A pilot study to record visual evoked potentials during prone spine surgery using the SightSaver™ photic visual stimulator.

This is a pilot study to assess the clinical safety and efficacy of recording real-time flash visual evoked potentials (VEPs) using the SightSaver TM Visual Stimulator mask during prone spine surgery. A prospective, observational pilot study. Twenty patients presenting for spine surgery (microdiscectomy, 1-2 level lumbar fusion, or > 2 levels thoraco-lumbar fusion) were enrolled. The SightSaver™ Visual Stimulator™ was used to elicit VEPs throughout surgery. Somatosensory evoked potentials (SSEPs) were simultaneously recorded. All patients underwent general anesthesia with a combination of intravenous and inhaled agents. The presence, absence, and changes in VEP were qualitatively analyzed. Reproducible VEPs were elicited in 18/20 patients (36/40 eyes). VEPs were exquisitely sensitive to changes in anesthesia and decayed with rising MAC of isoflurane and/or N2O. Decrements in VEPs were observed without concomitant changes in SSEPs. The mask was simple to apply and use and was not associated with adverse effects. The SightSaver™ mask represents an emerging technology for monitoring developing visual insults during surgery. The definitive applications remain to be determined, but likely include use in select patients and/or surgeries. Here, we have validated the device as safe and effective, and show that VEPs can be recorded in real time under general anesthesia in the prone position. Future studies should be directed towards understanding the ideal anesthetic regimen to facilitate stable VEP recording during prone spine surgery.

Evidence-based guideline update: intraoperative spinal monitoring with somatosensory and transcranial electrical motor evoked potentials: report of the Therapeutics and Technology Assessment Subcommittee of the American Academy of Neurology and the American Clinical Neurophysiology Society.

OBJECTIVE: To evaluate whether spinal cord intraoperative monitoring (IOM) with somatosensory and transcranial electrical motor evoked potentials (EPs) predicts adverse surgical outcomes. METHODS: A panel of experts reviewed the results of a comprehensive literature search and identified published studies relevant to the clinical question. These studies were classified according to the evidence-based methodology of the American Academy of Neurology. Objective outcomes of postoperative onset of paraparesis, paraplegia, and quadriplegia were used because no randomized or masked studies were available. RESULTS AND RECOMMENDATIONS: Four Class I and 8 Class II studies met inclusion criteria for analysis. The 4 Class I studies and 7 of the 8 Class II studies reached significance in showing that paraparesis, paraplegia, and quadriplegia occurred in the IOM patients with EP changes compared with the IOM group without EP changes. All studies were consistent in showing all occurrences of paraparesis, paraplegia, and quadriplegia in the IOM patients with EP changes, with no occurrences of paraparesis, paraplegia, and quadriplegia in patients without EP changes. In the Class I studies, 16%-40% of the IOM patients with EP changes developed postoperative-onset paraparesis, paraplegia, or quadriplegia. IOM is established as effective to predict an increased risk of the adverse outcomes of paraparesis, paraplegia, and quadriplegia in spinal surgery (4 Class I and 7 Class II studies). Surgeons and other members of the operating team should be alerted to the increased risk of severe adverse neurologic outcomes in patients with important IOM changes (Level A).

Propagation of epileptiform activity on a submillimeter scale.

Microseizures are highly focal low-frequency epileptiform-appearing events recorded from the neocortex of epilepsy patients. Because of their tiny, often submillimeter distribution, they may be regarded as a high-resolution window into the epileptic process, providing an excellent opportunity to study the fine temporal structure of their origin and spread. A 16 mm² 96-microelectrode array with 400-µm interelectrode spacing was implanted in seven patients undergoing invasive EEG monitoring for medically refractory epilepsy. Seven microdischarge populations were tested for a substantial contribution by volume conduction to the observed waveform amplitudes. Single-unit activity was examined for specific evidence of neural activity at multiple sites within the microdischarge fields. We found that microdischarges appear to originate at a highly focal source location, likely within a single cortical macrocolumn, and spread to local and more distant sites via neural propagation.

