Leslie Lee

F113. The methohexital challenge prior to intracranial endovascular embolization

Introduction The risks associated with endovascular embolization procedures are both mitigated and predicted with the use of pharmacologic provocative testing (PT) and intraoperative neurophysiologic monitoring (IONM). The utility of pharmacologic provocative testing has been reviewed in the literature, mostly in patients undergoing awake procedures with the use of neurologic examination and electroencephalogram. Here, we review our experience in patients undergoing intracranial endovascular embolization procedures with the use of IONM. Methods We reviewed our database to identify all patients undergoing endovascular procedures between January 1, 2014 and January 1, 2016. We included only patients who underwent cerebral endovascular embolization procedures with the use of methohexital PT. A retrospective chart review was performed to identify patients’ demographic factors, intraoperative findings, and postoperative examination details. We also reviewed perioperative notes to identify any adverse reactions to methohexital as well as other complications unrelated to the use of methohexital. Awake patients were tested with SSEP, EEG and real time neurologic examination while TcMEPs were performed in all anesthetized patients. BAEPs were performed in anesthetized patients if indicated. Methohexital was administered as an injection at a dose of 5 mg or 10 mg and repeat testing was performed if needed. Results A total of 64 endovascular procedures were performed in 31 patients with utilization of pharmacologic PT and were included in this review. PT was performed under general anesthesia in 54 procedures (84%) and awake testing was performed in 10 procedures (16%). PT was negative in 62 procedures and embolizations were performed in these patients. PT was positive in two procedures and the procedure was terminated without embolization in one patient. The other patient underwent embolization after repositioning the catheter at an alternative embolization site but repeat testing was not performed. There were no new postoperative neurologic deficits after any of these procedures. We calculated the specificity of pharmacologic PT as 100% as none of the patients with a negative provocative test developed a new postoperative neurologic deficit after embolization. Conclusion To the best of our knowledge, this is the first review on pharmacologic PT with methohexital and with the use of IONM under general anesthesia. Our results also indicate that IONM under general anesthesia can allow pharmacologic PT at a specificity comparable to awake testing. We also postulate that the absence of false negatives provides strong evidence that IONM accurately monitors the functional state of the brain during PT. If this were not the case, then new neurologic deficits should be seen in some of the cases with negative tests.

F111. Isolating C8 nerve root technique with focal digital stimulation

Introduction The American Clinical Neurophysiology Society guidelines recommend eliciting upper extremity somatosensory evoked potentials (SSEP) via median or ulnar nerve stimulation at the wrist. However, when monitoring cervical spine surgeries, it may be more appropriate to use ulnar nerve stimulation as the root contribution to the cortical SSEP originates at levels below C7, while median nerve SSEPs have contributions possibly as high as C5. We postulate that wrist stimulation reflects a combination of activated axons from both the median and ulnar nerves secondary to supramaximal stimulation typically used intraoperatively. It’s therefore difficult to precisely determine the level of SSEP nerve root entry. Isolated ulnar stimulation targeting only C8-T1 roots could remedy this issue. The authors tested the feasibility and reliability of pure ulnar C8-T1 activation through 5th digit stimulation. Methods This prospective study consented and enrolled eight patients without known evidence of peripheral neuropathy or cervical radiculopathy undergoing routine intraoperative neurophysiologic monitoring (IONM) including SSEP. Standard adhesive gel electrodes were used for wrist stimulation over the median and ulnar nerves, as well as the 5th digit. Ulnar nerve stimulating electrodes were placed at the wrist and 5th digit on the contralateral side of arterial line placement. Cortical SSEPs (montage of CPc-Fz and CPc-CPi) were analyzed for amplitude, latency and morphology. The intensity, pulse width, and repetition rate of stimulation to elicit the SSEP were also analyzed. SSEPs were obtained throughout surgery to ensure reproducibility. Results Stimulation of the median nerve at the wrist provided the highest amplitude cortical SSEP. Signals were consistently present in the ulnar nerve at wrist and 5th digit for each patient. The mean ulnar nerve amplitudes at the wrist and 5th digit were 42% and 78% smaller compared to the median amplitude, respectively. The average percent standard deviation of amplitudes for median nerve SSEP was 13.42% compared to 17.87% and 17.35% from ulnar nerve at the wrist and 5th digit, respectively. Conclusion These data confirm 5th digit stimulation may be a reproducible method of isolating C8-T1 root activation. Despite higher amplitude responses from median wrist stimulation, 5th digit ulnar stimulation remained consistent during surgical procedures and could be used to detect conduction changes. This technique avoids coactivation of the median nerve SSEP which can occur when stimulating for an ulnar nerve SSEP at the wrist, thereby preventing false negative responses. This was further confirmed during this study by the identification of a case, which was subsequently excluded from analysis, of a patient determined to have a preexisting ulnar neuropathy, diagnosed by outpatient nerve conduction studies. Intraoperative cortical SSEP was correctly absent from 5th digit stimulation and incorrectly present from wrist stimulation of the ulnar nerve in this patient.

