microsoft powerpoint - intraoperative neurophysiological
TRANSCRIPT
The Effect of TCI on Neuromonitoring Signal
Quality
Lin Bih-ChernChina Medical University Hospital
Intraoperative NeurophysiologicalMonitoring
Evoked potential monitoring includes somatosensoryevoked potentials (SSEP), brainstem auditory evoked potentials (BAEP), motor evoked potentials (MEP), neurogenic MEP (NMEP)and visual evoked potentials (VEP). Electromyography (EMG) also is used extensively during operative cases. Scalp electroencephalography (EEG) provides data for analysis monitor cerebral function during carotid or other vascular surgery. Electrocorticography (ECoG),EEG recorded directly from the pial surface.
SOMATOSENSORY EVOKED POTENTIALS
Technique
SSEPs are recorded by stimulating peripheral afferent nerves, recorded with scalp electrodes and averaged to improve signal-to-noise ratio. Median nerve at the wrist is the most common stimulation site for upper extremity monitoring. In the lower extremity, the posterior tibialnerve just posterior to the medial malleolusis used most commonly. Other sites that can be utilized include the ulnar and peroneal nerves.
RecordingNeedle electrodes generally are used to reduce artifactual signals. Recording electrodes are placed on the scalp and on the cervical spine. Additionally, electrodes can be placed at the Erb point for upper extremity SSEP recording and over the lumbosacralspine for lower extremity recording. If the operative field affords exposure, recording electrodes may be placed directly in the epidural space. Typically, the electrodes are placed just proximal to the lesion of concern.
Interference
Operating rooms are inundated with equipment that emits electromagnetic interference, which is greatest at the frequency of alternating current (60 Hz in the United States). Adequate filtering to remove this artifactis important. Additionally, shielding to reduce interference is essential.
Parameter monitoredAmplitude, shape, and latencies of the responses are monitored. Serially recorded responses are compared with laboratory norms. Establishing a reproducible baselinerecording prior to any positioning or surgical manipulation is important. Changes from the baseline responses are the indicators of neurological dysfunction. Keep in mind that anesthetics can alter the evoked responses significantly.
Table 1. Typical Responses to Median Nerve StimulationNameRecording
Site Latency Probable AnatomicalLocation
Erb point Erb point 9 ms Trunks of brachial plexus
Cervical A Cervical 11 ms Dorsal root entry zone of cervical roots
Cervical B Cervical 12-13ms Posterior columns
Cervical C Cervical 14 ms Brain stem
N18 Scalp 18 ms Subcortical structures
N20 Scalp 20 ms Somatosensory cortex
Table 2. Typical Responses to Tibial Nerve
StimulationNameRecording
Site Latency Probable AnatomicalLocation
N20 Lumbar 20 ms Spinal roots/cordP27 Cervical spine 27 ms Nucleus gracilis
N35 Scalp 35 ms Somatosensory cortex
P40 Scalp 40 ms Somatosensory cortex
Clinical usesSpinal surgery: Changes in latency and amplitude can be monitored during positional manipulations, including open or closed reduction of spinal deformities.Extradural manipulations, including surgery on disk or vertebral segments, or on epidural abscess or neoplasm, can be monitored with SSEP. Resection of intradural and intramedullary lesions, including tumors and arteriovenous malformations, also can be monitored.
LimitationRecognizing that SSEP recording monitors primarily the integrity of dorsal columns is important. Inability to test motor pathways, which probably are more important clinically than dorsal column integrity, is a significant limitation of the technique.
Cranial/vascular surgery
Carotid surgery including endarterectomy: Changes in SSEP recordings are sensitive for detection of cerebral ischemia. SSEP monitoring can be helpful in determining the need for shunting during the surgical procedure.Aortic cross-clamping: Changes in SSEP indicate a high risk of neurological injury, especially if the changes are immediate.
somatosensory evoked potentials, electroencephalography, carotid stump pressure,
transcranial Doppler sonography, jugular bulb oximetry
and near infrared spectroscopy
during carotid artery surgery for monitoring clamp related ischemia during carotid artery
surgery.
Comparison of
Cerebral aneurysm surgery:Changes may indicate occlusion of parent vessel branches, which potentially could be reversed by repositioning of aneurysm clips.SSEP monitoring can signal changes prior to irreversible cerebral ischemia. Amplitude and latency of the N20 peak, central conduction time (CCT), and latency difference between the N14 and N20 peaks are reliable indicators of cerebral hemispheric function in aneurysm surgery.
