Surgery and intraoperative neurophysiological monitoring for aneurysm clipping

21.1

Introduction

Intraoperative neuromonitoring for aneurysm surgery follows the rationale to detect pending ischemia related to the vascular territories of interest.

First reports about neuromonitoring in intracerebral aneurysm surgery date back into the mid-1980s, where solely somatosensory potentials have been used . This followed encouraging animal studies demonstrating a relation between electroencephalography, somatosensory potentials, and cerebral perfusion. Reports about sustained somatosensory-evoked potential (SEP) but postoperative motor deficit urged the request for motor-evoked potentials (MEPs). To follow their introduction, in 1993 the utilization and usefulness became of major attention . For posterior circulating aneurysms the use of brainstem auditory-evoked potentials (BAEPs) has been described. More recently also visual-evoked potentials (VEPs) are reported to be more reliable compared with previous reports. Reports about VEP utilization in aneurysm surgery are encouraging to implement the method in a broader range ( , (see Chapter 4 : Neurophysiology of the visual system: basics and intraoperative neurophysiology techniques)). Systematic comparison to other modalities such as intraoperative microvascular Doppler sonography, indocyanine angiography (ICG), or more recently intraoperative perfusion imaging by cranial computed tomography scans are still scarce. Lessons learnt in cerebrovascular surgery are templates for vascular injury related to brain tumor surgery.

21.2

Somatosensory-evoked potentials

Methodological aspects of SEP are provided in Chapter 3 , Monitoring somatosensory evoked potentials. Commonly, bilateral median and tibial nerve SEP with subcortical and cortical recordings are employed. It has to be emphasized that the recording of a noncephalic SEP is mandatory to distinguish between cerebral or noncerebral causes of amplitude alteration ( Fig. 21.1 ). Such median nerve SEP should also include N9-recording from Erb’s point or N11 respective N13 recordings from the lower (Cv7) or upper cervical spine (Cv1). In some instances the cervical N30 can also be recorded to follow stimulation of the tibial nerve.

Signal-to-noise ratios determine the amount of single-sweep averages needed to obtain a robust and reliable response. Multiple electrode recordings especially for tibial nerve SEP might be necessary for obtaining the optimal montage for recording ( , ( Chapter 3 : Monitoring somatosensory evoked potentials)). Optimal recordings are a perquisite for minimizing the amount of averages and a fast update. This allows for timely detection of signal alteration.

The cortical potential is generated within the gray matter and thus susceptible to cortical ischemia, less to subcortical and particular brainstem ischemia . This pioneering work demonstrated that the amplitude of the cortical response shows a close correlation with cortical cerebral perfusion: if the cerebral perfusion falls below 10 mL/mg brain parenchyma, SEP amplitudes show a close to linear decrement. SEPs are inevitable to detect cortical ischemia. Warning criteria were deducted from this work as well as from empirical experience and are commonly set to 50% amplitude decrement and/or 10% increment of the cortical latency (which relates to 2 ms in case of median nerve SEP and 4 ms in case of tibialis nerve SEP) or an increase of the central conduction time of 2 ms. In our experience imminent ischemia is usually indicated by parallel occurrence of both: increment in latency and amplitude decrement in consecutive recordings ( Fig. 21.1 ). This can occur within seconds of a vessel occlusion in the absence of sufficient collateral circulation. In some circumstances, ischemia related changes may appear with an up to 20 minutes or even longer delay. The rather large unforeseen variability, depending on each patient’s collateral circulation in the given surgical setting, entails continuing monitoring for at least 30 minutes to follow the last critical step.

Later SEP peaks, such as the N50, are more indicative for a lighter anesthesia and might help in judgment of the level of anesthesia.

The primary goal of recording SEP is to detect impairment of cortical perfusion by intended or inadvertent vessel occlusion. SEP changes are expected to also indirectly reflect perfusion of nonsensory eloquent areas such as the motor or speech cortex, as well as local events (pressure or traction from retractor) and systemic influences. Principally, SEP monitoring in supratentorial aneurysms is only reliably used with lesions of the anterior circulation. The territory supplied by the posterior cerebral artery (PCA) is neither overlapping with, nor adjacent to, the somatosensory cortex. The situation is different with posterior fossa aneurysms, since vascular supply of infratentorial pathways including the medial lemniscus and corticospinal tract stems from the basilar artery (BA) and vertebral arteries.