S116. Transient intraoperative peripheral nerve injury precipitated by rare iatrogenic lapse

Introduction Intraoperative neuromonitoring primarily tracks for disruptions of the neuraxis during cerebral surgical procedures. Occasionally, peripheral injury can mimic central injury. We report a case where an iatrogenic lapse triggered transient peripheral injury. Methods A 4 year old boy was diagnosed with a left-sided arteriovenous malformation and underwent cerebral angiogram with embolization. While acquiring vascular access to begin the procedure, the endovascular neurosurgeon advanced the femoral sheath into the right femoral artery towards the level of right common iliac artery and the distal aorta. Prior to guidewire insertion, the surgeon flushed the femoral sheath with heparinized saline to prevent blood clot formation. Within a few minutes, there was a significant right lower extremity somatosensory evoked potential (SSEP) amplitude decline of greater than 50%. Review of the SSEP traces displayed a decrease in both the cortical and peripheral potentials (popliteal fossa and C7), indicating a likely peripheral cause of the change. The surgeon was informed of the peripheral cause of the SSEP change, but no etiology was readily identified. Additional detailed investigation identified the saline flush being infused at a high rate into the right leg, instead of the typical rate of 1–2 drops per second. The flush was closed after an approximate total infused volume of 1000 mL. After closure of the flush, the lower extremity SSEP returned to baseline within a few minutes. The procedure continued and was completed without any other adverse events. He was neurologically intact postoperatively. Results This case provides an example of how the nervous system is at risk throughout a procedure, even during ”non-critical” periods when neurologic injury is not expected to occur. By quickly identifying and investigating neurophysiologic changes, the clinical team was able to correct a rare iatrogenic cause of peripheral limb ischemia and avoid potential complications or neurologic injury. Conclusion While infrequent, neurologic dysfunction and permanent damage may occur during the initial phases of any surgical procedure while positioning, access, or exposure is occurring. The etiology of these neurologic changes may not always be immediately recognizable and should encourage early and effective communication between the neurophysiologist, anesthesiologist, and surgeon.

T92. Diversity in clinical practice of “Continuous” intraoperative neurophysiologic monitoring (IONM)

Introduction Despite advances in neurophysiologic techniques and increased utilization of IONM across a wide diversity of surgical cases, there remain no formal guidelines or consensus about best practice parameters in the implementation of “continuous” monitoring. As critical IONM changes may occur at any time during a surgical procedure, even during “non-critical” periods, variations in total duration of neuromonitoring as well as frequency of evoked potential (EP) acquisition may adversely impact timely and reliable interpretation and communication of such neurophysiologic changes to the surgical team. It is our goal to evaluate the potential variability of IONM practice in both private and academic settings. Methods A practice survey was administered to attendees of the IONM Special Interest Group sessions at the annual ACNS meetings in 2016 (Part I) and 2017 (Part II). This included multiple choice as well as open-ended questions on topics such as the frequency of somatosensory and transcranial motor evoked potential (SSEP and tcMEP) acquisition in the OR, appropriate “start” and “end” times of neuromonitoring, effective communication between the IONM and surgical teams, as well as the number of simultaneous cases that are confidently monitored, and issues related to job satisfaction. These findings are summarized here. Results A total of 35 surveys were returned in 2016 and 30 surveys in 2017, respondents comprising IONM providers in academic, private, and mixed practices. The most popular answer for frequency of SSEP acquisition was every 3–5 min (48%), followed by every 5–7 min (23%). Most common responses regarding frequency of tcMEP acquisition were every 10–15 min (43%) followed by every 5–7 min (25%). Regarding the appropriate duration of an incision/exposure break, answers were especially variable, ranging from zero, to however long it takes to complete exposure (even if that exceeds 30 min). When asked about the maximum number of cases that are simultaneously monitored, responses also varied widely, ranging from 1 to 10 or more. For the question asking how many cases IONM providers felt comfortable monitoring simultaneously, the most common responses ranged from 1 to 6. The most frequent answer to the question of how many cases respondents felt confident monitoring simultaneously was 3 cases or less. A majority of respondents reported satisfaction most of the time with their current practice of IONM. Conclusion There is significant diversity in the clinical practice of IONM, including the frequency of evoked potential acquisition, duration of “continuous” neuromonitoring for a given procedure, and number of simultaneous cases monitored. By highlighting this variability across IONM providers, we are better able to evaluate clinical areas where future improvements and development of emerging practice parameters may be considered that have the potential to positively impact perioperative care and improve patient outcomes.

