innervation from a specific spinal nerve root. Myotomal activity can be spontaneously elicited by mechanical stimulation or triggered by electrical stimulation. Typically, the myotomal activity from several muscle groups is monitored at any given time and the activity is recorded using either surface or subdermal needle electrodes placed over or into various muscle groups. Recordings are commonly made from muscles of the lower extremities because screw placement is most often done in the lumbosacral area of the spine where lumbar and sacral nerve roots are placed at risk. The selection of the muscle groups to monitor is based on the spinal nerve roots that are at risk for irritation or injury. Muscles typically receive their innervation from several spinal levels although one spinal level generally predominates in terms of the amount of innervation it provides to any given muscle group.
Because of the mechanical advantages of pedicle fixation, the use of pedicle screw placement in the thoracic and cervical regions of the spine has gained popularity. In such cases, myogenic activity can be recorded from muscles innervated by cervical and/or thoracic nerve roots. Using this approach, monitoring of thoracic screw placement was first reported in 2001 . However it has been reported as having mixed success . In particular, the ability to detect medially misplaced screws using single-pulse stimulation techniques has been very variable and unlike screws placed in the lumbosacral region where only nerve root function is at risk, thoracic screws misplaced too medially can result in spinal cord injury. In order to detect such misplacements, a technique was developed that utilized constant current, high frequency, 4-pulse stimulus trains of variable intensity to stimulate a pedicle track. Using this technique, when a medial spinal breach occurs, the stimulation results in directly stimulating the spinal cord which results in electromyography (EMG) responses being elicited from lower extremity musculature . The neurophysiologic mechanism underlying train stimuli when applied to the spinal cord might be different as recently evidenced (see Chapter 11 : Brainstem mapping). Subsequent studies have reported on the efficacy of this technique .
A choice between several muscles is possible when selecting a muscle for monitoring purposes. Depending on the technique, the muscles commonly used for these purposes and their innervation are listed in Table 13.1 .
Table 13.1
Muscle groups commonly used to assess pedicle screw placements based on their innervation.
Reprinted from Toleikis JR. Electromyography. In: Koht A, Sloan TB, Toleikis JR, editors. Monitoring the nervous system for anesthesiologists and other health care professionals. 2nd ed. Springer; 2017. p. 103–24 with permission.
| Spinal region | Nerve root innervation | Muscle groups |
|---|---|---|
| Cervical | C2, C3, C4 | Trapezius, sternocleidomastoid |
| C5, C6 | Biceps, deltoid | |
| C6, C7 | Triceps, flexor carpi radialis | |
| C8, T1 | Abductor pollicis brevis, abductor digiti minimi | |
| Thoracic | T1, T2, T3, T4 | Intercostals |
| T5, T6 | Upper rectus abdominis, intercostals | |
| T7, T8 | Middle rectus abdominis, intercostals | |
| T9, T10, T11 | Lower rectus abdominis, intercostals | |
| T12 | Inferior rectus abdominis, intercostals | |
| Lumbar | L1 | Psoas |
| L2, L3 | Adductor magnus | |
| L3, L4 | Vastus medialis | |
| L4, L5 | Anterior tibialis | |
| L5, S1 | Peroneus longus | |
| Sacral | S1, S2 | Medial gastrocnemius |
| S2, S3, S4 | External anal sphincter |
13.2.1.1
Spontaneous or free-running electromyography
The responses that are elicited when nerve roots are mechanically or electrically stimulated are summed responses from many muscle fibers. These summed responses are known as compound muscle action potentials (CMAPs) and can be recorded using either pairs of surface or needle electrodes which are placed over or into the belly of a muscle. However it has been reported that intramuscular electrodes are preferred over surface electrodes for obtaining these recordings .
Monitoring recordings should be made continuously throughout a surgical procedure. Assuming that excessive amounts of muscle relaxants have not been administered to a patient during their surgery such that muscles are too relaxed, spontaneous or free-running activity will be elicited when mechanical activation results in nerve root irritation or injury. This activity can be recorded when train-of-four testing produces at least one twitch but having more than one twitch out of the train-of-four is desirable. The amplitude of the CMAPs is reduced by the muscle relaxants and it is possible that small CMAPs may not be detected if a patient is too relaxed. The activity will typically be elicited from one or more muscle groups depending on the activated nerve root, the muscle groups being monitored, and the placement of the recording electrodes on or in these muscle groups.
