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.

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%).

Evoked Potentials

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 averaging of repeated responses. EPs can be recorded from cerebral cortex, brainstem, spinal cord, and peripheral nerves. Usually, the term evoked potential is reserved for responses involving either recordings from, or stimulation of, central nervous system structures. Thus, evoked compound motor action potentials (CMAPs) or sensory nerve action potentials (SNAPs) as used in nerve conduction studies are generally not thought of as EPs, although they do meet the above definition. Clinically, the recorded electric potentials are evaluated for morphology, latency, and amplitude. These values may then be compared with laboratory-established norms or the contralateral side, or the patient may serve as his/her own control.

Visual Evoked Potentials

VEPs are generated by placing a recording electrode over or near to the visual cortex and applying visual stimuli such as a flashing light or a flickering checkerboard.³ Measured from the primary recording electrodes over the primary visual cortex, these stimuli typically produced a negative deflection at approximately 75 milliseconds (N75) and a positive deflection at approximately 100 milliseconds (P100).

Somatosensory Evoked Potentials

This test assesses the integrity of the dorsal column-lemniscal system.⁴ This pathway projects via the dorsal column of the spinal cord to the cuneate nucleus in the lower brainstem, to the ventroposterior lateral thalamus, to the primary somatosensory cortex, and then to a wide network of cortical areas involved in somatosensory processing. Median and tibial nerves are most often stimulated in SSEP testing, although others (eg, ulnar, peroneal) may be used when appropriate.

The stimulus for SSEPs is a brief electric pulse delivered by a pair of electrodes placed on the skin above the nerve. To minimize artifact produced by electric stimulation, a ground electrode is placed between the stimulation site and the recording site. Both standard surface disc electrodes and needle electrodes can be used as recording electrodes. For upper limb SSEP recording, electrodes are placed at the clavicle between the heads of the sternoclidomastoid muscles (Erb point) and on the skin overlying cervical bodies 6-7. On the scalp, electrodes are placed at CP3 and CP4 of the International 10-20 System. For tibial SSEPs, electrodes are placed in the popliteal fossa and over the lumbar vertebra. At least two bipolar channels (eg, CPz–Fpz and CP3–CP4) should be used to record the cortical component. The SSEP waveform is obtained by averaging typically from 500 to 2000 stimuli; it is necessary to repeat at least two independent averages to demonstrate reproducibility. SSEPs are recorded using a broad pass-band with high-pass and low-pass filters set typically to 30 and 2000 Hz, respectively. Notch filters to eliminate power line noise (50 or 60 Hz) should be used with caution because of their tendency to create “ringing” oscillatory artifact.

A normal median SSEP waveform is shown in Figure 18-1 and normal values in Table 18-1. The purpose of obtaining peripheral potentials (ie, N9, Erb point) is to differentiate peripheral causes of conduction delays as seen in peripheral neuropathy from central ones. The P14 is generated in the caudal medial lemniscus within the lower medulla, and the N20 reflects activation of the primary somatosensory receiving area, located in the posterior bank of the rolandic fissure in Brodmann area 3b. Middle and late latency potentials are less frequently used. A delayed or absent signal at the Erb point may suggest a brachial plexus injury. A delayed or absent potential at the base of the brain may suggest pathology in the upper cervical cord or brainstem. If the typical brainstem potentials (ie, N14) are present, but cortical potentials like the N20 are not seen, this suggests disruption to the thalamicocortical projections, which may be related to pathology such as tumors, anoxic injury, or ischemia.

Motor Evoked Potentials

Other evoked potentials typically used in the OR are MEPs (Figure 18-2). These EPs are generated using magnetic stimulation at or close to the primary motor cortex and have the recording electrode placed in the relevant muscle.

Describe BAEPs

BAEPs are produced by an audible clicking stimulus. The first 10 milliseconds on the response elicited from the audio stimulus represents the electric transmission of that signal through the brainstem and thus is known as the BAEP.³ Recording electrodes are between the Cz (central midline placement of electrodes in EEG) and the ipsilateral ear. The normal BAEP typically shows 5 to 6 peaks, which are labeled with corresponding Roman numerals (Figure 18-3). Although generators of individual peaks are still somewhat controversial, commonly identified generators include the distal auditory nerve (wave I), the auditory nerve as it exits the porus acousticus or the cochlear nucleus (wave II), the cochlear nucleus or ipsilateral superior olivary nucleus (wave III), the superior olive or nucleus or axons of the lateral lemniscus (wave IV), and the inferior colliculus and ventral lateral lemniscus (wave V). Because waves II and IV are less reliably recorded across individuals, clinical interpretation is based primarily on assessment of waves I, III, and V.

How Can BAEPs be interpreted?

Absence or delays in wave V with a normal wave I latency can be seen with conduction abnormalities central to the distal portion of cranial nerve VIII, whereas absence of wave I with preserved wave V may reflect a problem with the peripheral hearing apparatus or with the auditory nerve, but commonly reflects technical difficulty of recording wave I. Functional disruption of the brainstem may cause loss of waves II to V with perseveration of wave I. Unilaterally abnormal BAEPs most often reflect ipsilateral brainstem damage. More rostral structures, including generators within the primary auditory cortex and surrounding areas, are responsible for longer latencies components;⁵ however, similar to other EPs, the later components of the BAEPs are substantially affected by state and are less commonly used in the OR or ICU setting.⁶

Practical Use of EPs in the OR

In the OR, EPs are used for two related purposes: (1) to minimize postoperative injury and (2) to guide treatment. Monitoring may detect functional impairment of neuronal function before permanent deficit occurs. Intraoperatively neuronal function is at risk from a number of mechanisms, including direct surgical transsection, transmission of heat from surgical devices, stress, compression, and compromise of blood supply. In addition to EPs, recording needles may be placed in muscles of pertinent myotomes to look for spontaneous electromyographic (EMG) activity caused by stimulation of the relevant nerve. The surgeon may also have a specialized dissecting device that also serves as a stimulating electrode. EMG activity in the muscles of the face may, for example, signal the surgeon that he/she is dissecting too close to the facial nerve while trying to remove a pontomedullary tumor. A surgeon may also apply an electric stimulus to a screw placed during back surgery. The corresponding EMG activity detected in the relevant muscle may give the surgeon an idea of how close the screw is to the nerve root.

ORs are electrically noisy environments with abundant artifact from use of surgical equipment such as a Bovie device and also ungrounded electric equipment. Consideration must also be given to the type of stimulation and recording electrodes that are used. It is also important to note the placement of the ground electrode on the patient. Stray current from various surgical device may travel along the surface of the skin and cause burns at the site of electrodes. Specific consideration should be given to the type of electrodes used. Surface electrodes are appropriate for awake patients, but they may be easily dislodged, and replacement may be very challenging in a patient who is prepared and draped for surgery. Needle electrodes are less likely to be dislodged and have a very low infection risk. Neurologic injury may occur from positioning and baseline studies, after induction but before the final positioning, and may be helpful to differentiate EP changes present at baseline from those due to positioning or the surgical intervention.

Subcortical components (ie, waves I to V of the BAEP) are relatively unaffected by most medications commonly used in the OR and the ICU, whereas cortical components are more susceptible to medication effects, especially at high doses. Sedative medications, including benzodiazepines, propofol, barbiturates, nitrous oxide, and halogenated inhalational agents, all depress the cortical components of EPs in a dose-related manner.^7–9 Interpeak changes may be more stable (ie, I to V interpeak latencies for BAEPs), and comparing left- and right-sided recordings may be useful, particularly for SSEPs. Neuromuscular blockade should not affect SSEP,² but will affect MEPs.