Does Neurologic Electrophysiologic Monitoring Affect Outcome?

INTRODUCTION

Neurologic injury from surgery results in substantial increased morbidity, mortality, and cost. More importantly, it is devastating to patients and their families. Thus techniques to lessen, reverse, and even avoid neurologic injury are very valuable. Neurologic intraoperative electrophysiologic monitoring (NIOM) can identify impending or ongoing intraoperative injury, allowing for interventions. Changes to a patient’s neurologic electrophysiologic baseline during the procedure alert the operative team that a potential injury may be occurring. The goal of NIOM is to detect dysfunction caused by ischemia, mass effect, stretch, heat, or direct injury in real time before it causes permanent neurologic injury. Monitoring may also be useful for identifying and preserving neurologic structures during a procedure where they are at risk (mapping).

There are several challenges to establishing the efficacy of NIOM. The first is that blinded or randomized trials assessing the efficacy of NIOM in humans are lacking. Unfortunately, a substantial trial will likely never examine this issue. The reason behind the lack of high-level evidence is that monitoring is well established and accepted in clinical practice. Moreover, it is generally extremely low risk to the patient. The general consensus in the surgical community is that monitoring is useful and there would be ethical and medico-legal dilemmas in withholding monitoring in patients who are at potential risk for injury, although this opinion is not universal. ^, A second limitation in establishing outcomes for NIOM is that the goal of monitoring is to reverse a significant change if one is seen during a procedure. Monitoring may detect an impending injury, which is reversed by intraoperative intervention, but the benefit can never be confirmed because the patient wakes up with a normal examination. The utility of monitoring is based on animal studies and case series with comparisons to historical control subjects. The utility of NIOM may be supported by establishing that monitoring can, in fact, detect injury in cases where injury has occurred (true-positive outcomes), while limiting false-negative outcomes (injury occurred and was not detected) and persistent false-positive outcomes (injury was predicted by NIOM at the end of a procedure but did not occur). Multimodality monitoring is possible, so the ability of different NIOM techniques to predict injury can be compared in the same patient.

MODALITIES OF MONITORING AND MAPPING

Various parts of the nervous system can be monitored using NIOM techniques. The specific neurologic tissues at risk, and the type of potential injury, vary with different surgical procedures. Specific techniques include electroencephalography (EEG), electromyography (EMG), nerve conduction studies (NCS)/triggered EMG, and evoked potentials, including brainstem auditory evoked potentials (BAEPs), visual evoked potentials (VEPs), somatosensory evoked potentials (SSEPs), transcranial electrical motor evoked potentials (TcMEPs), and intraoperative mapping of cortical structures.

EEG is a measure of spontaneous electrical brain activity recorded from electrodes placed in standard patterns (“montages”) on a patient’s scalp or directly on the cortex with sterile electrode strips or grids (also called electrocorticography [ECoG]). Time-varying electrical potential differences between individual electrodes are amplified and then recorded as continuous wavelets that have different frequencies and amplitudes. These data can be displayed as raw traces on a display in a series of channels or processed in real time into their component frequencies and displayed as a spectral analysis. A change in a patient’s background EEG activity from baseline during a procedure may indicate ischemia of the cerebral cortex either focally or through a generalized loss of activity over the entire cortex. A 50% decrease in EEG amplitude is generally considered a significant change. Increased focal or generalized slowing may also indicate early focal ischemia or global cerebral hypoperfusion, respectively. EEG is routinely used intraoperatively during carotid endarterectomy (CEA), cerebral aneurysm, and arteriovenous malformation surgeries, or in other procedures that place the cortex at risk. ^,

EMG is used to assess the integrity of the peripheral and cranial nerves. Monitoring is performed by placing electrodes into muscles innervated by the nerves of interest and observing for spontaneous activity from the muscles, which may indicate that the nerves supplying them are suffering unexpected injury. Peripheral nerves are at risk for crush, stretch, ligation, ischemic, and hyperthermic injury during many surgical procedures because of mispositioning, electrocautery, or direct injury. Cranial motor nerves are also often monitored in this fashion. Monitoring has been performed on oculomotor, trochlear, abducens, trigeminal, facial, glossopharyngeal, vagus, spinal accessory, and hypoglossal motor nerves. The facial nerve is often monitored during posterior fossa procedures, where it is at high risk for injury, and during parotid gland procedures or other ear, nose, and throat (ENT) procedures involving the face, ear, or sinuses. The external branch of the superior laryngeal nerve (EBSLN) and recurrent laryngeal nerve (RLN) can be injured during thyroidectomies and other ENT procedures in the anterior neck and have been monitored by detection of movement in the vocal cords.

NCSs involve determination of whether a specific length of nerve will conduct electrical activity between a stimulating and recording electrode and can be performed to localize peripheral and cranial nerves (triggered EMG mapping) and further assess their integrity. Electrodes are placed into muscles innervated by the nerves of interest, as in EMG. For mapping, tissue is interrogated by electrically stimulating it, and if compound motor action potentials (CMAPs) are generated in the muscle, the associated peripheral or cranial nerve can be identified. Stimulation can also be performed to test the function of an identified nerve or assess the proximity of another structure near a nerve, such as a pedicle screw. If a nerve is stimulated and no CMAPs are generated, this may indicate that it has been significantly injured along its course. Monitoring of peripheral nerves can aid in localizing and protecting nervous tissue during nerve repairs or during spinal surgery for structural repairs or tumor resections.

