Neurophysiologic monitoring in neurosurgery

Direct brainstem somatosensory evoked potentials for cavernous malformations

OBJECTIVE

Brainstem cavernous malformations (CMs) often require resection due to their aggressive natural history causing hemorrhage and progressive neurological deficits. The authors report a novel intraoperative neuromonitoring technique of direct brainstem somatosensory evoked potentials (SSEPs) for functional mapping intended to help guide surgery and subsequently prevent and minimize postoperative sensory deficits.

METHODS

Between 2013 and 2019 at the Stanford University Hospital, intraoperative direct brainstem stimulation of primary somatosensory pathways was attempted in 11 patients with CMs. Stimulation identified nucleus fasciculus, nucleus cuneatus, medial lemniscus, or safe corridors for incisions. SSEPs were recorded from standard scalp subdermal electrodes. Stimulation intensities required to evoke potentials ranged from 0.3 to 3.0 mA or V.

RESULTS

There were a total of 1 midbrain, 6 pontine, and 4 medullary CMs-all with surrounding hemorrhage. In 7/11 cases, brainstem SSEPs were recorded and reproducible. In cases 1 and 11, peripheral median nerve and posterior tibial nerve stimulations did not produce reliable SSEPs but direct brainstem stimulation did. In 4/11 cases, stimulation around the areas of hemosiderin did not evoke reliable SSEPs. The direct brainstem SSEP technique allowed the surgeon to find safe corridors to incise the brainstem and resect the lesions.

CONCLUSIONS

Direct stimulation of brainstem sensory structures with successful recording of scalp SSEPs is feasible at low stimulation intensities. This innovative technique can help the neurosurgeon clarify distorted anatomy, identify safer incision sites from which to evacuate clots and CMs, and may help reduce postoperative neurological deficits. The technique needs further refinement, but could potentially be useful to map other brainstem lesions.

Effects of rocuronium dosage on intraoperative neurophysiological monitoring in patients undergoing spinal surgery

What is known and objective

Intraoperative neurophysiological monitoring (IONM) has been widely used in clinical practice. Therefore, the influence of neuromuscular blockers essential for spinal anaesthesia on IONM is worthy of our attention, but no randomized study has evaluated the dose‐response effect. This study investigated the effects of different doses of rocuronium bromide on the intraoperative monitoring of motor evoked potentials (MEPs).

Methods

We conducted a randomized, double‐blind trial to assess the effects of three rocuronium bromide doses (6.0, 9.0, 12 μg·kg⁻¹·min⁻¹) combined with intravenous infusion of propofol 6–8 mg·kg⁻¹·h⁻¹ and remifentanil 10 μg·kg⁻¹·h⁻¹ on the amplitudes of somatosensory evoked potentials (SEPs) and MEPs at the time of the baseline recording (T₁), before pedicle screw placement (T₂) and before spinal canal decompression (T₃). Secondary outcomes included measurement of neuromuscular function, the occurrence of unexpected intraoperative body movement and recovery of spontaneous breathing.

Results and discussion

A total of 123 patients were enrolled, and 120 patients were ultimately analysed. No differences were observed in the amplitude of SEPs among the three groups (p > 0.05). The MEP amplitude differences at T₁, T₂ and T₃ in all limbs did not differ in patients receiving rocuronium at 6.0 μg·kg⁻¹·min⁻¹ and 9.0 μg·kg⁻¹·min⁻¹ (p > 0.05). However, when rocuronium was administered at 12.0 μg·kg⁻¹·min⁻¹, MEP amplitudes at the time point T₃ were significantly attenuated compared with the time points T₁ and T₂ in both right upper limb and left lower limb (p = 0.002, p = 0.025, respectively). In patients treated with rocuronium 6.0 μg·kg⁻¹·min⁻¹, the incidence of unexpected body movement was significantly higher (p = 0.026), and the train‐of‐four count (TOF count) showed a significant increase at T₂ and T₃ (p < 0.001) compared to other doses.

What is new and conclusion

Rocuronium bromide at a rate of 9.0 μg·kg⁻¹·min⁻¹ provided suitable and adequate muscle relaxation without inhibiting IONM; thus, this dose is recommended for spinal surgery.

