Monitoring Corticocortical Evoked Potentials During Intracranial Vascular Surgery

Background

Monitoring of corticocortical evoked potentials (CCEPs) during brain tumor surgery of patients under anesthesia was recently reported to be effective in assisting in preservation of speech function. The aim of this study was to investigate whether CCEPs can be reproducibly measured between the frontal and temporal lobes during standard intracranial vascular surgery under general anesthesia; whether dynamic changes in CCEPs caused by reduced focal cerebral blood flow can be measured; and whether CCEPs can be used to monitor speech function, particularly associated with the left side of the brain.

Methods

We monitored CCEPs during 58 vascular surgeries (42 clipping procedures; 15 bypasses, 1 of which overlapped with clipping; and 2 hematoma removals from the left frontal and temporal lobe) at Kashiwaba Neurosurgical Hospital from October 2016 to January 2018.

Results

CCEPs could be reproducibly and routinely monitored in bilateral vascular surgeries. None of the patients experienced any postoperative symptoms or showed any ischemic lesions on postoperative magnetic resonance imaging; however, 5 patients temporarily demonstrated reduced CCEPs intraoperatively that were caused by transient obstructions of blood flow. Motor evoked potentials and somatosensory evoked potentials were simultaneously monitored intraoperatively and did not show any changes.

Conclusions

The results of our pilot study show that CCEPs can be routinely monitored during bilateral intracranial vascular surgery and that they are sensitive to ischemia. CCEPs on the left side could serve as unique intraoperative monitoring of speech function under anesthesia.

Introduction

Both safety and efficacy must be considered for the surgical treatment of various intracranial vascular lesions. In aneurysmal clipping surgery, the causes of intraoperative ischemia are mainly divided into 1) insufficiency of cerebral blood flow owing to steno-occlusive lesions of the parent artery and 2) disturbance of the nearby perforating arteries.¹ The former is caused by permanent aneurysmal clipping or prolonged obstruction of arteries by temporary clipping.1, 2 Intraoperative monitoring of motor evoked potentials (MEPs) or somatosensory evoked potentials (SEPs) is extremely effective in preventing motor and sensory impairments caused by ischemia and is indispensable in lowering the risk of postoperative complications on actually proceeding with the treatment.3, 4, 5 However, we cannot completely predict ischemic events only by detecting motor or sensory impairment during vascular surgery regardless of the injury to the artery.1, 2 This is because individual ischemic lesions depend on existence of collateral blood flow and eloquence of the area supplied by the injured artery.³

The connecting fascicles among the cortices in the deep white matter can be observed using tractography with diffusion tensor imaging, a magnetic resonance imaging (MRI)–based technique, thus allowing us to assess how the brain anatomy relates to various brain functions. On tractograms, MEPs and SEPs relate to only a small part (the pyramidal tracts and spinothalamic tract) of the deep white matter. It is uncertain whether MEPs and SEPs can be used to detect ischemic lesions occurring in the other dominant parts of the brain. It may be useful to spread the electrophysiological net to detect more ischemia and as many ischemic complications as possible during surgery.

Postoperative deterioration of speech function is difficult to predict and is an important consideration of neurosurgeons. Intraoperative monitoring of speech is feasible during surgery for awake patients; however, it depends on the degree of preoperative or recovered consciousness of each individual patient.6, 7 Other methods of monitoring speech function are difficult to apply when a patient is under general anesthesia. Recently, anatomic and functional relationships of the deep white matter have been reported following electrical intraoperative investigations.7, 8, 9, 10, 11 These studies reported the effectiveness of intraoperative monitoring of the dorsal language pathway by using corticocortical evoked potentials (CCEPs) to monitor language function during brain tumor surgery.6, 7, 8, 10, 11, 12 The investigators demonstrated that CCEPs can be bidirectionally and specifically associated with the dorsal language pathway of the arcuate fasciculus.7, 8, 10, 11, 13, 14 This pathway was first reported by Matsumoto et al.¹⁵ as a reciprocal connection between Broca (area 44/45) and Wernicke (area 22/40) areas. CCEPs comprise the electrical conducting pathway from the cortical area through connecting fibers in the arcuate fasciculus. A neuroimaging study has shown that the arcuate fasciculus runs widely and longitudinally through the deep white matter between the frontal and temporal lobes.¹⁶ Therefore, we planned to use CCEPs to intraoperatively monitor patients to detect ischemic complications in a wider range during bilateral intracranial vascular surgery than MEPs and SEPs. Our objectives were 1) to consistently and reproducibly measure the amplitudes of CCEPs during bilateral general intracranial vascular surgery when patients were under anesthesia, 2) to observe any sharp changes of electrical conduction that could be caused by the partial reduction of cerebral blood flow, and 3) to intraoperatively monitor patients' speech function by using CCEPs.