Localization of sensorimotor cortex:
Localization of the motor cortex is important to minimize the risk of contralateral motor deficits resulting from surgical procedures in its vicinity.When recording SSEP, the primary sensory cortex and motor cortex generate potentials that are mirror images of each other. This “phase reversal” across the central sulcus is a highly reproducible characteristic that can aid in the localization of primary motor cortex.
Intraoperative photograph showing orientation of monitoring electrode
for intraoperative SSEP. 1, Location of phase reversal
Imaging studies obtained in a patient with metastatic adenocarcinoma.
Preoperative axial contrast-enhanced MR images.
The red lines indicate the central sulcus based on radiographic landmarks.
Intraoperative photograph showing the location of tumor within sensorimotor
cortex.
1. Paper tickets identify areas of positive stimulation.
2. 1, motor for thumb;3. 2, sensory for hand; 4. 3, motor for face.
BRAINSTEM AUDITORY EVOKED POTENTIALS
Brainstem auditory evoked potentials (BAEP) record cortical responses to auditory stimuli.This allows monitoring of the function of the entire auditory pathway including acoustic nerve, brain stem, and cerebral cortex.
TechniqueRecordings are obtained by stimulating with auditory clicks in the ear.Standard EEG cortical montage is used with recordings obtained from scalp electrodes.Best responses are obtained from electrodes near the ears (A1, A2) referenced to the vertex (Cz). Auditory clicks are delivered in a repetitive pattern, often at 11 Hz, with a frequency that does not coincide with the 60-Hz noise of electrical AC current.
InterpretationPositive deflections are termed waves I-VII.Waves I, III, and V are the waves most consistently seen in healthy subjects (obligate waves). Wave V is the most reliably seen wave, particularly in patients with hearing impairment or undergoing surgery. A shift in latency of 1 millisecond or a drop in amplitude of 50% could be significant and should be reported to the surgeon.
Table 3. Interpretation of BAEP Waves
Name Probable Anatomical LocationP1(wave I) Action potential of distal acoustic nerve
P2 (wave II) Proximal acoustic nerve/cochlear nucleus
P3 (wave III) Lower pons
P4 (wave IV) Mid/upper pons
P5 (wave V) Lower midbrain
Clinical usesCerebellopontine angle surgery: This includes surgery for acoustic neuroma or meningioma, or for microvascular decompression for tic douloureux or hemifacial spasm. Important parameters to monitor include peak amplitude of waves III and V, latency of wave V, latency of waves I-V, and latency of waves I-III. If changes occur, they may be due to improper retraction on the cerebellum and brain stem; these may be reversible with a change of position of the retractors by the surgeonMonitoring of visual pathways has potential utility in surgery performed in proximity to the visual apparatus, especially in the parasellar region.
VISUAL EVOKED POTENTIALSTumors that arise in this area include craniopharyngiomas, pituitary adenomas, and suprasellar meningiomas. Resection of these tumors carries significant risk of visual impairment.
It has potential usefulness in assessing integrity of visual pathway including optic nerves; however, it cannot detect the presence of visual field defects.
Technique
Visual stimulation is given by flashing light-emitting diodes (LED) or strobe lights. Potentials are recorded with scalp electrodes. Signal-averaging and noise-reduction techniques are used.
Interpretation
Typically 3 negative peaks (N1, N2, N3) and 3 positive peaks (P1, P2, P3) are seen. The P1-N2-P2 complex typically is monitored during surgery. Latency and amplitude changes are recorded.
Clinical uses
Experiences described in the literature regarding the clinical utility of intraoperative VEP have been conflicting. Monitoring has been performed in tumor resections that require manipulation of the optic apparatus, but its use has not yet become standard practice.
MOTOR EVOKED POTENTIALSSSEP has been the standard of intraoperativ monitoring,
with excellent ability to assess dorsal column and lateral sensory tract function; it probably also can detect changes in function of anterior motor tracts by stimulating mixed sensorimotor peripheral nerves. However, significant motor deficits have been seen in patients undergoing spinal surgery despite normal SSEPs. MEPs were developed to better assess the motor neurophysiological pathways. Note that anesthetic agents can severely diminish the motor evoked responses.
