Evoked potentials are the electrophysiologic responses of the nervous system to various stimuli. They can prove to be a valuable diagnostic tool to assess the functional integrity of the nervous system intraoperatively. Commonly monitored evoked potentials are sensory, motor, visual, and auditory. Surgical procedures associated with possible neurological injury include carotid endarterectomy, spinal fusion with instrumentation, abdominal aortic aneurysm repair, craniotomy, and cardiopulmonary bypass. A change in evoked potentials (usually a critical decrease in amplitude or an increase in latency) can serve as an early sign of neural structures at risk. It also guides the surgeons and the anesthesiologists whether their subsequent interventions to prevent permanent damage is adequate.
SENSORY EVOKED POTENTIALS
Sensory evoked potentials (SEPs) are the electrophysiologic manifestation of the nervous system response to sensory stimulation. Auditory, visual, and somatosensory evoked potentials are examples of SEPs. Evoked potentials are often low in amplitude (0.1–20 μV), which is far less than the voltage of an EEG recording. SEPs therefore must be extracted from the background EEG and artifacts (such as electromagnetic interference). This separation is accomplished by recording systems that use summation and computer signal averaging techniques. Since the SEP is elicited at the same time interval after the stimulus, by averaging the signal, using summation, the amplitude of the SEP signal is augmented and background noise such as the EEG is attenuated. Typically, all SEPs have a post-stimulus latency with a subsequent waveform (Figure 26-1). The conduction velocity and the central conduction time are derived from the distance from the stimulus site and the post-stimulus latency.
Amplitude and latency of SSEP. (Reproduced with permission from Banoub M, Tetzlaff JE, Schubert A. Pharmacologic and physiologic influences affecting sensory evoked potentials: implications for perioperative monitoring. Anesthesiology. 2003;99:716–737.)
Somatosensory Evoked Potentials
Subdermal electrodes are inserted to deliver electric shocks to sensory peripheral nerves, producing a somatosensory evoked potential (SSEP). A common placement of stimulation includes the median nerve, common peroneal nerve at the knee, and posterior tibial nerve at the ankle. SSEPs traverse a great extent of the central nervous system. After the transmission of the electrical signal via the peripheral nerve, it enters the cell bodies of the large fiber sensory system in the dorsal root ganglia. These impulses travel up in the posterior columns of the spinal cord and to the dorsal column nuclei at the cervical medullary junction. Second-order fibers cross to the opposite side and travel to the thalamus through the medial lemniscus. Third-order fibers continue from the thalamus to the fronto-parietal sensory cortex. Ideally, the closer the recording electrodes are to the neural generator, the larger the amplitude of the recorded SSEPs. However, interference with the surgical field poses limitations. Therefore, recording electrodes are typically placed on the scalp using the international 10-20 system. ...