The EMG activity from each electrode pair is recorded using differential amplification and is filtered using a wide band-pass filter (30 Hz–3 kHz). When spontaneous myogenic activity is being recorded in order to detect mechanical nerve root irritation, the data acquisition system should be set to operate in free-run mode. In this mode, the sweep time is typically 1 second and any elicited activity can easily be visualized and evaluated. Typically, several channels of myogenic activity should be monitored simultaneously. This will depend on the number of channels available but should be six or more. When nerve root irritation occurs, the nerve root innervation will determine how many muscle groups are actually activated ( Fig. 13.1 ).
When interpreting spontaneous activity, there are several factors to take into consideration because not all nerve roots react in the same manner when they are irritated. First, healthy nerve roots and injured or regenerating nerves in-continuity react differently to mechanical forces. When mechanical forces are statically or rapidly applied to healthy nerve roots, they induce no nerve root activity or trains of impulses of short duration . When the same forces are applied to injured or regenerating nerves, they induce long periods of repetitive impulses. When interpreting intraoperative motor nerve root activity, it is important to understand the pathophysiological mechanisms of nerve root injury and the response of normal and pathological nerve to not only different types of mechanical force but also to electrical stimulation.
Normally, nerve root irritation is an infrequent occurrence. Therefore the recordings of spontaneous activity will generally be flat and consist of little or no CMAP activity. However when preexisting nerve root irritation has been present, the recordings will often consist of low-amplitude, low-frequency activity even prior to mechanical irritation. Mechanically elicited activity consists of either short bursts of activity that can last a fraction of a second or long trains of activity that can last up to several minutes. The short bursts of activity are common and are often associated with nerve root decompression and the result of tugging and displacement, irrigation, electrocautery, metal-to-metal contact, or application of soaked pledgets. Although attention should be paid to this type of activity, it is normally not a cause for alarm and is rarely indicative of a neural insult. However the long trains are more serious and may be indicative of neural injury. Train activity is commonly related to sustained traction and compression. The more sustained the activity, the greater the likelihood of nerve root damage. When such sustained train activity occurs, the surgeon must be notified so that corrective measures can immediately be taken. For both spinal and intracranial tumor removal surgeries, it has been reported that the presence of sustained spontaneous EMG activity correlates with postoperative motor deficits . The presence of such activity has also been used as a means for determining when and where spinal decompression is surgically required and even to determine the adequacy of decompression . On the other hand, the presence of continuous spontaneous activity during surgery for tethered cord syndrome has been reported to be a poor predictor of post-operative outcome with a sensitivity of 100% but only a specificity of 19% . These reported findings suggest that the monitoring of continuous spontaneous EMG activity and the understanding of its significance remain an evolving technique.
In general, spontaneous EMG has been found to be relatively insensitive to spinal nerve root injury. It is recommended that it best be used as an adjunct procedure and should never be used as a sole modality for spinal nerve root monitoring . When its significance has been questionable, it has been reported that the combined acquisition of motor evoked potentials (MEPs) and EMG activity provides a means for corroborating the significance of the EMG activity. An indication for the acquisition of an MEP would be when there is sustained or sudden loss of EMG activity . It has been reported that the complementary use of both these monitoring techniques provides a means for detecting and minimizing the incidence of both nerve root and spinal cord injuries .
13.2.1.2
Triggered electromyography
The technique of intraoperative EMG monitoring using indirect nerve root stimulation via stimulation of bone or spinal instrumentation to assess pedicle screw placements was first described by Calancie et al. in 1992 . It was introduced as a means for objectively monitoring the placement of pedicle screws in order to provide an objective basis for notifying the surgeon when a screw placement appeared to have breached the medial pedicle cortex and might result in nerve root injury. Testing was performed by first placing recording electrodes in muscle groups that received their innervation from those nerve roots at risk for injury. The placement of a pedicle screw was then tested by placing a stimulus probe in contact with the screw and increasing the pulsatile constant current stimulus intensity until EMG activity was reached or a predetermined maximum intensity was achieved. The stimulus parameters that they utilized were a 3.1 Hz stimulation rate with pulse durations of 200 µs, and a maximum intensity of 40 mA. Based on their experience, they reported that when screw stimulation resulted in no EMG responses at a stimulus intensity of 10 mA, this was consistent with a screw placement well enclosed within intact cortical bone. Calancie et al. reported that the technique was sensitive and reliable in the detection of pedicle wall perforations. Rose et al. later reported similar findings when using a constant voltage stimulus intensity of 20 V.