TechniqueMEPs are elicited by either electrical or magnetic stimulation of the motor cortex or the spinal cord. Recordings are obtained either as neurogenic potentials in the distal spinal cord or peripheral nerve, or as myogenic potentials from the innervated muscle.
Electrical stimulationTranscranial electrical stimulation involves stimulation of electrodes on the scalp, or if the brain is exposed by a craniotomy, stimulation of electrodes placed directly on the brain surface. Electrical stimulation also can be applied directly over the spinal cord when a laminectomy affords exposure proximal to the lesion in question. Distal neurogenic potentials then can be recorded.
Magnetic stimulationTranscortical magnetic stimulation delivers a pulsed magnetic field over the scalp in the regionof the primary motor cortex. The basis for electrical stimulation generated by applying a magnetic field is based on the Faraday law, which states that a changing magnetic field induces an electric current in a nearby conductor.Unfortunately, generating good signals in the operating room with this technique is difficult; also, the devices necessary to apply strong magnetic fields can be a hindrance in surgery.
EFFECTS OF ANESTHETICS ON EVOKED POTENTIALS AND EEG
Anesthetics exert their effects on the brain by depressing cerebral metabolism. This results in alteration of EEG recordings of the brain.Each type of anesthetic agent alters the evoked response in different ways
Volatile anestheticsThe volatile agents, which include the halogenated anesthetics and nitrous oxide, produce a dose-dependent depression of cerebral metabolism. They have the most potentially deleterious effect of all anesthetics. All cause similar depression of evoked potentials and prolongation of latencies. They affect cortically evoked responses more than subcortical, spinal, or peripherally evoked responses. At high concentrations, most also can suppress epileptiform discharges.
SSEP monitor under Desflurane
Barbiturates
These may decrease evoked potential amplitude and lengthen latency, but typically recordings can be obtained despite high doses. They also increase beta frequency activity.
Etomidate
In low doses, etomidate can increase evoked potential amplitude but prolong latencies. At induction doses, amplitude may be reduced.
Ketamine
Ketamine either does not affect or may increase evoked potential amplitude.
Narcotics
Narcotics cause mild reduction in amplitude of evoked potentials but usually allow consistent monitoring.
Benzodiazepines
Benzodiazepines usually result in decreased amplitude with little effect on latencies. Like barbiturates, they increase beta activity (more over normally than abnormally functioning cortex), but they typically decrease rather than increase epileptiform activity.
Neuromuscular blockers
These agents have no significant effect on evoked potentials. Muscle relaxation reduces artifactual signals from spontaneous muscle activity and, if complete, suppresses evoked muscular responses as well.
Waters, A.; Mahmoud, M.; Goldschneider, K.; Sadhasivam, S.: Comparison of patient controlled analgesia with and without dexmedetomodine following spine surgery in children. Presented at the SPA Winter Conference; February 16-19, 2006; Fort Myers, FL.
Susceptibility of Motor-Evoked Potentials to Varying Targeted Blood Levels of Dexmedetomidine
Reduction of the spinal cord injuries during scoliosis surgery is a major goal of the anesthesia and surgical team. Despite improvement inscoliosis surgery over the years, the development of neurological deficits remains the most feared complication of spine surgery. During scoliosis surgery it is very important to monitor the spinal cord to detect spinal cord injury with surgical manipulation. Continuous or intermittent intraoperative electrophysiological monitoring (neuron-monitoring) is used routinely during these procedures to provide the surgeon with information concerning the integrity of neurological structures at risk. All neuron-monitoring modalities are affected by the anesthetic regimen used. Of the various intravenous anesthetic drugs, the combination of propofol, remifentanil and dexmedetomidine appearto impact neuron-monitoring the least. The current anesthetic practice is to use the three drugs in combination at doses that do not depress the signals but there is no data relating targeted dexmedetomidine and propofol blood levels to neuron-monitoring signals. The lack of data results in wide variability in dosing with consequent variability in patient response.Hypothesis: Clinically relevant blood levels of dexmedetomidine will affect the amplitude of transcranial motor-evoked potentials (TcMEP) either independently or by interactionwith propofol in a dose dependent manner.
NMEP with low dose propofol
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C7-T1 Spinal tumor