Spatial characterization of interictal high frequency oscillations in epileptic neocortex.

Interictal high frequency oscillations (HFOs), in particular those with frequency components in excess of 200 Hz, have been proposed as important biomarkers of epileptic cortex as well as the genesis of seizures. We investigated the spatial extent, classification and distribution of HFOs using a dense 4 x 4 mm(2) two dimensional microelectrode array implanted in the neocortex of four patients undergoing epilepsy surgery. The majority (97%) of oscillations detected included fast ripples and were concentrated in relatively few recording sites. While most HFOs were limited to single channels, approximately 10% occurred on a larger spatial scale with simultaneous but morphologically distinct detections in multiple channels. Eighty per cent of these large-scale events were associated with interictal epileptiform discharges. We propose that large-scale HFOs, rather than the more frequent highly focal events, are the substrates of the HFOs detected by clinical depth electrodes. This feature was prominent in three patients but rarely seen in only one patient recorded outside epileptogenic cortex. Additionally, we found that HFOs were commonly associated with widespread interictal epileptiform discharges but not with locally generated 'microdischarges'. Our observations raise the possibility that, rather than being initiators of epileptiform activity, fast ripples may be markers of a secondary local response.

Electrographic seizures and periodic discharges after intracerebral hemorrhage.

OBJECTIVE: To determine the frequency and significance of electrographic seizures and other EEG findings in patients with intracerebral hemorrhage (ICH). METHODS: We reviewed 102 consecutive patients with ICH who underwent continuous electroencephalographic monitoring (cEEG). Demographic, clinical, radiographic, and cEEG findings were recorded. Using multivariate logistic regression analysis, we determined factors associated with 1) electrographic seizures, 2) periodic epileptiform discharges (PEDs), and 3) poor outcome (death, vegetative or minimally conscious state) at hospital discharge. RESULTS: Seizures occurred in 31% (n = 32) of patients with ICH, prior to cEEG in 19 patients. Eighteen percent (n = 18) of patients had electrographic seizures; only one of these patients also had clinical seizures while on cEEG. After controlling for demographic and clinical predictors, only an increase in ICH volume of 30% or more between admission and 24-hour follow-up CT scan was associated with electrographic seizures (33% vs 15%; OR 9.5, 95% CI 1.7 to 53.8). PEDs were less frequently seen in those with hemorrhages located at least 1 mm from the cortex (8% vs 29%; OR 0.2, 95% CI 0.1 to 0.7). PEDs were independently associated with poor outcome (65% vs 17%; OR 7.6, 95% CI 2.1 to 27.3). In patients with electrographic seizures, the first seizure was detected within the first hour of cEEG monitoring in 56% and within 48 hours in 94%. CONCLUSIONS: Seizures occurred in one third of patients with intracerebral hemorrhage (ICH) and over half were purely electrographic. Electrographic seizures were associated with expanding hemorrhages, and periodic discharges with cortical ICH and poor outcome. Further research is needed to determine if treating or preventing seizures or PEDs might lead to improved outcome after ICH.

Detection of electrographic seizures with continuous EEG monitoring in critically ill patients.

