Indications for intraoperative monitoring of evokedpotentials (EPs) include surgical procedures asso-ciated with possible neurological injury: spinal fusion with instrumentation, spine and spinal cord tumor resection, brachial plexus repair, thoracoab-dominal aortic aneurysm repair, epilepsy surgery, and cerebral tumor resection. Ischemia in the spi-nal cord or cerebral cortex can be detected by EPs. EP monitoring facilitates probe localization during stereotactic neurosurgery. Auditory EPs have also been used to assess the effects of general anesthesia on the brain. The middle latency auditory EP may be a more sensitive indicator than BIS in regard to anesthetic depth. The amplitude and latency of this signal following an auditory stimulus is influenced by anesthetics.
Although there are no specific contraindications for somatosensory-evoked potentials (SEPs), this modality is severely limited by the availability of monitoring sites, equipment, and trained personnel. Sensitivity to anesthetic agents can also be a limit-ing factor, particularly in children. Motor-evoked potentials (MEPs) are contraindicated in patients with retained intracranial metal, a skull defect, and implantable devices, as well as after seizures and any major cerebral insult. Brain injury secondary to repetitive stimulation of the cortex and inducement of seizures is a concern with MEPs.
EP monitoring noninvasively assesses neural func-tion by measuring electrophysiological responses to sensory or motor pathway stimulation. Commonly monitored EPs are brainstem auditory evoked responses (BAERs), SEPs, and increasingly, MEPs (Figure 6–11).
For SEPs, a brief electrical current is delivered to a sensory or mixed peripheral nerve by a pair of electrodes. If the intervening pathway is intact, a nerve action potential will be transmitted to the contralateral sensory cortex to produce an EP. This potential can be measured by cortical surface electrodes, but is usually measured by scalp elec-trodes. To distinguish the cortical response to a specific stimulus, multiple responses are averaged and background noise is eliminated. EPs are repre-sented by a plot of voltage versus time. The result-ing waveforms are analyzed for their poststimulus latency (the time between stimulation and potential detection) and peak amplitude. These are compared with baseline tracings. Technical and physiological causes of a change in an EP must be distinguished from changes due to neural damage. Complications of EP monitoring are rare, but include skin irrita-tion and pressure ischemia at the sites of electrode application.
EPs are altered by many variables other than neural damage. The effect of anesthetics is complex and not easily summarized. In general, balanced anesthetic techniques (nitrous oxide, neuromuscular block-ing agents, and opioids) cause minimal changes, whereas volatile agents (halothane, sevoflurane, desflurane, and isoflurane) are best avoided or used at a constant low dose. Early-occurring (spe-cific) EPs are less affected by anesthetics than are late-occurring (nonspecific) responses. Changes in BAERs may provide a measure of the depth of anes-thesia. Physiological (eg, blood pressure, tempera-ture, and oxygen saturation) and pharmacological factors should be kept as constant as possible.
Persistent obliteration of EPs is predictive of postoperative neurological deficit. Although SEPs usually identify spinal cord damage, because of their different anatomic pathways, sensory (dorsal spinal cord) EP preservation does not guarantee normal motor (ventral spinal cord) function (false negative). Furthermore, SEPs elicited from pos-terior tibial nerve stimulation cannot distinguish between peripheral and central ischemia (false posi-tive). Techniques that elicit MEPs by using transcra-nial magnetic or electrical stimulation of the cortex allow the detection of action potentials in the mus-cles if the neural pathway is intact. The advantage of using MEPs as opposed to SEPs for spinal cord monitoring is that MEPs monitor the ventral spi-nal cord, and if sensitive and specific enough, can be used to indicate which patients might develop a postoperative motor deficit. MEPs are more sensi-tive to spinal cord ischemia than are SEPs. The same considerations for SEPs are applicable to MEPs in that they are affected by volatile inhalational agents, high-dose benzodiazepines, and moderate hypothermia (temperatures less than 32°C). MEPsrequire monitoring of the level of neuromuscu-lar blockade. Close communication with a neu-rophysiologist is essential prior to the start of any case where these monitors are used to review the optimal anesthetic technique to ensure monitoring integrity. MEPs are sensitive to volatile anesthet-ics. Consequently, intravenous techniques are often preferred.