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Case

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Evoked potentials (EPs) are well established as diagnostic and monitoring tools in the operating room (OR) as well as the intensive care unit (ICU) setting. They may aid clinicians to detect injury to peripheral nerves and the spinal cord during surgery and help to prognosticate outcome after traumatic brain injury (TBI) and cardiac arrest.

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A 55-year-old woman has complained for several years of neck stiffness and pain for which she was medicating herself with nonsteroidal anti-inflammatory drugs. Over the past several months, she notes that she has had more difficulty getting around and increasing clumsiness. She denies any bowel or bladder symptoms. Her examination reveals increased tone in all of her extremities with bilateral brisk reflexes and sustained ankle clonus. Her strength is full power on confrontation testing throughout. No sensory level is appreciated. Her primary care physician sends her for a computed tomography (CT) scan of the cervical spine, which reveals significant spondylosis and canal stenosis that is worse at C5-6 and C6-7, with posteriorly displaced discs. A cervical magnetic resonance image (MRI) reveals cord compression with cord signal changes at C5-6. After getting these results, the primary care physician refers her to a local orthopedic surgeon who recommends decompression of C5-6 and C6-7. He mentions that he will be using intraoperative neurophysiologic monitoring during the case.

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What is the role of intraoperative neurophysiologic monitoring?

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During operative procedures requiring anesthesia resulting in depressed consciousness, surgeons have limited means to assess the integrity of the nervous system using clinical examination techniques alone. Monitoring techniques during surgery or interventions (such as interventional neuroradiologic procedures) may allow documentation of acute, but still reversible, changes in neurologic function. Additionally, these techniques can be used intraoperatively to assist in identifying important neural structures. Techniques commonly used for intraoperative monitoring include electroencephalography (EEG) and EPs. EPs include somatosensory evoked potentials (SSEPs), brainstem auditory evoked potentials (BAEPs), visual evoked potentials (VEPs), and motor evoked potentials (MEPs). Each of these specifically assesses different sensory or motor pathways and can be selected based on the individual clinical scenario. Wiedemayer et al 20021 reported that of 423 operations with intraoperative monitoring, surgical decisions were successfully modified in 5.2%. Using both SEP and BAEP monitoring, the rates were as follows: true positive findings with intervention, 42 cases (9.9%); true-positive findings without intervention, 42 cases (9.9%); false-positive findings, 9 cases (2.1%); false-negative findings, 16 cases (3.8%); and true-negative findings, 314 cases (74.2%).

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Evoked Potentials

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EPs are electric potentials recorded from the nervous system following presentation of a stimulus, which can be auditory, visual, or electric. EPs are orders of magnitude smaller in amplitude than EEG signals and require signal averaging and precise localization of the recording electrode to measure a response.2 The recorded potential is time-locked to the stimulus, and most of the noise occurs randomly, allowing the noise to be averaged out with ...

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