OBJECTIVE: To identify patients most likely to have seizures documented on continuous EEG (cEEG) monitoring and patients who require more prolonged cEEG to record the first seizure. METHODS: Five hundred seventy consecutive patients who underwent cEEG monitoring over a 6.5-year period were reviewed for the detection of subclinical seizures or evaluation of unexplained decrease in level of consciousness. Baseline demographic, clinical, and EEG findings were recorded and a multivariate logistic regression analysis performed to identify factors associated with 1) any EEG seizure activity and 2) first seizure detected after >24 hours of monitoring. RESULTS: Seizures were detected in 19% (n = 110) of patients who underwent cEEG monitoring; the seizures were exclusively nonconvulsive in 92% (n = 101) of these patients. Among patients with seizures, 89% (n = 98) were in intensive care units at the time of monitoring. Electrographic seizures were associated with coma (odds ratio [OR] 7.7, 95% CI 4.2 to 14.2), age <18 years (OR 6.7, 95% CI 2.8 to 16.2), a history of epilepsy (OR 2.7, 95% CI 1.3 to 5.5), and convulsive seizures during the current illness prior to monitoring (OR 2.4, 95% CI 1.4 to 4.3). Seizures were detected within the first 24 hours of cEEG monitoring in 88% of all patients who would eventually have seizures detected by cEEG. In another 5% (n = 6), the first seizure was recorded on monitoring day 2, and in 7% (n = 8), the first seizure was detected after 48 hours of monitoring. Comatose patients were more likely to have their first seizure recorded after >24 hours of monitoring (20% vs 5% of noncomatose patients; OR 4.5, p = 0.018). CONCLUSIONS: CEEG monitoring detected seizure activity in 19% of patients, and the seizures were almost always nonconvulsive. Coma, age <18 years, a history of epilepsy, and convulsive seizures prior to monitoring were risk factors for electrographic seizures. Comatose patients frequently required >24 hours of monitoring to detect the first electrographic seizure.

Continuous EEG monitoring and midazolam infusion for refractory nonconvulsive status epilepticus.

BACKGROUND: Although cIV-MDZ has emerged as a popular alternative to barbiturate therapy for refractory status epilepticus (RSE), experience with its use for this indication is limited. OBJECTIVE: - To evaluate the efficacy of continuous intravenous midazolam (cIV-MDZ) for attaining sustained seizure control in patients with RSE. METHODS: The authors reviewed 33 episodes of RSE treated with cIV-MDZ in their neurologic intensive care unit over 6 years. All patients were monitored with continuous EEG (cEEG). MDZ infusion rates were titrated to eliminate clinical and EEG seizure activity; cIV-MDZ was discontinued once patients were seizure-free for 24 hours. Acute treatment failures (seizures 1 to 6 hours after starting cIV-MDZ), breakthrough seizures (after 6 hours of therapy), post-treatment seizures (within 48 hours of discontinuing therapy), and ultimate treatment failure (frequent seizures that led to treatment with pentobarbital or propofol) were identified. RESULTS: All patients were in nonconvulsive SE at the time cIV-MDZ was started; the mean duration of SE before treatment was 3.9 days (range 0 to 17 days). In addition to benzodiazepines, 94% of patients had received at least two antiepileptic drugs (AED) before starting cIV-MDZ. The mean loading dose was 0.19 mg/kg, the mean maximal infusion rate was 0.22 mg/kg/h, and the mean duration of cIV-MDZ therapy was 4.2 days (range 1 to 14 days). Acute treatment failure occurred in 18% (6/33) of episodes, breakthrough seizures in 56% (18/32), post-treatment seizures in 68% (19/28), and ultimate treatment failure in 18% (6/33). Breakthrough seizures were clinically subtle or purely electrographic in 89% (16/18) of cases and were associated with an increased risk of developing post-treatment seizures (p = 0.01). CONCLUSIONS: Although most patients with RSE initially responded to cIV-MDZ, over half developed subsequent breakthrough seizures, which were predictive of post-treatment seizures and were often detectable only with cEEG. Titrating cIV-MDZ to burst suppression, more aggressive treatment with concurrent AED, or a longer period of initial treatment may reduce the high proportion of patients with RSE who relapse after cIV-MDZ is discontinued.

Attenuation of somatosensory evoked potentials during positioning in a patient undergoing suboccipital craniectomy for Chiari I malformation with syringomyelia.

Intraoperative electrophysiologic monitoring can diminish the risk of neurologic injury by enabling the detection of injury at a time when it can be reversed or minimized. This report describes a 14-year-old girl with a Chiari's malformation type I and syringomyelia who underwent a suboccipital decompression and dural patch grafting with concurrent somatosensory evoked potentials. When the patient was turned into the prone position and the neck was flexed, the left-sided somatosensory evoked potential deteriorated. After the patient's neck was repositioned, the left median nerve potential improved but did not return to baseline. Postoperatively, the patient had decreased proprioception of her left arm, which completely resolved at 2-week follow-up. This single case report does not establish the need for routine somatosensory evoked potential monitoring. Nevertheless, deterioration of the potential in this case led directly to a change in the surgical positioning, which may have significantly reduced the chances of a permanent neurologic injury.

