Intraoperative neuromonitoring and complex spine surgery

28.1

Introduction

While surgical interventions upon the spine have been performed since ancient times, there has been an exponential increase in the number of spine surgeries over the past 35 years. There are many reasons for this phenomenon: increasing longevity, changing expectations of the elderly population regarding physical activity and technical advances in several fields, from metallurgy to anesthesiology to radiological imaging, enabling more aggressive surgeries for patients who were not candidates initially. Certain procedures such as en bloc spondylectomy for primary spinal tumors or the surgical correction of cervical or adult degenerative deformity were impossible even 40 years ago; these are not only performed today but particularly surgery for adult spinal deformity is becoming increasingly popular . On the other hand, technical advances have also made possible the development and increasing popularity of so-called minimally invasive spine surgery, allowing for minimized tissue trauma and hoping to achieve improved outcome with faster recovery . An essential part in this revolution in spine surgery was played by the development of reliable implants to allow for spinal stabilization. These implants allowed for more aggressive decompressions, correction of alignment, and effective fusion in different surgical approaches to every segment of the spine, for a variety of indications.

As spine surgery evolved and became more aggressive, more popular, and arguably more effective, different technological resources were developed to assist the surgeon in performing the surgery itself and the rest of the care team to successfully care for the patient during the perioperative period. Examples of these resources range from imaging devices, such as computed tomography (CT) or magnetic resonance imaging (MRI), to perioperative care protocols such as complex deformity pathways . One of these tools is intraoperative neurophysiology monitoring (ION). Its utilization is still a matter of heated debate for the varied indications of spine surgery: it may range from a virtual standard of care, such as in surgery for pediatric or adult deformity surgery, to an option (such as in surgery for spondylotic cervical myelopathy) to unnecessary such as in simple decompressive procedures of the lumbar spine. There is considerable bias in the literature as to its value, depending significantly on the educational background, training, and academic opinions of whoever produced the literature . It also has very significant legal implications. We will succinctly review the literature and present our appraisal of the utility of ION for certain scenarios pertaining to complex spinal instrumentation, deformity correction, and the occipitocervical junction. These authors have no intellectual bias toward or against ION, and we will start this chapter by stressing our view that ION, as any other surgical adjuvant tool, can only help as much as the operator(s) behind it who analyzes the information provided and acts (or not) upon it.

28.2

Complex spine instrumentation and deformity surgery

Development of effective and durable spinal implants for a range of approaches and indications is one of the foundations of the development of modern spine surgery. The basic principle of spinal fixation is to stabilize vertebrae until solid osseous arthrodesis occurs, which may typically take upward of 6 or 12 months. If the osseous arthrodesis does not occur and the implant is continuously subjected to elevated stress, it will eventually fail, either at the bone–implant interface (loosening, pullout) or the implant itself (implant fracture, disconnections) . There are specific situations where fixations may be employed without inducing solid arthrodesis: these are typically restricted to trauma in which the primary spinal injury is expected to heal and bear physiologic loads eventually without the assistance of the spinal implant—it may even be removed afterward—or oncological applications with limited expected survival.

Implants can be classified in several manners, but one such way is an anatomical classification based upon the Denis’ “spinal columns” concept for the thoracolumbar spine: implants may involve one, two, or all three of the Denis’ columns. In 1984 Denis divided the thoracolumbar spine into three columns—anterior (anterior two-thirds of the vertebral body), middle (posterior third of the vertebral body and posterior longitudinal ligament), and posterior (facet joints and posterior elements)—in order to provide a systematic approach to spinal trauma. His theory was that injuries to a single column would be stable; his concepts of spinal stability have been widely extrapolated to degenerative and tumoral pathology and even to the cervical spine, where four columns are thought to exist . The first implants developed for use in the spine where adapted from general construction materials and employed a fixation interface limited to one of the Denis’ columns of the spine. Examples of such implants include the Harrington and Luque rods and the Cotrel–Dubousset system, when utilizing laminar or transverse process hooks or sublaminar wires, anterior cervical plating or anterior lumbar instrumentation such as the Kaneda rod system ( Fig. 28.1 ). Fixation could be employed in a nonsegmental (only at the cranial and caudal ends of the construct) or segmental (with intervening fixation points) manner, with segmental fixation obviously providing stronger support. By virtue of their one-column, usually posterior, fixation, however, the amount of vertebral manipulation was significantly limited, particularly in the axial and sagittal planes. These limitations became evident as their application increased and their use was attempted for pathologies other than trauma and adolescent idiopathic deformity. By the mid-1980s a few groups developed more or less simultaneously a fixation system that utilized the vertebral pedicles as the anatomical anchor . These were popularized as “pedicle screws,” initially being connected by plates and then by rods, and now consisting of the most widely employed method of intervertebral fixation. Being inserted posteriorly through the pedicle and into the vertebral body, they cross all three of Denis’ columns, and if the osseous quality will allow it, they can be manipulated in all three planes to provide correction of the most complex deformities ( Fig. 28.2 ).