21.3

Motor-evoked potentials

With the introduction of total intravenous anesthesia (TIVA) and the modification of the transcranial and direct cortical stimulation (DCS), MEP became applicable during procedures where motor pathways are at risk , for technical details see Chapter 2 , Intraoperative neurophysiology and methodologies used to monitor the functional integrity of the motor system. While using only SEP it became soon apparent that these only indirectly cover motor pathways and that pure hemiparesis might occur without SEP alteration . The reliability to detect impending ischemia resulting into motor deficits was shown in several studies: permanent MEP loss is always associated with long-term severe motor deficit, whereas unaltered MEPs are not followed by motor deficits . In contrast to MEP–motor–outcome relation in spine surgery, where the all-or-nothing principle applies, results of the abovementioned studies showed, that prolonged transient and permanent MEP alterations not including losses can also be followed by permanent motor deficits ( Table 21.1 ). These findings were supported by the analysis of sole MEP alteration in their relation to postoperative motor deficit and magnetic resonance imaging (MRI) . This led to the refinement of MEP warning criteria in supratentorial surgery . Many warning criteria—mostly stemming from empirical evidence and deducted from the relation between MEP alteration and motor outcome—are described. Most commonly in supratentorial surgery a 50%–80% amplitude decrement is applied. The combination of alteration in amplitude and stimulation intensity with regard to bilateral changes has been used. Termed as “bilateral threshold criterion”, it was just recently elaborated in supratentorial surgery and seems to enhance warning criteria . But it has to be emphasized that MEP warning criteria are a still matter of controversy as MEP-waveform and -amplitudes underlie physiologic fluctuations, which are aggravated during surgery. Of concern are warning criteria which are either too sensitive, for example, solely amplitude decrements not exceeding the physiological amplitude fluctuation of consecutive MEP, resulting into too many false-positive warnings. On the other hand the criterion of “amplitude loss” results into too many false-negatives. In supratentorial surgery, care has to be taken that the fibers of the corticospinal tract are not activated deep within the white matter, which has been described with far lateral and suprathreshold stimulation . One criterion for more focal stimulation is tailored transcranial stimulation with midline-hemispheric or hemispheric stimulation, which especially for the upper extremity muscles MEP allows to minimize stimulation intensity and movement alike .

To prevent deep white matter activation of fast conducting corticospinal tract neurons as close as possible at the gray/white matter border, DCS has been used . The subdural placement of a several-contact strip electrode has a risk of subdural bleeding in 2% of the cases, whereas the risk of seizures was not significantly different: 0.85% for transcranial electric stimulation (TES) and 1% for DCS. Edema in subarachnoid hemorrhage (SAH), subdural bridging veins, and subdural scarring due to previous surgeries can preclude the placement of a strip electrode. With the pterional approach, it is possible to place the strip electrode on the hand area of the motor cortex, but the distance to the parasagittal leg motor representation area is mostly too far to be reached . Despite a more focal stimulation, DCS allows only unilateral assessment after dura-opening. TES in contrary is performed bilaterally throughout the surgery. Assessment of both hemispheres is mandatory for all midline lesions and bilateral vascular supply, for example, from the anterior communicating artery. Bilateral TES recording also allows for the timely detection and discrimination of general effects, such as anesthesia, on the generation of MEP. When comparing alteration and the intraoperative course of MEP elicited by slightly suprathreshold DCS to those elicited with slightly suprathreshold TES, changes in DCS-MEP were more likely (12% vs 8% in TES) without reaching statistical significance .

21.4

Early auditory-evoked potentials

The early components of auditory-evoked potential (AEP) reflect the conductivity along the auditory pathway between the cochlear nerve and the upper mesencephalon. AEP are an essential method for neuromonitoring in posterior fossa surgery. Commonly, AEP are elicited with click sound of 60 dB above hearing level with sound-masking of the other ear. Most systems use a click sound of a frequency band of 2000–4000 Hz, which reflect one part of the audible frequencies. In patients with hypoacusis, the maximal stimulation intensity is usually not sufficient for eliciting a reliable BAEP, a problem being intensified by a low signal-to-noise ratio. For intraoperative interpretation, mostly signal alteration of wave-complex IV/V is judged. A 50% decrement of wave V and a latency increment of more than 0.5 ms are used as warning criteria. The abolition of wave V relates to postoperative hearing impairment in 30% of the affected patients . Despite the repeated crossing and interconnection of the auditory pathways, the ipsilateral wave V resp. the IV/V-complex is usually more affected but exceptions occur . Consequently, bilateral recording increases the likelihood of detecting signal alteration and diagnostic accuracy in circumscribed brainstem lesions.