Spike detection II: automatic, perception-based detection and clustering.

OBJECTIVES: We developed perception-based spike detection and clustering algorithms. METHODS: The detection algorithm employs a novel, multiple monotonic neural network (MMNN). It is tested on two short-duration EEG databases containing 2400 spikes from 50 epilepsy patients and 10 control subjects. Previous studies are compared for database difficulty and reliability and algorithm accuracy. Automatic grouping of spikes via hierarchical clustering (using topology and morphology) is visually compared with hand marked grouping on a single record. RESULTS: The MMNN algorithm is found to operate close to the ability of a human expert while alleviating problems related to overtraining. The hierarchical and hand marked spike groupings are found to be strikingly similar. CONCLUSIONS: An automatic detection algorithm need not be as accurate as a human expert to be clinically useful. A user interface that allows the neurologist to quickly delete artifacts and determine whether there are multiple spike generators is sufficient.

Evidence for unconscious memory processing during elective cardiac surgery.

BACKGROUND: Many anesthetic drugs have been shown to disrupt conscious recall (explicit memory) in volunteers. However, unconscious processing (implicit memory) of intraoperative auditory material may occur during general anesthesia and may provide an opportunity for intraoperative therapeutic intervention. In this study, we examined patients undergoing elective cardiac surgery for evidence of intraoperative implicit and explicit memory. METHODS AND RESULTS: Twenty-five subjects provided written informed consent and underwent general anesthesia and cardiopulmonary bypass for cardiac surgery. During the operation, patients were randomized to receive 1 of 2 different audiotapes of associated word pairs. Postoperatively, a blinded observer conducted a standardized interview to determine the extent of intraoperative implicit and explicit memory. With the use of free association, significant intraoperative implicit memory was found. In contrast, no patient had spontaneous or directed recall of intraoperative events, and we did not find evidence of intraoperative explicit memory with a recognition task. CONCLUSIONS: Patients undergoing general anesthesia for cardiac surgery were reliably able to reinforce associations between word pairs solely on the basis of their intraoperative presentation. This provides further evidence that patients are capable of processing intraoperative auditory information.

Evoked potentials in clinical trials for multiple sclerosis.

Multiple sclerosis produces disruption of conduction in the central nervous system by a variety of mechanisms, relating, in part, to loss of the myelin sheath. Although often not well correlated with the clinical course of the disease in individual patients, the resulting evoked potential (EP) disturbances can serve as measures of an accumulating disease burden, particularly in longitudinal population studies. Accordingly, EPs can serve as useful instruments for assessing the effectiveness of therapeutic agents which may alter the course of the multiple sclerosis. Furthermore, since EPs measure conduction within the central nervous system, they provide a means of directly assessing symptomatic treatments designed to improve central conduction.

Intraoperative monitoring of motor evoked potentials: a review of 116 cases.

We reviewed the results of motor evoked potential (MEP) and somatosensory evoked potential (SEP) monitoring during 116 operations on the spine or spinal cord. We monitored MEPs by electrically stimulating the spinal cord and recording compound muscle action potentials from lower extremity muscles and monitored SEPs by stimulating posterior tibial or peroneal nerves and recording both cortical and subcortical evoked potentials. We maintained anesthesia with an N2O/O2/opioid technique supplemented with a halogenated inhalational agent and maintained partial neuromuscular blockade using a vecuronium infusion. Both MEPs and SEPs could be recorded in 99 cases (85%). Neither MEPs nor SEPs were recorded in eight patients, all of whom had preexisting severe myelopathies. Only SEPs could be recorded in two patients, and only MEPs were obtained in seven cases. Deterioration of evoked potentials occurred during nine operations (8%). In eight cases, both SEPs and MEPs deteriorated; in one case, only MEPs deteriorated. In four cases, the changes in the monitored signals led to major alterations in the surgery. We believe that optimal monitoring during spinal surgery requires recording both SEPs and MEPs. This provides independent verification of spinal cord integrity using two parallel but independent systems, and also allows detection of the occasional insults that selectively affect either motor or sensory systems.