In parallel to the development of different types of vertebral anchors, interbody implants have also continuously evolved and are now ubiquitous in spine surgery. Initially created as a tool to enhance arthrodesis—osseous surfaces submitted to axial loads induce the formation of more bone according to Wolff’s law—they were perceived to allow for intervertebral distraction, indirect decompression of the exiting nerve root and the canal itself, and the correction of deformity . These intervertebral implants were initially developed to be applied from an anterior approach (and continue to be so applied in the cervical spine) but have since evolved to be more commonly placed in the lumbar spine from a posterior approach.

When coupled together with spinal osteotomies, these implants allow for independent and segmental manipulation of the vertebrae. The deformed spine can then be repositioned into a more appropriate shape with a number of maneuvers such as three-point bending or cantilevering. With the advent of pedicle screw fixation, these maneuvers can put significant stress upon the neural elements and it becomes imperative that the surgeon ensures their integrity before the patient leaves the operating room. The general sequence of events in a deformity case would then be exposure, implant placement, decompression/osteotomies, cage placement (if necessary), and realignment. As in any surgical process, the surgeon relies on a number of different cues and strategies to ensure the process goes smoothly. The first and most important of those is obviously anatomy: visual and sometimes tactile examination of the field. If an implant is visibly misplaced or the spinal cord is visibly compressed, no additional information is necessary, and the surgeon should address it as soon as possible. Radiological input, commonly in the form of intraoperative two-dimensional fluoroscopy but more and more frequently with CT and computer-assisted navigation, also a mainstay of spinal surgery and coupled with anatomical examination, provides a morphological picture of the spine and neural elements. What is not addressed with these methods is the physiological aspect, which may be of crucial importance when in a borderline situation or when a structural problem is not evident to the surgeon. There is a risk of iatrogenic injury to the spinal cord with any surgical correction of spinal deformity, which can result in temporary or permanent neurological impairment. Most complications are associated with these corrections arise from ischemia and mechanical injury to the spinal cord with the correction of large curves. The risk of ischemia is greater with distraction than with compression as has been shown in animal models .

The first attempt to evaluate neurological function during surgery was the so-called wake-up test. Although popularized by Stagnara, it was initially described by Vauzelle in 1973 . Awakening the patient during surgery and verifying whether legs and arms move is the ultimate form of neurologic testing. It is a complicated, time-consuming process that can be most commonly employed only once during a procedure. All the spinal work must cease for a long period of time, and obviously no work may be performed and quickly checked again. Most importantly, neurological damage may have occurred at any point prior to the “wake-up” test, and by the time it is perceived, the neurological deficit may have become established. It has thus been largely substituted for continuous somatosensory-evoked potential (SEP) and motor-evoked potential (MEP) monitoring, the “wake-up” test being employed only with equivocal ION results, when ION is not available and only rarely as routine at the end of the procedure.

Nash et al. pioneered ION for spine surgery in 1977 with SEP . Continuous SEP monitoring, however, only evaluates a small portion of the spinal cord bundles and it is a relatively common occurrence that changes to the anterior tracts go undetected with SEP-only monitoring. The prototypical example is that of anterior spinal artery infarct. Even injuries to the posterior tracts will only become evident after several minutes of delay. Thus appropriate spinal cord monitoring for deformity surgery is composed of SEP and trans cranially electrically elicited MEP, with the addition of free-running electromyography (EMG) depending on the level being operated on. The downside of avoiding muscle relaxation is easily overcome by the benefit of MEP monitoring, which provides evidence of physiological disruption of the anterior tracts within seconds to a couple of minutes. Although not formally recommended for routine lumbar fusion especially below the level of the conus, if there is a deformity correction component to the surgery being performed, we would recommend its routine use because of the rare instance of tethering producing traction on the spinal cord, particularly in revision cases.

