Neurological Monitoring During Anesthesia

Abstract

Intraoperative neurophysiological monitoring has been utilized in attempts to minimize neurological morbidity from operative manipulations. The goal of such monitoring is to identify changes in brain, spinal cord, and peripheral nerve function prior to irreversible damage. Intraoperative monitoring also has been effective in localizing anatomical structures, including peripheral nerves and sensorimotor cortex, which helps guide the surgeon during dissection. Cerebral monitoring of patients generally focuses on intracranial pressure ICP and CPPmonitoring. Recently, several new techniques have become available for more detailed routine monitoring of cerebral function, oxygenation and metabolism.ICP measurement plays an important role in the management of patients with head injury and neurosurgical patients. Microtransducer technology has contributed to the contemporary widespread usage of ICP monitoring following several types of ABI. ICP monitoring is used not only to monitor and manage ICP, but also to quantify CPP. Elevated ICP has been recognized as one of the most important factors affecting morbidity and mortality rates in patients who have had TBI; therefore, ICP monitoring has become routine in the management of severe head injuries. Hemodynamic monitoring in the care of the critically ill patient provides information that assists the clinician in minimizing secondary neuronal injury.rCBF is considered an important upstream monitoring parameter that is indicative of tissue viability. Continuous monitoring of rCBF could provide the opportunity to diagnose and to correct insufficient rCBF before deficits in tissue oxygenation and metabolism are recognised.There are a variety of CBF measurement techniques available, such as stable xenon-enhanced computed tomography, single-photon-emission computed tomography, MRI and PET; however, these methods are hampered by several clinical and practical drawbacks. Although laser- Doppler flowmetryand thermal diffusion flowmetry based measurement techniques provide continuous bedside monitoring of CBF, their clinical acceptance has been very low due to enduring technical drawbacks. Recently, the TD-rCBF microprobe, which is implanted intra-parenchymallyproved to circumvent the major drawbacks of the old systems that have been in use so far. Entropy as an estimate of complexity of the electroencephalogram is an effective parameter for monitoring the depth of anesthesiaduring surgery. Multiscale entropy (MSE) is useful to evaluate the complexity of signals over different time scales.Also, BIS monitor can also be used and it is the first quantitative EEG index used in clinical practice and to allow better titration of anesthesia resulting in lower hypnotic drug use, improved recovery andto assess the depth of anesthesia. Evoked potentials and many other scales have also been used to monitor brain functions and obtain optimum depth of anesthesia to ensure better patient outcomes. It is of clinical importance to monitor cerebral metabolism as cerebral energy metabolism and indicators of cellular damage.The technique of cerebral microdialysis provides the opportunity for continuous monitoring of metabolic changes in the tissue before they are reflected in peripheral blood chemistry or in systemic physiological parameters. Cerebral oxygenation can be determined in various ways including monitoring of SjvO2 in which it’s clinical value lies inthe fact that even if cerebral metabolic requirements are altered bydisease or by anesthesia, and even if cerebral oxygen delivery isabnormally high or low, the oxygen content of jugular venous bloodcontinues to reflect the relative balance between oxygen consumption and supply. NIRS-based cerebral oximetry is a non-invasive technology which can monitor the regional oxygen saturation of the frontal cortexand therefore, permits early detection of cerebral hypoxia and guides therapy to restore it. The goal of multimodality neurophysiologic monitoring is rapid detection of any neurological insult to the spinal cord sensory or motor function that can result in neurological deterioration during surgical intervention on the spine and prompt early intervention to systematic thus reversing the insult and avoiding adverse sequels. The first intraoperative test of spinal cord function was the Stagnara wake-up test which is used for direct evaluation of the patient’s motor functions without depending on computers or specialized equipment. The first electrophysiologic test of intraoperative spinal cord function was the somatosensory evoked potential (SSEP). Recently, TcMEP and intraoperative EMG have been included. In most cases, a combination of SSEP and tceMEP monitoring provides optimal safety. The level of the surgery will determine which nerves and muscles will be monitored. In procedures that are limited to the lumbar spine, free-run EMG may be appropriate. The use of MEP techniques is thought to provide additional information not obtained with SSEP concerning the integrity of all neurological tracts of the spinal cord.So, their combined use is thought to provide the most comprehensive information on the status of the spinal cord. These tests provide full coverage of the patient’s sensory and motor functions during the surgical procedure. The dermatomal somatosensory evoked potential tends to be unreliable in the operating room and is not recommended for monitoring surgical procedures. There have been many advances and improvements in neurological monitoring.The sensitivity and specificity of these monitors should be determined before we can regard any of them as the new ‘gold standard’. It is highly possible that not one of them is the new ‘gold standard’, but the next step for improving monitoring is the combination of several of them to be multimodality monitoring. Multimodality monitoring can be useful to improve patient outcomes postoperatively or in ICU and can be used in many fields as in manangement of TBI, neurosurgical procedures, after cardiac surgery and for assuring the integrity of the spinal cord sensory and motor functions during and after spinal procedures.