Tetanic stimulation of the peripheral nerve augments motor evoked potentials by re-exciting spinal anterior horn cells

Tetanic stimulation of the peripheral nerve, immediately prior to conducting transcranial electrical stimulation motor evoked potential (TES-MEP), increases MEP amplitudes in both innervated and uninnervated muscles by the stimulated peripheral nerve; this is known as the remote augmentation of MEPs. Nevertheless, the mechanisms underlying the remote augmentation of MEPs remain unclear. Although one hypothesis was that remote augmentation of MEPs results from increased motoneuronal excitability at the spinal cord level, the effect of spinal anterior horn cells has not yet been investigated. We aimed to investigate the effect of tetanic stimulation of the peripheral nerve on spinal cord anterior horn cells by analyzing the F-wave. We included 34 patients who underwent elective spinal surgeries and compared the changes in F-waves and TES-MEPs pre- and post-tetanic stimulation of the median nerve. F-wave analyses were recorded by stimulating the median and tibial nerves. TES-MEPs and F-wave analyses were compared between baseline and post-tetanic stimulation time periods using Wilcoxon signed-rank tests. A significant augmentation of MEPs, independent of the level corresponding to the median nerve, was demonstrated. Furthermore, F-wave persistence was significantly increased not only in the median nerve but also in the tibial nerve after tetanic stimulation of the median nerve. The increased F-wave persistence indicates an increase of re-excited motor units in spinal anterior horn cells. These results confirm the hypothesis that tetanic stimulation of the peripheral nerve may cause remote augmentation of MEPs, primarily by increasing the excitability of the anterior horn cells.

Anaesthesia and multimodality intraoperative neuromonitoring in carotid endarterectomy. Chronological evolution and effects on intraoperative neurophysiology

Contingency data was retrospectively collected to evaluate the historical and current ability to provide multimodality intraoperative neurophysiological monitoring during carotid endarterectomy under two conditions: total intravenous anaesthesia (TIVA) and low dose halogenated anaesthesia (SEVO). 229 patients were monitored during carotid endarterectomy procedures under general anaesthesia between 2012 and 2020. 121 Patients were monitored with SEVO at a minimum alveolar concentration less than 0.7 and 108 were monitored using TIVA, according to common anaesthetic practice standards in our hospital across the years. Multimodality IONM was established with electroencephalography, somatosensory evoked potentials and motor evoked potentials. As compared to TIVA, patients monitored with SEVO showed significantly higher motor evoked potential thresholds (313.52 ± 77.74 SEVO and 218.93 V ± 103.2 V TIVA p < 0.05) and lower reproducibility. Electroencephalography and somatosensory evoked potentials showed no significant differences among the groups. When using SEVO, multimodality intraoperative neurophysiological monitoring during carotid endarterectomy could mask or miss a motor isolated change in patients in spite of low dose minimum alveolar concentration and of apparently adequate electroencephalography and somatosensory evoked potentials for monitoring. Given these difficulties, we believe the chronological transfer to TIVA could have improved our ability to establish multimodality intraoperative neurophysiological monitoring during carotid endarterectomy in recent times.

The evolution of cranial meningioma surgery-a single-center 25-year experience

BACKGROUND

There have been major developments in diagnostic and surgical and non-surgical techniques used in the management of meningiomas over last three decades. We set out to describe these changes in a systematic manner.

METHOD

Clinical and radiological data, surgical procedures, complications, and outcome of 817 patients who underwent surgery for primarily diagnosed meningioma between 1991 and 2015 were investigated.

RESULTS

Median age at diagnosis increased significantly from 56 to 59 years (p = .042), while tumor location and preoperative Karnofsky performance status did not change during the observation period. Availability of preoperative MRI increased, and rates of angiography and tumor embolization decreased (p < .001, each). Median duration of total, pre-, and postoperative stay was 13, 2, and 9 days, respectively, and decreased between 1991 and 2015 (p < .001, each). Median incision-suture time varied annually (p < .001) but without becoming clearly longer or shorter during the entire observation period. The use of intraoperative neuronavigation and neuromonitoring increased, while the rates of Simpson grade I and III surgeries decreased (p < .001). Rates of postoperative hemorrhage (p = .997), hydrocephalus (p = .632), and wound infection (p = .126) did not change, while the frequency of early postoperative neurological deficits decreased from 21% between 1991 and 1995 to 13% between 2011 and 2015 (p = .003). During the same time, the rate of surgeries for postoperative cerebrospinal fluid leakage slightly increased from 2 to 3% (p = .049). Within a median follow-up of 62 months, progression was observed in 114 individuals (14%). Progression-free interval did not significantly change during observation period (p > .05). Multivariate analyses confirmed the lack of correlation between year of surgery and tumor relapse (HR: 1.1, p > .05).