The reliability of quantitative electroencephalography as an indicator of cerebral ischemia.

The electroencephalogram (EEG) has been used to detect episodes of cerebral ischemia during various surgical procedures. Recently, computerized systems for recording and interpreting the quantitative EEG (QEEG) have been used by anesthesiologists because of their ease of application, clarity of display, and reported ability to identify ischemic EEG changes. However, the extent to which automated techniques of QEEG interpretation reliably differentiate cerebral ischemia from the confounding effects of anesthetics and other sources of "artifact" is not completely established. In this study, EEGs were recorded before and after defibrillator testing in patients undergoing implantable cardioverter defibrillator (ICD) placement and during analogous time periods in control patients undergoing abdominal surgery. EEGs were subjected to standard visual inspection by an experienced electroencephalographer and QEEG analysis with a commercially available system was used for automated EEG interpretation in order to evaluate the reliability of this quantitative technique. The CIMON technique identified episodes which met previously defined criteria for QEEG cerebral dysfunction and ischemic pattern in both groups, despite the presumed absence of cerebral ischemia in the control patients. Since there was no evidence of cerebral ischemia in the raw EEGs of either the ICD patients or the controls, these QEEG changes were not confirmed by conventional techniques of EEG interpretation. Our results suggest that caution is warranted when using automated systems for intraoperative interpretation of EEG.

Propagation patterns of temporal spikes.

In standard EEG recordings, spikes appear as single events characterized mainly by the scalp location of the their peak voltage. The signal-to-noise ratio of raw EEG is usually too high to permit more detailed analysis. We used spike averaging to improve the resolution of interictal spikes in 40 patients with temporal lobe epilepsy. Spikes were identified visually in raw, digitally stored EEG. When multiple spike types were present in a patient, they were grouped separately. Spikes were synchronized for averaging by aligning their negative peaks in a designated channel. Sixteen patients demonstrated spike propagation from anterior temporal to posterior temporal electrode locations. Thirty-six patients demonstrated spread of spikes from anterior temporal to fronto-polar electrode sites. While anterior temporal and fronto-polar spikes were often synchronous, fronto-polar spikes followed anterior temporal discharges in 25% of cases and preceded them in 13%. Spike averaging revealed propagation patterns not apparent on visual inspection of raw EEG. We speculate that these patterns may reflect inherent physiological properties of temporal and frontal neuronal circuits, possibly utilized by the epileptogenic process.

Implantable cardioverter-defibrillator. Evaluation of clinical neurologic outcome and electroencephalographic changes during implantation.

During placement of implantable cardioverter-defibrillators, ventricular arrhythmias are induced to test the function of the devices. Although cerebral hypoperfusion and ischemic electroencephalographic changes occur in patients while implantable cardioverter-defibrillators are being tested, no investigation has assessed neurologic outcome in these patients. Nine patients having either implantation or change of an implantable cardioverter-defibrillator underwent neurologic examination and neuropsychometric tests before and after the operation. After induction of general anesthesia and insertion of implantable cardioverter-defibrillator leads (when needed), ventricular fibrillation, ventricular flutter, or ventricular tachycardia, was induced by means of programmed electrical stimulation. Implantable cardioverter-defibrillator testing continued until satisfactory lead placement was confirmed. The intraoperative electroencephalographic recording was analyzed for evidence of ischemic change. In all, an electroencephalogram was recorded during 50 periods of circulatory arrest. Mean duration of the arrest periods was 13.6 seconds. By means of conventional visual inspection of the raw electroencephalogram, high-amplitude rhythmic delta or theta, voltage attenuation, or loss of fast frequency activity was observed in 30 of the arrests. By means of an automated technique of electroencephalographic interpretation based on power spectral analysis, electroencephalographic changes were correctly identified in 26 of the arrests. The incidence of these electroencephalographic changes was dependent on the arrest duration. The mean interval from arrest onset to electroencephalographic change was 7.5 seconds (standard deviation +/- 1.8 seconds). In patients with electroencephalographic changes during multiple arrests, no downward trend in this interval was detected in later arrests and no evidence of persistent ischemic change was observed in electroencephalograms recorded after the conclusion of implantable cardioverter-defibrillator testing. Postoperative neurologic and neuropsychometric testing was completed in eight patients, none of whom exhibited a new neurologic deficit, exacerbation of a preexisting neurologic condition, or significant deterioration in neuropsychometric performance. We conclude that the brief arrest of cerebral circulation induced during insertion of an implantable cardioverter-defibrillator is not associated with permanent neurologic injury.

