Efficient extraction of data from intra-operative evoked potentials: 1.-Theory and simulations.

UNLABELLED Quickly and efficiently extracting evoked potential information from noise is critical to the clinical practice of intraoperative neurophysiologic monitoring (IONM). Currently this is primarily done using trained professionals to interpret averaged waveforms. The purpose of this paper is to evaluate and compare multiple means of electronically extracting simple to understand evoked potential characteristics with minimum averaging. A number of evoked potential models are studied and their performance evaluated as a function of the signal to noise level in simulations. METHODS which extract the least number of parameters from the data are least sensitive to the effects of noise and are easiest to interpret. The simplest model uses the baseline evoked potential and the correlation receiver to provide an amplitude measure. Amplitude measures extracted using the correlation receiver show superior performance to those based on peak to peak amplitude measures. In addition, measures of change in latency or shape of the evoked potential can be extracted using the derivative of the baseline evoked response or other methods. This methodology allows real-time access to amplitude measures that can be understood by the entire OR staff as they are small, dimensionless numbers of order unity which are simple to interpret. The IONM team can then adjust averaging and other parameters to allow for visual interpretation of waveforms as appropriate

Anesthesia and intraoperative neurophysiological spinal cord monitoring

Purpose of review We will explain the basic principles of intraoperative neurophysiological monitoring (IONM) during spinal surgery. Thereafter we highlight the significant impact that general anesthesia can have on the efficacy of the IONM and provide an overview of the essential pharmacological and physiological factors that need to be optimized to enable IONM. Lastly, we stress the importance of teamwork between the anesthesiologist, the neurophysiologist, and the surgeon to improve clinical outcome after spinal surgery. Recent findings In recent years, the use of IONM has increased significantly. It has developed into a mature discipline, enabling neurosurgical procedures of ever-increasing complexity. It is thus of growing importance for the anesthesiologist to appreciate the interplay between IONM and anesthesia and to build up experience working in a team with the neurosurgeon and the neurophysiologist. Safety measures, cooperation, careful choice of drugs, titration of drugs, and maintenance of physiological homeostasis are essential for effective IONM

The bispectrum of the human electroencephalogram

It was the purpose of this thesis to investigate the utility of the bispectrum in the analysis of the human electroencephalogram (EEG). First 95% confidence thresholds were determined for the normalized bicoherence and phase bicoherence. From these 95% confidence thresholds we studied the ability of these measures to extract significant interactions in varying degrees of noise as compared to existing measures. Investigations into interactions in the visual system were carried out to demonstrate the utility of the bicoherence as a signal tracer through the nervous system. Bispectra, bicoherence, and biphase were calculated for eight subjects observing the visual stimulus monocularly. Both phase vs frequency and biphase vs frequency plots were made to determine weighted time delays from stimulus application to signal appearance at the EEG electrodes. Bispectral analysis revealed non-linear interactions between visual fields occurring at the level of the cortex with weighted delay times of 410 $\pm$ 58 msec while the non-interactive components propagated with weighted time delays of 202 $\pm$ 39 msec. Combining these results with the predictions of various existing models we conclude that the interaction does not occur in the retina. This conclusion was confirmed by a second set of experiments, in which each stimulus was presented to a different visual field in separate eyes. Investigations into the inter-ictal EEG of patients afflicted with epilepsy were carried out to determine if the side of the cortex that contained the seizure focus could be differentiated from the side that did not contain the seizure focus. The emphasis was on patients who had seizures emanating from the anterior temporal lobe. It was found that bispectral analysis of this area could not differentiate the side containing the ictal focus from the side not containing the ictal focus during the inter-ictal EEG; however during the ictal period, the bispectrum identifies harmonic order in the EEG signal that is not found either visually or in the power spectrum. These results show the bispectrum as a effective tool in the analysis of the EEG