Indirect nerve root stimulation is the most widely used technique for monitoring pedicle screw placements. Some surgeons favor such testing during every aspect of pedicle screw placement. This includes testing of the probe used to make the initial hole into the pedicle for marker placement, the markers, the taps used to make the holes for the pedicle screws, the pedicle screw holes, and the pedicle screw placements themselves. Other surgeons may prefer to test only the screw placements. The assessment criteria are similar in all cases.
Several articles have been published by various investigators, which have reported on the efficacy of this technique . The published stimulation parameters that have been used by these investigators have sometimes varied from those originally used by Calancie et al. . They have included both constant current and constant voltage stimulation to assess screw placements. Although similar, the utilization of these two forms of stimulation is not equivalent. Voltage is the driving force that causes electrical charge to flow through the resistance or impedance of biological tissue. However it is the flow of electrical charge or current through an area of tissue (the current density) which causes a nerve or nerve root to depolarize. When using constant voltage stimulation, current density will vary with changes in tissue impedance whereas with constant current stimulation, it will not. When testing a pedicle screw’s placement, tissue impedance includes that of the pedicle and vertebral body bone in addition to the impedance of muscle, vascular tissue, and blood. Although the latter impedances probably remain relatively constant between individuals, bone density and therefore bone impedance is known to vary between individuals as a result of osteoporosis and other factors. Therefore it takes more or less voltage to cause the same current to flow in various individuals. As a result it would be expected that using constant voltage stimulation for testing pedicle screw placements would be more variable than constant current stimulation. Several investigators have already reported this . Constant current stimulation therefore appears to be a better choice for assessing pedicle screw placements.
As indicated earlier, various stimulation parameters and techniques have been used to electrically assess pedicle screw placements. Typically a ball-tipped probe of some type, such as a nasopharyngeal electrode, functions as the cathode and is placed within the pedicle screw holes and/or on hardware and a needle electrode is typically placed in muscle near the surgical site. The needle electrode is used as the anode and provides a return path for the stimulation current ( Fig. 13.2 ). Historically, rates of pulsatile stimulation have ranged from 1 to 5 Hz with pulse durations of 50–300 µs . Typically when testing, the intensity of the stimulation is gradually increased from 0 mA until a current threshold intensity is reached where a reliable and repeatable EMG response is elicited from at least one of the monitored muscle groups or a predetermined maximum stimulus intensity (such as 40 or 50 mA) is reached.
When testing a pedicle screw placement using the Calancie et al. technique, the current intensity that is needed to elicit an EMG or CMAP response simply provides an indication of whether a breach in the pedicle wall has occurred and if it has, whether the position of the screw could potentially cause nerve root injury. As indicated earlier, when Calancie et al. introduced this technique for assessing pedicle screw placements, they reported that screw stimulation that resulted in no EMG responses at a stimulus intensity of 10 mA (using a 200 µs pulse width) was consistent with a screw placement enclosed within intact cortical bone. If EMG responses were elicited at stimulus intensities lower than this predetermined “warning threshold,” the surgeon was advised to remove the screw and the pedicle screw hole was reexamined with a probe. This sometimes led to the discovery of previously undetected pedicle perforations. Depending on whether such perforations were acceptable, the screw was either reinserted, the screw position was revised, or the surgeon chose not to place a screw at that site. A “warning threshold” of 10 mA is often used to assess whether such a breach has occurred which is then verified either visually, via palpation, or from radiographs. Although other thresholds varying between 6 and 11 mA have also been recommended , the findings associated with the use of radiographs as a detection technique can be misleading. From my experience, radiographs have sometimes been suggestive of adequate screw placements but when these placements were tested using pedicle screw stimulation, low-stimulation thresholds resulted in visual inspection or a post-op CT, which revealed that the placement needed to be revised ( Fig. 13.3 ).
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