AEP are affected by decreased brainstem perfusion , and waveform alteration related to circumscribed brainstem infarcts is described . In aneurysm surgery, signal alteration seems to occur more frequently and earlier in MEP, followed by SEP and AEP alteration .

21.5

Visual-evoked potentials

Visual deficits—either of acuity or visual field deficits—are related to the surgery of paraclinoid aneurysms resp. aneurysms of the ophthalmic segment, which refers to internal carotid artery (ICA) aneurysms between the cavernous sinus exit and the posterior communicating artery . In those aneurysm locations, monitoring of vision—either acuity or visual field deficits—is of great concern, and the rate of visual loss has been presented up to 24%. Alterations of acuity, visual field deficit, or blindness are given with a cumulative rate of as high as 24%. After disappointing results in the mid-1980, intraoperative VEPs were abandoned . With the introduction of TIVA and flash light-emitting diodes (LED), VEPs were reintroduced into the operating room (see Chapter 4 : Neurophysiology of the visual system: basics and intraoperative neurophysiology techniques). Transient VEP-loss while temporary occlusion of the hypophyseal artery without any postoperative alteration of the visual function was a powerful demonstration of potential VEP use . In aneurysm surgery, VEP alteration related to prechiasmatic lesions has been shown and seems considerably useful . The recovery of the potential has been reported without any visual field deficit, but false-positives are not reported. Preoperative impairment of acuity or visual field loss hampers VEP recordings . Systematic larger studies of VEP during cerebral aneurysm surgery are still lacking. A most recent meta-analysis stated that “Visual evoked potentials showed marked methodologic improvement in recent studies. Predictive power for visual deterioration after surgery was approximately 60% and reached 100% when coupled with simultaneous monitoring of electroretinography” . Nevertheless, the reliability of VEP monitoring in lesions affecting the optic radiation and primary visual cortex still remains a matter of conflicting results.

Commonly, VEPs are elicited with flash LED, and the separate analysis of the response to the on- (light-emitting phase) and off-phase showed that the VEP responses became more stable using responses to the off-phase . The most commonly used warning criteria are 50% decrement of the response. In our own experience the individual VEP configuration and amplitudes are very variable, which are prone to amplitude reductions and phase shifting during the course of surgery, and there is the need for more studies to establish a common warning criterion.

Several attempts for improvement of intraoperative VEP were made: optic nerve–evoked potentials (ONEP) have been described, although the term is misleading: stimulating electrodes are placed epidurally to the optic nerve after unroofing of the optic canal, while recording electrodes are placed close to the chiasm where “ONEP” were recorded . But direct stimulation and recording from the ON resembles nerve action potentials; “optic nerve action potential” (ONAP) would be a more appropriate term. In an experimental setting, there was a good correlation between stepwise nerve transection and the ONAP-amplitude. Later, the technique was refined by epidural stimulation and cortical recordings, such being true for ONEP with a positive occipital cortical potential at 20 ms (P20) and a negative deflection at 30 ms (N30). Amplitude reduction related to optic nerve manipulation in tumor surgery . While writing this chapter, using ONAP or ONEP is not reported for larger series in aneurysm surgery. If vision is at high risk, the necessity to unroof optic canal for placement of stimulating electrodes might be debated, although only be limited and associated with a significant risk for affection of the optic nerve by itself.

The significance of intraoperative VEP compared with other scarcely used methods as intraoperative MRI or ICG-angiography and each of their sole or combined impact on neurosurgical decision still remains to be demonstrated. In summary, intraoperative VEP and ONEP have been shown in limited publications to have some benefit in protecting the anterior visual pathway.

21.6

General remarks for safety considerations and anesthesia