Evoked potentials are measures of nervous system electrical activity resulting from a specific stimulus that is applied to the patient. They can be recorded in different locations along the pathways of interest as the evoked activity propagates down its course.

BAEPs are wavelets generated by the auditory nerve and brainstem in response to repetitive clicks delivered to the ear. Typically, five wavelets are recorded from electrodes placed near the ear: the first and second wavelets represent the responses from the peripheral cochlear nerve, and the next three wavelets are generated from ascending structures in the brainstem. Responses are averaged, and changes in latency and amplitude of these five waves are used to assess the integrity of the auditory pathway during procedures that put them at risk. BAEPs are commonly used in posterior fossa neurosurgical procedures, such as acoustic neuroma resections, which place the eighth nerve at risk for either ischemia or stretch injury. BAEPs may also be useful in identifying and preventing injury in procedures such as tumor resections, cerebral aneurysm repairs, or arteriovenous malformation repairs that place the brainstem itself at risk because of ischemia or mass effect.

VEPs are wavelets generated by the occipital cortex in response to visual stimuli (typically flashing lights delivered with light-emitting diode [LED] goggles in the operative setting). Responses are recorded from electrodes overlying the occipital cortex and averaged, and the resulting VEPs provide information about the integrity of the visual pathway during procedures. VEPs have been monitored during neurosurgical procedures involving mass and vascular lesions near the optic nerve and chiasm.

SSEPs are produced by electrical stimulation of a peripheral nerve, recording over various points along the afferent sensory system. Averaged potentials are generated using repetitive stimulation to increase the signal-to-noise ratio of the evoked signals. SSEP waveforms are recorded from the peripheral nerve, spinal cord, brainstem, and primary somatosensory cortex. Recording waveforms at sequential locations along the complete afferent sensory system allows for localization of dysfunction during procedures. This dysfunction could be caused by ischemia, mass effect, or local injury, and may represent injury unrelated to the surgical site, such as stretch injury during positioning or systemic effects. SSEPs recorded from stimulation of the median nerve are used intraoperatively during CEAs and intracranial surgeries for anterior circulation vascular lesions. ^, SSEPs recorded from stimulation of the posterior tibial nerve in the leg are used during intracranial surgeries involving vascular lesions in the posterior cerebral circulation. Monitoring both upper and lower extremity SSEPs during procedures that place the spinal cord at risk may be useful in procedures to treat scoliosis, spinal tumors, or descending aortic repairs. The accepted criterion for significant SSEP change, suggesting a potential injury, is a decrease of spinal or cortical amplitudes by 50% or an increase in latency by 10% from baseline.

TcMEPs are generated by stimulating the motor cortex with electrical current using scalp electrodes and are recorded from epidural electrodes near the spine itself (D-waves) or intramuscular electrodes in the periphery (muscle motor evoked potentials, which are CMAPs). There has been some evidence presented in the literature that loss of D-wave responses with loss of myogenic CMAPs may represent an irreversible spinal cord injury, whereas persistence of the D-wave with loss of the myogenic response may indicate a spinal cord injury that is recoverable either intraoperatively or postoperatively. ^, Motor evoked potentials (MEPs) may also be recorded by direct electrical stimulation of the motor cortex after craniotomy (as a means of functional mapping of the motor cortex) or via transcranial magnetic stimulation. TcMEPs provide a real-time assessment of the descending motor pathway from the cortex to muscle during procedures that place the corticospinal tracts at risk. TcMEPs are increasingly being used in advanced neurosurgical, aortic, and orthopedic centers for monitoring motor pathways of the brain and spinal cord during procedures. TcMEPs may have a superior temporal resolution for detection of ischemia compared with SSEPs with delays of up to 30 minutes seen in SSEPs. This is likely because TcMEPs measure spinal gray matter, which is very sensitive to ischemia, in addition to spinal motor myelinated tracts. One downside is that no clear criteria exist in the literature to define a critical change implying that injury is occurring. Studies have used different levels of losses in CMAP amplitude (25% vs. 50% vs. 80%) or threshold changes (i.e., the amount of stimulation it takes to obtain the CMAP) to signify a critical change. The ability to perform TcMEPs is also limited by its sensitivity to anesthetics, paralytic agents, and temperature. The use of paralytic agents is discouraged and, if used at all, should be extremely limited and kept relatively constant (at <20% neuromuscular blockade). This also means that patients are at higher risk for injury from spontaneous movements or stimulation during their procedures. Another limitation is the major concern that TcMEPs are often difficult to obtain from the leg. Whether this is because of technical limitations of the modality or preexisting injury in patients is unclear. ^{, , ,} Complications are of greater concern than in other modalities because of the stimulus intensity required to induce the response and actual patient movement; complications may include rare instances of seizures and tongue lacerations. Direct intraoperative mapping of brain structures can be performed during open craniotomies to localize brain function both cortically and subcortically. This technique uses direct focal stimulation of the brain to identify language, motor, or sensory functions in asleep or awake patients. Mapping can be used to try to avoid or limit injury to “eloquent” brain structures while performing surgery for epileptogenic, vascular or oncologic lesions. Stimulation, recording, and testing techniques vary based on the areas of the brain that are at risk and consciousness of the patient, but ultimately the goal of mapping is optimal resection with acceptable functional loss of clinical neurologic function.

NIOM modalities are often combined during procedures and may increase the ability to detect and prevent injury. For example, a recent survey of the Scoliosis Research Society indicated that 80% of the surgical membership use both SSEPs and TcMEPs during their scoliosis repairs.

EVIDENCE