Answer
Answer
Recording electrode sites are identified by anatomical landmarks. Those on the head are defined using the international 10-20 system, or its extension, the 10-10 system. Electrode CP3 is midway between C3 and P3, and electrode CP4 is midway between C4 and P4. CPi denotes either CP3 or CP4, whichever is ipsilateral to the stimulated limb; CPc is the contralateral centroparietal scalp electrode. CPz is midway between Cz and Pz.
Recording electrodes over the spine are placed in the midline and can be labeled with the name of the vertebral body they are placed over followed by the letter S, for example, C5S or T10S.
Recording montages for cortical SEP components are either cephalic bipolar, in which both electrodes are placed over the head, or referential, in which a reference electrode is placed at a noncephalic site. Cephalic bipolar montages yield signals with less noise contamination and are preferred for routine clinical use.
For upper limb SEP studies, electrodes are placed over Erbs point (ie, the angle between the clavicular head of the sternocleidomastoid muscle and the clavicle), both ipsilateral and contralateral to the stimulus (labeled EPi and EPc, respectively). For lower limb SEP studies, IC denotes an electrode placed over an iliac crest.
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Media Gallery
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Normal median nerve somatosensory evoked potentials (SEPs) recorded using the minimal (4-channel) recording montage recommended by the American EEG Society (AEEGS) guidelines. Negativity at input 1 is shown as an upward deflection. Courtesy of American Electroencephalographic Society, 1994.
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Normal posterior tibial nerve somatosensory evoked potentials (SEPs) recorded using the minimal (4-channel) recording montage recommended by the American EEG Society (AAEGS) guidelines. Note that the second channel from the bottom is specified as Fpz-C5S, so that the far-field potentials appear as a P31 followed by an N34. Negativity at input 1 is shown as an upward deflection. Courtesy of American Electroencephalographic Society, 1994.
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Somatosensory evoked potentials (SEPs) recorded during resection of a posterior fossa tumor (intradural extension of a clear-cell tumor of the right middle ear) in a 46-year-old woman. The peripheral nerve compound action potentials (CAPs) to left median nerve stimulation, recorded at the elbow, and the simultaneously recorded cortical SEPs both displayed marked amplitude attenuation. The stimulating electrodes at the left wrist were replaced; the peripheral nerve and cortical SEPs both returned to their baseline values and remained there through the end of the operation. Courtesy of Legatt, 1995.
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Somatosensory evoked potentials (SEPs) to stimulation of the left median nerve, recorded from a ring of electrodes placed around the neck at the level of SC5 posteriorly and the superior border of the thyroid cartilage anteriorly. Courtesy of Emerson et al, 1984.
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Cortical (left) and cervicomedullary N14 (right) somatosensory evoked potentials (SEPs) to stimulation of the right median nerve, recorded during the initial phases of surgery for resection of a right vestibular schwannoma. The cortical SEPs show prominent anesthetic-related changes. While the waveforms recorded in the A2-Fpz channel contain some volume-conducted cortical SEPs, the N14 far-field component (arrowhead) is unaffected by the changes in the anesthetic regimen. Courtesy of Legatt, 1995.
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Diagram showing generation of the N20 component of the median nerve somatosensory evoked potentials (SEP) in the primary somatosensory cortex located in the posterior bank of the central sulcus producing a horizontal dipole with a postcentral N20 and precentral P20.
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Somatosensory evoked potentials (SEPs) to median nerve stimulation recorded from cortical surface electrodes prior to resection of a right parietal arteriovenous malformation (AVM) in a 35-year-old man. Note the inversion of the N20/P20 component (arrowheads) across the central sulcus; the amplitude is largest over the postcentral gyrus, where the component is negative in polarity. The longer latency surface-positive component has a different distribution. The arrows indicate 2 large veins draining the AVM. Courtesy of Legatt, 1991.
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Somatosensory evoked potentials (SEPs) recorded simultaneously over multiple vertebral levels to posterior tibial nerve stimulation, with an iliac crest reference. The amplitude of the stationary lumbar potential (SLP) is maximal at the T12 level. Negativity at input 1 is shown as an upward deflection. Courtesy of Legatt et al, 1986.
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Serial somatosensory evoked potentials (SEPs) recorded during spinal instrumentation and fusion surgery in a 13-year-old girl with scoliosis. Note the attenuation of the cortical SEPs resulting from administration of an intravenous bolus dose of 50 mg of fentanyl given at 1:53 pm. The far-field SEPs were relatively unaffected. In addition to the far-field components, the C2S-Fpz waveforms (labeled "SC2-Fpz") contain a volume-conducted contribution from the cortical SEPs; the contribution also was attenuated by the fentanyl. Nitrous oxide (60%) and isoflurane (0.6-0.8%) were being administered throughout these recordings. Positivity at input 1 is shown as an upward deflection in this picture.
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Diagram showing 2 possible locations for the foot area of the somatosensory homunculus (shaded area of cerebral cortex), which generates the P37 cortical component of the posterior tibial somatosensory evoked potential (SEP). The arrows represent the equivalent dipoles of the cortical SEP generators; the arrowhead marks the positive side of the dipole field. A: The maximum P37 amplitude is in the midline. B: The maximum P37 amplitude is over the hemisphere ipsilateral to the stimulus, and the negativity can be recorded over the contralateral hemisphere.
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Cortical somatosensory evoked potentials (SEPs) to stimulation of the left posterior tibial nerve in 2 different healthy subjects, showing the variability of scalp topography. The SEPs were recorded from the coronal chain of electrodes; negativity at the active electrode as compared to the Fpz reference is shown as an upward deflection. A: The cortical positivity (labeled "P38") is maximal in the midline at the vertex. B: The cortical positivity is maximal over the hemisphere ipsilateral to the stimulus and is inverted to a negativity over the contralateral hemisphere. Courtesy of Emerson, 1988.
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Cortical somatosensory evoked potentials (SEPs) to left posterior tibial nerve stimulation, showing a secondary cortical positivity (open arrow) that is much larger than the P37 primary cortical SEP component (solid arrow). If only a single Pz-Fpz channel were used to record the cortical SEP, the secondary component might be identified erroneously as a markedly delayed cortical SEP. Negativity at input 1 is shown as an upward deflection.
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Desynchronized, polyphasic somatosensory evoked potentials (SEPs) to posterior tibial nerve stimulation, recorded from the spinal cord during removal of an intradural extramedullary neuroma that was compressing the spinal cord in a 44-year-old woman. Cervical SEPs were highly inconsistent and not suitable for monitoring; cortical SEPs were absent. The bipolar recording electrodes were placed on the dorsal surface of the spinal cord rostral to the lesion. Note the reversible changes with manipulation of the spinal cord and with irrigation of the cord with cold fluids. Courtesy of Legatt, 1991.
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