Positive pharmacologic provocative testing with methohexital during cerebral arteriovenous malformation embolization

A middle-aged patient underwent staged endovascular embolization of a Spetzler-Martin grade V right parietal arteriovenous malformation(AVM).In the fifth endovascular embolization, after methohexital 10 mg injection into a right posterior choroidal artery feeding the AVM nidus, there was an immediate change in the electroencephalogram (EEG) with simultaneous loss of motor evoked potentials (MEPs) in the bilateral upper and lower extremities and a delayed change in somatosensory evoked potential responses (SSEPs). No embolization was made and procedure was terminated. This case demonstrates the utility of intraoperative neurophysiologic monitoring (IONM) with pharmacologic provocative testing in predicting and mitigating the risks prior to the proposed embolization.

Detection of acute femoral artery ischemia during neuroembolization by somatosensory and motor evoked potential monitoring.

Neuromonitoring can be used to map out particular neuroanatomical tracts, define physiologic deficits secondary to specific pathology or intervention, or predict postoperative outcome and proves essential in the detection of central and peripheral ischemic events during neurosurgical intervention. Herein, we describe an instance of elective balloon-assisted coiling of a recurrent basilar tip aneurysm in a 61-year-old woman, where intraoperative somatosensory evoked potentials (SSEPs) and transcranial motor evoked potentials (TcMEPs) were lost in the right lower extremity intraoperatively. We aim to highlight that targeted use of monitoring proves advantageous in both the open surgical and endovascular setting, even in the avoidance of potential iatrogenic peripheral nerve damage and limb ischemia as documented herein. Consideration of the increased risk for peripheral ischemia in the neurointerventional setting is especially imperative in particular populations where blood vessels might be of diminished size, such as in infants, young children, and severely deconditioned adults.

Detection of inferolateral trunk syndrome by neuromonitoring during catheter angiography with provocative testing

Background and importance It is not uncommon that endovascular balloon test occlusion (BTO) is performed to assess collateral blood flow and risk of injury of permanent occlusion of the internal carotid artery (ICA). This case is the first reported of detection and reversal of the inferolateral trunk (ILT) syndrome in an awake patient during provocative BTO; prompt recognition of the syndrome effectively prevented permanent neurologic deficits. Clinical presentation The case of a 42-year-old woman is reported who had a left sphenoid wing meningioma with extension into the cavernous sinus and who underwent awake catheter angiography with provocative BTO of the ICA. Serial examinations by intraoperative monitoring neurologists and neurointerventionalists detected acute progressive left retro-orbital pressure followed by sudden inability to adduct the left eye, or a left medial rectus palsy, indicative of the ILT syndrome which led to immediate balloon deflation and resolution of the deficits. The hypothesis was that hypoperfusion of the ILT, an arterial branch of the ICA which provides blood supply to several cranial nerves (CN) III, CN V1 and CN V2, caused her acute symptoms. Conclusion Although cerebral ischemia is a well known complication of endovascular procedures, CN ischemia is a rare potential risk. Knowledge of cerebrovascular anatomy and serial examinations prevented neurologic deficits; this case underscores the added utility of examinations by intraoperative monitoring neurologists and interdisciplinary collaboration.