The bispectrum of the human electroencephalogram

It was the purpose of this thesis to investigate the utility of the bispectrum in the analysis of the human electroencephalogram (EEG). First 95% confidence thresholds were determined for the normalized bicoherence and phase bicoherence. From these 95% confidence thresholds we studied the ability of these measures to extract significant interactions in varying degrees of noise as compared to existing measures. Investigations into interactions in the visual system were carried out to demonstrate the utility of the bicoherence as a signal tracer through the nervous system. Bispectra, bicoherence, and biphase were calculated for eight subjects observing the visual stimulus monocularly. Both phase vs frequency and biphase vs frequency plots were made to determine weighted time delays from stimulus application to signal appearance at the EEG electrodes. Bispectral analysis revealed non-linear interactions between visual fields occurring at the level of the cortex with weighted delay times of 410 $\pm$ 58 msec while the non-interactive components propagated with weighted time delays of 202 $\pm$ 39 msec. Combining these results with the predictions of various existing models we conclude that the interaction does not occur in the retina. This conclusion was confirmed by a second set of experiments, in which each stimulus was presented to a different visual field in separate eyes. Investigations into the inter-ictal EEG of patients afflicted with epilepsy were carried out to determine if the side of the cortex that contained the seizure focus could be differentiated from the side that did not contain the seizure focus. The emphasis was on patients who had seizures emanating from the anterior temporal lobe. It was found that bispectral analysis of this area could not differentiate the side containing the ictal focus from the side not containing the ictal focus during the inter-ictal EEG; however during the ictal period, the bispectrum identifies harmonic order in the EEG signal that is not found either visually or in the power spectrum. These results show the bispectrum as a effective tool in the analysis of the EEG

Safety issues during surgical monitoring

While intra-operative neuro-physiologic assessment and monitoring improve the safety of patients, its use may also introduce new risks of injuries. This chapter looks at the electric safety of equipment and the potential hazards during the set-up of the monitoring. The physical and functional physiologic effects of electric shocks and stimulation currents, standards for safety limits, and conditions for tissue damage are described from basic physical principles. Considered are the electrode-tissue interface in relation to electrode dimensions and stimulation parameters as applied in various modalities of evoked sensory and motor potentials as to-date used in intra-operative monitoring, mapping of neuro-physiologic functions. A background is given on circumstances for electric tissue heating and heat drainage, thermal toxicity, protection against thermal injuries and side effects of unintended activation of neural and cardiac tissues, adverse effects of physiologic amplifiers from transcranial stimulation (TES) and excitotoxicity of direct cortical stimulation. Addressed are safety issues of TES and measures for prevention. Safety issues include bite and movement-induced injuries, seizures, and after discharges, interaction with implanted devices as cardiac pacemaker and deep brain stimulators. Further discussed are safety issues of equipment leakage currents, protection against electric shocks, and maintenance.</p

Failure of Sequential Compression Device Detected by Neuromonitoring during Minimally Invasive Posterior Scoliosis Surgery

Intraoperative neuromonitoring is recommended as standard practice for corrective scoliosis surgery. Common methods include somatosensory-evoked potentials (SSEPs) and transcranial motor-evoked potentials (TcMEPs), which have been shown to have a high diagnostic accuracy in detecting new neurological deficits postoperatively. Sequential compression devices (SCDs) are a common method for thromboprophylaxis in spine surgery and are not known to have many device-related complications. To date, there have been no reports of lower extremity ischemia secondary to SCD deflation failure detected by multimodality neuromonitoring during minimally invasive posterior spine surgery. We, therefore, present a case report of an 18-year-old male with adolescent idiopathic scoliosis who underwent minimally invasive posterior spinal fusion with instrumentation. Intraoperative decrease in SSEPs and TcMEPs were noted in the left leg shortly after incision before any instrumentation or reduction occurred. Further examination revealed that the left leg was hypoperfused compared with the right leg and that the left SCD was not properly deflating. Bilateral SCDs were removed, and perfusion and neuromonitoring returned to baseline immediately. Bilateral SCDs and the machine were replaced, and neuromonitoring remained within normal limits for the rest of the surgery. The patient had no postoperative neurologic or vascular deficits. Early detection of lower extremity ischemia by neuromonitoring resulted in the prompt identification and addressing of SCD malfunction, sparing devastating neurological and vascular injury to the patient's leg. This case reinforces the importance of neuromonitoring within spine surgery