CONCLUSIONS

Preoperative diagnosis and surgery of meningiomas have been substantially evolved. Although early neurological outcome has improved, long-term prognosis remains unchanged.

Prognosis of pituitary adenomas in the early 1970s and today-Is there a benefit of modern surgical techniques and treatment modalities?

OBJECTIVE

Neurosurgical techniques for the treatment of sellar pathologies have been evolving continuously over the last decades. Additionally to the innovation of approaches and surgical techniques, this progress yielded to the application of modern intraoperative surgical tools as well as peri- and intraoperative imaging. Until now, no long-term analysis of the impact of new therapy concepts on the patient's outcome exists. Aim of this study was to analyse the impact of new operative approaches on perioperative mortality and morbidity as well as the long-term outcome after pituitary surgery.

PATIENTS AND METHODS

Three groups of patients were compared in this retrospective analysis of surgically treated pituitary adenomas between the years of 1963 and 2014. Group A contains 93 patients, treated between 1963-1980 with a mean follow-up of 12.1 years (±14.3years), group B comprises 89 patients treated between 1990 and 2000 with a mean follow-up of 10.1 years (±8.1years) and group C consists of 95 patients treated between 2011-2014 with a mean follow-up of 3.4 years (±1.9years).

RESULTS

The surgical treatment was performed significantly earlier today on smaller tumors with less preoperative complaints (p<0.01). Panhypopituitarism was detected only in 9.5% of the cases in group C compared to 50.8% in group A (p<0.01). Also, the incidence of revision surgery (5.6 vs. 2% vs 0%), postoperative hemorrhage (10.8% vs. 3.4% vs. 1%) and diabetes insipidus (34.4% vs. 11.2% vs. 5.2%) was decreased (p<0.01). Moreover, a significant postoperative improvement of ophthalmological complaints was detected (p<0.001). The long-term follow-up showed 40% of the entire recurrence rate occurring after the ninth postoperative year. The progression-free survival time increased significantly from group A to group B (p<0.05).

CONCLUSIONS

The results demonstrate a benefit of the recent developments of pituitary surgery in the short-term results as well as in the long-term outcome. The prognosis of pituitary adenoma patients could be improved by the introduction of new surgical approaches and techniques in the last decades. Also the perioperative morbidity and mortality rate has been reduced clearly since the 1970s. Furthermore our results emphasise the necessity of lifelong follow-up of all patients with successfully treated pituitary adenomas.

[Neuroanesthesia]

Anesthesiological challenges during craniotomy result from the anatomically related low compensatory capacity of the intracranial space in response to increased volume and the low ischemic tolerance of brain tissue. The anesthetic agents used should therefore not increase the intracranial volume and improve the ischemic tolerance. An acute life-threatening increase of intracranial pressure can be temporarily treated by hyperventilation until measures, such as osmotherapy and infusion of intravenous anesthetics become effective. During an operation the homeostatic parameters including blood volume, blood pressure, partial pressure of carbon dioxide and oxygen in blood, plasma glucose concentration and core body temperature have to be closely monitored and kept normal (6 Ns). Optimal implementation of anesthesia necessitates a detailed knowledge of the surgical approach and potential complications. Postoperatively, patients should be extubated as soon as possible to closely monitor cognitive function so that potential deterioration can be detected.

Effect of peripheral nerve tetanic stimulation on the inter-trial variability and accuracy of transcranial motor-evoked potential in brain surgery

OBJECTIVES

The aim of this study was to evaluate and compare the advantages of post-tetanic motor-evoked potential (p-MEP) and conventional motor-evoked potential (c-MEP) in terms of MEP inter-trial variability and accuracy.

METHODS

c-MEP and p-MEP were quantified in subjects who underwent brain surgery. c-MEP was generated by transcranial electrical stimulation (TES). p-MEP was generated using a preconditioning process involving tetanic stimulation at the left tibial nerve followed by TES. The presence of significant MEP deterioration was monitored during major surgical process. An additional 5-8 MEP obtained after major surgical process were used to analyze amplitude parameters such as mean, standard deviation, range, coefficient of variation (CV), and range to mean ratio.

