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Visual Evoked Potentials (VEPs)
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These 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).
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Somatosensory Evoked Potentials (SSEPs)
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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.
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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 sternocleidomastoid 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 bandpass filter 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 artifacts.
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A normal median SSEP waveform is shown in Figure 16-1 and normal values in Table 16-1.5 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.
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Motor Evoked Potentials (MEPs)
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Other evoked potentials typically used in the operating room are MEPs (Figure 16-2). 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.
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A 42-year-old man presented with several months of persistent headaches and decreased hearing on the right. Examination by his primary physician revealed an otherwise normal neurologic examination. The headaches were not controlled with pain medication and were progressively becoming more severe. An MRI was ordered and he was found to have a 3 cm × 2.5 cm pontomedullary mass. He was referred to a neurosurgeon for further evaluation. The neurosurgeon planned to surgically remove the tumor using intraoperative monitoring with brainstem auditory evoked potentials (BAEPs).