ION is not without its problems; false-positive changes in MEPs, and particularly SEPs, can happen for a variety of reasons and must be considered within the context of the patient’s prior neurological exam and stage of the procedure being performed. A severely diabetic patient who is positioned for surgery and then found to have reduced amplitudes of MEPs in lower extremities is likely at his or her baseline. It is important at the beginning of every case that the surgeon clearly establishes a “baseline” SEP and MEP recording with the neurophysiologist and deviations from this baseline, particularly if representing greater than 50% amplitude or if asymmetric (e.g., lower extremities only) are more clinically relevant. ION use in deformity cases is so attractive because of the possibility of reversing the correction maneuver that led to the ION change, thus offering a “fixable” change ( see Cases 1 and 2 below ). Albeit supported by limited evidence, strong recommendations are also in place for surgery for intramedullary tumors, in which a change in ION may dictate a decision to limit resection in order to preserve neurological function—thus offering the surgeon another cue to directly influence surgical course . On the other hand, ION use in situations where there is no possible reversal or no direct intervention to be performed, for example, SEP and MEP in decompressive operations or simple lumbar fusions without correction of deformity, is less attractive, not supported by available evidence or guidelines and may actually adversely affect surgical course due to false positives . There is debate whether there may be value in knowing beforehand that an injury has occurred, but surgeons must be careful to understand the context of ION change and possibility of a structural solution . Multimodal monitoring thus presents with elevated sensitivity and specificity for the correct indications . Finally, it is essential that every member of the operative team understands the value of and actions necessary in the event of changes in ION signals. We highly encourage surgical teams to develop their own ION change protocol and present our own with case examples at the end of the chapter.

An important point to consider and discuss with the patient preoperatively when performing MEPs is the issue of tongue and lips trauma—in rare cases, stimulation associated with MEPs may cause tongue lacerations or hematomas. Bite blocks are necessary in order to prevent tongue injury while monitoring MEPs, but there is no agreement as to which bite blocks are the best . The stimulation for MEPs can activate the temporalis and/or masseter muscle, and the prone position for a prolonged period of time may cause the tongue to swell. The mechanism of injury can involve stimulation with pulse trains and direct muscle/trigeminal nerve stimulation since clenching can also occur with single pulses. The use of a hard bite block may cause temporomandibular joint dysfunction or tissue necrosis if used for a prolonged period of time .

Bite blocks should be placed prior to the start of monitoring MEPs in order to prevent teeth occlusion. Since rigid bite blocks may cause pressure injuries to the oropharyngeal airway, soft bite blocks are recommended . They need to be placed in a manner as to prevent occlusion by the molars and the front teeth, all while keeping the tongue midline. The ACNS guidelines suggest the use of “padding or soft bite blocks to mitigate or prevent mouth injury or tracheal tube damage” during MEP runs . At our institution, we use two bite blocks made from three to four tightly rolled 4 ×4 cm gauze held together with plastic tape. After placement the tape is then placed across the opening of the mouth making sure to cover the bite blocks and adhered to the cheeks. All blocks are made by the neurophysiology team and placed by the anesthesiology team. It is recommended to check the bite blocks periodically during the case to make sure they have not moved or become dislodged. Using lower voltage and less frequent stimulation, along with the aforementioned suggestions, can help to greatly reduce patient bite injuries during MEP monitoring.

Another form of ION that has some accumulated evidence is triggered EMG for pedicle screw placement (see Chapter 13 : Electromyographic monitoring for pedicle screw placements, for a more detailed description of EMG methods). Conduction of electric current through the screw and detection at the monitored muscle indicates a screw in contact or close proximity with the nerve. If the nerve is, however, completely severed, EMG may still be silent. Raynor et al. have famously established that the likelihood of screw misplacement and need for repositioning are higher with decreasing trgEMG amplitudes . While rectus abdominis monitoring is less feasible and sensitive for thoracic pedicle screw placement, trgEMG is a very useful technique for lumbar pedicle screw placement that does not require much more in terms of setup than the regular MEP setup and a direct stimulation probe . Free-running EMGs are less sensitive and useful for a nerve root injury and also present a significant rate of false negatives. There is, however, a specific indication for which it is considered a standard of care: lateral transpsoas interbody fusion. This is a technique created in the early 2000s, which offers the benefit of quickly increasing foraminal height and placing a large interbody graft for fusion at multiple lumbar levels, while avoiding a large posterior approach or nerve manipulation in the spinal canal. It has the disadvantage of utilizing a working channel through the psoas muscle, where elements of the lumbosacral plexus and, particularly, the femoral nerve are exposed to injury during the approach . Different monitoring techniques have been utilized for this approach, but, at a minimum, triggered and free-running EMG must be used when dilating through the psoas and docking the retractor on the spine ( Fig. 28.3 ). Special tools are utilized that normally enable conduction of the electrical current through the retractor blades, and ultimately the field is inspected for the presence of neural elements. Even with attentive ION, rates of transient sensory and motor deficits with this approach are 20%–25%, the majority of which are resolved by 1 year postoperatively .