Ebook A concise guide to intraoperative monitoring: Part 2
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Ebook A concise guide to intraoperative monitoring: Part 2
chapter 7Evoked Activity7.1IntroductionA clinically important tool in assessing the integrity of conical and subcortical neuronal relays is the study Ebook A concise guide to intraoperative monitoring: Part 2 of evoked responses (ERs) which result from external stimulation of a neural pathway.The rationale for using ERs intraoperatively is very simple: all naturally occurring external stimuli detected by the sense organs, such as sounds and lights, are transmitted to the brain in the form of electrical Ebook A concise guide to intraoperative monitoring: Part 2signals through various sensory neural pathways. If these pathways are structurally and functionally intact, the signals reaching the brain give riseEbook A concise guide to intraoperative monitoring: Part 2
to certain patterns of activity. Thus, like the natural stimuli, the delivery of experimental stimuli, such as tones or electrical pulses, and the simchapter 7Evoked Activity7.1IntroductionA clinically important tool in assessing the integrity of conical and subcortical neuronal relays is the study Ebook A concise guide to intraoperative monitoring: Part 2g between the stimulation and recording sites.ERs can be subdivided further into Averaged and nonave raged responses, examples of which are the familiar evoked potentials (EPs) and the electrically triggered EMG. respectively. In this chapter we present details on the use. features, stimulation, and Ebook A concise guide to intraoperative monitoring: Part 2 recording procedures, as well as interpretation criteria of the various kinds of averaged and nonaveraged ERs.7.2Evoked PotentialsAn EP is the electrEbook A concise guide to intraoperative monitoring: Part 2
ical response of the nervous system to external stimulation. There are two major types of EPs. sensory and motor. In the former category, a stimulus ichapter 7Evoked Activity7.1IntroductionA clinically important tool in assessing the integrity of conical and subcortical neuronal relays is the study Ebook A concise guide to intraoperative monitoring: Part 2s is delivered centrally (e.g.. at the cortex) and the resulting response is recorded peripherally (e.g.. at a leg nerve or muscle).Depending on the stimulus modality, sensory EPs are divided into somatosensory, auditory, and visual, indicated as SEPs, AEPs. and VEPs, respectively. Early AEPs are re Ebook A concise guide to intraoperative monitoring: Part 2ferred to as brainstem auditory' evoked responses (BAERs). Motor EPs can be90chapter?: Evoked Activityfurther divided into neurogenic and myogenic, deEbook A concise guide to intraoperative monitoring: Part 2
pending on whether the response is recorded at a nerve or at a muscle.Single-trial evoked responses are not readily apparent in the background activitchapter 7Evoked Activity7.1IntroductionA clinically important tool in assessing the integrity of conical and subcortical neuronal relays is the study Ebook A concise guide to intraoperative monitoring: Part 2itive components (waves or peaks) of particular morphology, amplitude, and latency. These three characteristics are the variables to be monitored intraoperatively.For averaged responses, regardless of the stimulus modality, the stimulation rate should be relatively high, so that data are collected f Ebook A concise guide to intraoperative monitoring: Part 2ast enough and average responses are updated sufficiently often to allow early detection of possible response changes. However, this rate should not eEbook A concise guide to intraoperative monitoring: Part 2
xceed a certain critical value, to avoid degradation of response amplitude and morphology. Moreover, the interval between successive stimuli should nochapter 7Evoked Activity7.1IntroductionA clinically important tool in assessing the integrity of conical and subcortical neuronal relays is the study Ebook A concise guide to intraoperative monitoring: Part 2 artifact. To avoid this synchronization problem, a noninteger stimulation rate should be used, such as. for instance. 4.7 Hz.Also, in all modalities the analysis time or time base, that is. the length (in msec) of the segment of signal collected following each stimulus, is another factor to conside Ebook A concise guide to intraoperative monitoring: Part 2r in selecting the stimulation rate. If the interval between successive stimuli is shorter than the analysis lime, a stimulus artifact will be presentEbook A concise guide to intraoperative monitoring: Part 2