RESULTS

When only irreversible MEP deteriorations were considered as positive results, the false-positive rate was identical for p-MEP and c-MEP. When total MEP deteriorations were considered as positive results, the false-positive rate of p-MEP was lower and p-MEP had higher specificity than c-MEP. The mean amplitude of p-MEP was significantly higher than that of c-MEP. The CV and range to mean ratio of p-MEP were less than those of c-MEP.

CONCLUSION

The p-MEP technique is useful for augmenting MEP amplitude and reducing inter-trial variability.

SIGNIFICANCE

p-MEP has clinical significance as a useful technique for intraoperative monitoring.

Transient and reproducible loss of motor-evoked potential signals after intravenous levetiracetam in a child undergoing craniotomy for resection of astrocytoma

Transcranial electrical motor-evoked potential (tceMEP) monitoring is used in complex intracranial and spinal surgeries to detect and prevent neurological injury. We present a case of transient, reproducible loss of tceMEPs after an infusion of levetiracetam during craniotomy and tumor resection in a child. Cessation of the infusion resulted in restoration of baseline tceMEPs. When the infusion was resumed at the end of the procedure, a similar decrease in tceMEPs was seen as before, after the infusion was stopped. The surgery and postoperative course proceeded without incident, and the patient experienced a full recovery.

Is hemifacial spasm a phenomenon of the central nervous system? --The role of desflurane on the lateral spread response

OBJECTIVE

A signature EMG feature of hemifacial spasm (HFS) is the lateral spread response (LSR). Desflurane is a common anesthetic with potent effects on synaptic transmission. We tested the hypothesis that the LSR is mediated by corticobulbar components by comparing the LSR during total intravenous anesthesia (TIVA) or TIVA plus desflurane during microvascular decompression (MVD) surgery.

METHODS

22 HFS patients undergoing MVD surgery participated in this prospective study. The LSR data was recorded from the o. oculi, o. oris and mentalis muscles prior to opening dura. LSR onset latencies and amplitudes were determined under TIVA and TIVA/desflurane (0.5 and 1MAC). Facial muscle LSRs and EEG were analyzed.

RESULTS

Desflurane (1MAC) significantly decreased the LSR amplitude in all 3 facial muscles (p<0.01). Pooled LSR data from all facial muscles showed desflurane inhibited the LSR amplitude by 43% compared to TIVA (p<0.001). No effects on the latency of the LSR or on EEG state were observed.

CONCLUSIONS

LSR inhibition by desflurane suggests a central mechanism involvement in the genesis of this signature HFS response.

SIGNIFICANCE

This study demonstrates that facial nerve vascular compression and plastic changes within the CNS are part of the pathophysiology of HFS.

Effect of mild hypothermic cardiopulmonary bypass on the amplitude of somatosensory-evoked potentials

BACKGROUND

Several neurophysiological techniques are used to intraoperatively assess cerebral functioning during surgery and intensive care, but the introduction of hypothermia as a means of intraoperative neuroprotection has brought their reliability into question. The present study aimed to evaluate the effect of mild hypothermia on somatosensory-evoked potentials' (SSEPs) amplitude and latency in a cohort of cardiopulmonary bypass (CPB) patients as the temperature reached the steady-state.

MATERIALS AND METHODS

The amplitude and latency of 4 different SSEP signals--N9, N13, P14/N18 interpeak, and N20/P25--were evaluated retrospectively in 84 patients undergoing CPB during normothermic (36°C±0.43°C) and mild hypothermic (32°C±1.38°C) conditions. SSEPs were recorded in normothermia immediately after the induction of anesthesia and in hypothermia as the temperature reached its steady-state, specifically, when the nasopharyngeal temperature was equivalent to the rectal temperature (±0.5°C). A paired-samples t test was performed for each SSEP to test the differences in latencies and amplitudes between normothermic and hypothermic conditions.

RESULTS

Compared with normothermia, hypothermia not only significantly increased the latency of all SSEPs, N9 (P<0.001), N13 (P<0.001), P14/N18 (P<0.001), and N20/P25 (P<0.001), but also the amplitude of N9 (P<0.001) and N20/P25 (P<0.001).

CONCLUSIONS

The increased amplitude in particularly of cortical SSEPs (N20/P25), detected specifically during steady-state hypothermia, seems to support the clinical utility of this methodology in monitoring the brain function not only during cardiac surgery with CPB, but also in other settings like therapeutic hypothermia procedures in an intensive care unit.