P032: MONITORING CENTRAL NERVOUS SYSTEM INTEGRITY VIA EVOKED POTENTIALS
Robert Stoker, MD; Department of Anesthesiology, University of Florida College of Medicine
Introduction: Surgery performed on the central nervous system (CNS) may result in permanent changes from baseline. Neuro-monitoring can provide a warning to the surgeon that integrity of the CNS has been compromised, which in turn may provide the surgeon time to reverse a previous step to decrease the likelihood of permanent damage to the CNS.
Case Report/Example: Our patient is a 58-year-old female with a past medical history of hypertension, asthma, and recent unilateral vision impairment who presented to an outside hospital after experiencing two syncopal events in the past 2 weeks. Work-up revealed an aneurysm of the anterior communicating artery with mass effect to the left optic nerve. Cardiac work-up was essentially normal. The patient was transferred to our hospital for further treatment. She underwent a left craniotomy for anterior communicating artery clipping with intraoperative angiogram. A temporary clip was placed over the left middle cerebral artery to decrease flow to the area of the aneurysm while a permanent clip was placed at the base of the aneurysm. Fluoroscopy was performed that revealed a small amount of blood flow still able to bypass the permanent clip and enter the aneurysm. A second permanent clip was placed proximal to the first. The patient’s sensory and motor neural pathways were monitored via motor-evoked potentials (MEPs) and somatosensory-evoked potentials (SSEPs). Within minutes, the SSEP waveform on the patient’s right foot had a decrease in amplitude of >50%. The surgeon quickly removed the second permanent clip and SSEP waveform returned to baseline. This is one example in which intraoperative neuro-monitoring improved a patient’s outcome by providing a warning that impending neurologic injury would most likely occur if a surgical change did not take place. Upon completion of the aneurysm clipping, the patient was able to move all four extremities and over the next 5 days in the hospital, she returned to baseline.
Discussion: SSEPs, MEPs, and brainstem auditory-evoked potentials are used intraoperatively to alert the surgical team and anesthesiologist to changes in the integrity of the CNS. The approach to anesthesia can either enhance or attenuate the evoked potentials. In general, inhalational agents, including nitrous oxide, have a depressant effect on EP monitoring when compared to intravenous anesthetics. Monitoring of MEPs requires that no long-acting neuromuscular blockers are used. MEPs are particularly sensitive to inhaled anesthetics. Total intravenous anesthesia is the ideal anesthetic technique for monitoring of MEPs. Ketamine may enhance the amplitude of MEPs, whereas opioids and benzodiazepines have little effect on EPs. Propofol attenuates the amplitude of all modalities of EPs, but MEPs can still be monitored when propofol is used. Sudden changes in anesthetic depth should be avoided, as these can cause misinterpretation of the change of EPs.