in the averaged response. The analysis lime is selected so that all peaks of interest fall within the analysis window.During the course of surgery, ochapter 7Evoked Activity7.1IntroductionA clinically important tool in assessing the integrity of conical and subcortical neuronal relays is the study Ebook A concise guide to intraoperative monitoring: Part 2of the patient and before any surgical manipulation. However, if after the incision and before any surgical maneuvering, the responses have changed excessively due, for example, to drastic changes in anesthesia regime, such as use of different anesthetic agents or induction of hypotension, then the Ebook A concise guide to intraoperative monitoring: Part 2baselines should be reestablished.Baseline recordings should be of familiar morphology, should contain clear and reliable components, and should alsoEbook A concise guide to intraoperative monitoring: Part 2
be consistent with the clinical picture of the patient. However, one should keep in mind that the purpose of intraoperative monitoring is to detect rechapter 7Evoked Activity7.1IntroductionA clinically important tool in assessing the integrity of conical and subcortical neuronal relays is the study Ebook A concise guide to intraoperative monitoring: Part 2ghout the case.Like the ongoing activity presented in Chapter 6. evoked responses are affected by anesthetic agents, blood pressure, and body temperature, since all these factors can alter blood perfusion and metabolic rate in neural cells. In the following sections we concentrate on different types Ebook A concise guide to intraoperative monitoring: Part 2 of evoked activity typically recorded during the course of neurological, orthopedic, or vascular surgery and we give details regarding the generationEbook A concise guide to intraoperative monitoring: Part 2
, information content, recommended electrode locations, and typical acquisition parameters. A quick summary of the various factors that affect the recchapter 7Evoked Activity7.1IntroductionA clinically important tool in assessing the integrity of conical and subcortical neuronal relays is the study Ebook A concise guide to intraoperative monitoring: Part 2assist with the interpretation of the results.7.3 Somatosensory Evoked Potentials917.3Somatosensory Evoked Potentials7.3.1 GenerationSomatosensory evoked potentials (SEPs) can be elicited by electrical stimulation of a peripheral nerve, such as the median nerve at the wrist or the posterior tibial n Ebook A concise guide to intraoperative monitoring: Part 2erve at the ankle. The location of these nerves is schematically shown in Figure 7.1.(a)(b)Figure 7.1 Schematic diagram of (a) the median nerve at theEbook A concise guide to intraoperative monitoring: Part 2
wrist and (b) the posterior tibial nene at the ankle.These nerves are part of the somatosensory system, a schematic diagram of which is shown in Figuchapter 7Evoked Activity7.1IntroductionA clinically important tool in assessing the integrity of conical and subcortical neuronal relays is the study Ebook A concise guide to intraoperative monitoring: Part 2e impulses first to the brainstem, then to the thalamus and. finally, to the primary sensory-cortex. Ascending volleys of SEPs can be recorded at any point along this pathway.More specifically, activity within the spinal cord is conveyed by the dorsomedial tracts, and remains ipsilateral to its side Ebook A concise guide to intraoperative monitoring: Part 2 of entry. A first synapse is formed in the medulla, the inferior portion of the brainstem, in the nucleus gracilis for fibers from the lower portionEbook A concise guide to intraoperative monitoring: Part 2
of the body and in the nucleus cuneatus for fibers from the upper portion of the body. Fibers leaving the medulla decussate to form the contralateral chapter 7Evoked Activity7.1IntroductionA clinically important tool in assessing the integrity of conical and subcortical neuronal relays is the study Ebook A concise guide to intraoperative monitoring: Part 2atotopic arrangement. Legs are represented close to the midline, whereas arms and hands are represented more laterally. A diagram of the somatotopic arrangement of the primary sensory-cortex is shown in Figure 7.3.92chapter 7: Evoked A Ct i V ityFigure 7.2 Schematic diagram of the somatosensory syst Ebook A concise guide to intraoperative monitoring: Part 2em.7.3.2 UseSomatosensory evoked potentials are used intraoperatively to:•Monitor blood perfusion of the cortex or the spinal cord (e.g., during an anEbook A concise guide to intraoperative monitoring: Part 2
eurysm clipping).•Monitor the structural and functional integrity of the spinal cord during orthopedic or neurological surgery (e.g.. for scoliosis orchapter 7Evoked Activity7.1IntroductionA clinically important tool in assessing the integrity of conical and subcortical neuronal relays is the study Ebook A concise guide to intraoperative monitoring: Part 2ts (e.g.. during decompression in radiculopathy), and peripheral nerve structures (e.g., the brachial plexus).chapter 7Evoked Activity7.1IntroductionA clinically important tool in assessing the integrity of conical and subcortical neuronal relays is the studyGọi ngay
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