Monitoring of intraoperative motor-evoked potentials under conditions of controlled neuromuscular blockade.

Motor-evoked potentials were recorded after electrical spinal cord stimulation in 19 patients undergoing neurosurgical or orthopedic procedures. Anesthesia was maintained with nitrous oxide, opioids, and inhaled anesthetics. Vecuronium was infused sufficient to eliminate 90% of twitch tension. The spinal cord was stimulated using either epidural or subarachnoid electrodes. Compound muscle action potentials were recorded from quadriceps and tibialis anterior muscles. Well-formed, stable motor-evoked potentials were recorded in all but one patient, in whom a preexisting myelopathy was felt to preclude recording. Intraoperative deterioration of motor-evoked potentials occurred in one patient who had a postoperative neurologic deficit. This study demonstrates the feasibility and utility of intraoperative motor tract monitoring using direct spinal cord stimulation. Controlled neuromuscular blockade permits recording of compound muscle action potentials while eliminating patient motor activity that could interfere with surgery.

Monitoring during supratentorial surgery.

Electroneurophysiological monitoring is employed during various supratentorial surgical procedures. EEG and evoked potential monitoring are used to detect and to facilitate the timely correction of cerebral ischemia during carotid endarterectomy and aneurysm surgery. Direct cortical recording and stimulation is used to identify areas or cortex that would be likely to produce clinical deficits if removed. Electrocorticography is used to identify epileptogenic cortex intraoperatively during surgical treatment of epilepsy.

Repetitive high magnetic field stimulation: the effect upon rat brain.

The effect on rat brain of a large number of stimulations with a high strength pulsed magnetic field was investigated in 31 rats: 10 naive controls, 10 anesthetized controls, and 11 stimulated and anesthetized rats. An investigational magnetic stimulating device with a circular 5.5 inch diameter stimulating head was used. The stimulating coil was energized by 1000 V, 8000A, 200 microseconds half sine pulses at a rate of 8 Hz. The peak field strength was 3.4 T, and the peak field flux was approximately 53,000 T/sec. Stimulation was performed for 20 min at a rate of 8/stimuli per second, for a total of at least 10,000 stimulations. The rats were sacrificed after 8 days, and their brains were examined using light microscopy with hematoxylin and eosin staining, or electron microscopy. Histological samples were taken from the neocortex, the hippocampus, the basal ganglia, and the cerebellum. No significant changes were seen.

Normal brain-stem auditory evoked potentials with abnormal latency-intensity studies in patients with acoustic neuromas.

Brain-stem auditory evoked potentials (BAEPs) are highly sensitive for detecting acoustic neuromas but false-negative results occur. We studied BAEPs preoperatively in 39 cases of acoustic neuroma. Absolute and interpeak latencies ipsilateral to the tumor, and interaural latency differences, were normal in four patients with small tumors. In three of these, however, results of latency-intensity studies were abnormal. In one patient, the latency-intensity result became normal postoperatively. If acoustic neuroma is suspected, and BAEPs are normal by usual criteria, latency-intensity functions should be examined to maximize chances of detecting a small tumor.