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16 Somatosensory-Evoked Potentials Misulis-16 11/5/02 1:59 PM Page 221 16 Somatosensory-Evoked Potentials The somatosensory-evoked potential (SEP) is the response to electrical stimu- lation of peripheral nerves. Stimulation of almost any nerve is possible, although the most commonly studied nerves are: • Median • Ulnar •Peroneal •Tibial Brief electric pulses are delivered to the peripheral nerve with the cathode proximal to the anode. The stimulus cannot selectively activate sensory nerves, so a small muscle twitch is seen. There are no effects of the retrograde motor volley in the motor nerves on central projections of the sensory fibers. Intensity of the stimulus is adjusted to activate low-threshold myelinated nerve fibers, which in the motor fibers elicits a small twitch of the innervated muscles. The compound action potential is conducted through the dorsal roots and into the dorsal columns. The impulses ascend in the dorsal columns to the gracile and cuneate nuclei where the primary afferent fibers synapse on the second-order neurons. These axons ascend through the brainstem to the thalamus. Thalamocortical projections are extensive, as are secondary intracortical associative projections. Table 16.1 shows common stimulus and recording parameters. The recording is made from various levels of the nervous system, including: • Afferent nerve volley • Spinal cord •Brain Recording of the afferent nerve volley ensures that the stimulus is ade- quate, and determines whether there is a defect in peripheral conduction that would interfere with interpretation of the central waveforms. Recordings from the spinal cord measure electrical activity in white-matter tracts and relay nuclei. Recordings from brain will measure the projections from relay nuclei to the cortex. Short-latency responses are used for clinical interpretation; long-latency responses are too variable to be helpful. 221 Misulis-16 11/5/02 1:59 PM Page 222 TABLE 16.1 Somatosensory-evoked potential common stimulus and recording parameters Parameter Value Stimulus rate 45–7/sec Low-frequency filter 5–30 Hz (−3 dB) High-frequency filter 2,500–4,000 Hz (−3 dB) The anatomy of the projections from the peripheral nerve to cortex is complex. Unfortunately, there is not a precise anatomical correlate to each of the waves in the SEP. This complicates interpretation, unfortunately. SEPs are particularly useful for evaluation of function of the spinal cord. Lesions of the cord may be invisible to routine imaging, including magnetic resonance imaging and myelography, yet may have devastating effects on cord function. Transverse myelitis, multiple sclerosis (MS), and cord infarction are only three of the potential causes that can be missed on structural studies. Although almost any nerve can be studied, median and tibial will be discussed here. Additional information on other techniques can be seen in Spehlmann’s Evoked Potential Primer (Misulis and Fakhoury, Butterworth- Heinemann, 2001). Median SEP Methods Stimulating electrodes are placed over the median nerve at the wrist with the cathode proximal to the anode. Stimuli are square-wave pulses given at rates of 4–7/sec. Recording electrodes are placed at the following locations: • Erb’s point on each side (EP) •Over the second or fifth cervical spine process (C2S or C5S) • Scalp over the contralateral cortex (CPc) and ipsilateral cortex (CPi) • Noncephalic electrode (Ref) Erb’s point is 2–3 cm above the clavicle, just lateral to the attachment of the sternocleidomastoid muscle. Stimulation at Erb’s point produces abduction of the arm and flexion of the elbow. The second and fifth spinous processes are identified by counting up from the seventh, notable by its prominence at the base of the neck. CPc and CPi are scalp electrodes halfway between C3 and P3 or C4 and P4, where CPc is contralateral to the stimulus and CPi is ipsilateral to the stimulus. These electrodes are over the motor-sensory cortex. EPi is Erb’s point ipsilateral to the stimulus. The recommended montage is: • Channel 1: CPc-CPi • Channel 2: CPi-Ref 222 EVOKED POTENTIALS Misulis-16 11/5/02 1:59 PM Page 223 • Channel 3: C5S-Ref • Channel 4: Epi-Ref Details of the recommended stimulus and recording parameters are pre- sented in Table 16.2, including analysis time and filter settings. These param- eters are usually not changed for individual studies. The number of trials averaged for adequate waveform identification is 500–2,000, but more may occasionally be needed. Duplicate trials overlaid on the display help greatly with waveform identification. Waveform Identification and Interpretation Figure 16.1 shows a normal median SEP. The recorded potentials are from Erb’s point, spinal cord, and scalp. The waves of interest are: • N9 = from the Epi channel • P14 = from the neck channel • N20 = from scalp channels N9: The potential recorded from Erb’s point is sometimes called EP, but because of the obvious abbreviation confusion, N9 is a preferable designa- tion. The N9 potential is a compound action potential from the axons stimu- lated by the median nerve stimulation. While the N9 potential includes both orthograde sensory nerve potentials and retrograde motor nerve potentials, all of the subsequent waves are due to sensory activation, alone. TABLE 16.2 Median nerve somatosensory-evoked potential stimulus and recording parameters Parameter Value Number of trials 500–2,000 Analysis time 40 ms Bandpass 30–3,000 Hz (−6 dB) Stimulating electrodes Cathode Median nerve 2 cm proximal to the wrist crease Anode 2 cm distal to cathode Recording electrodes CPc Contralateral scalp, between C3/4 and P3/4 CPi Ipsilateral scalp, between C3/ and P3/4 C5S Over C5 spinous process Epi Erb’s point ipsilateral to the stimulus Ref Noncephalic reference, such as distal arm Montage Channel 1 CPc-CPi Channel 2 CPi-Ref Channel 3 C5S-Ref Channel 4 EPi-Ref Measurements Peak latency of N9 potential Peak latency of P14 in neck–scalp channel Peak latency of N20 in scalp–scalp or scalp–ear channel Somatosensory-Evoked Potentials 223 Misulis-16 11/5/02 1:59 PM Page 224 FIGURE 16.1 Normal median nerve SEP. P14: The neck potentials include N13 and P14, with the latter used for clinical interpretation. The origin is thought to be the caudal medial lemniscus. N20: Scalp potentials include N18 and N20, but the latter is used for clinical interpretation. Origin is thought to be the thalamocortical radiations. Calculated Data Table 16.3 shows normal SEP data and Table 16.4 shows probable waveform origins for the SEP potentials. When the N9, P14, and N20 measurements have been made, calculations are made to assess conduction between the regions. N9–P14 interval represents the time for conduction between the brachial plexus and cervical spine. P14–N20 interval represents the conduction between the cervical spine and the brain. This is called the brain conduction time (BCT). The median SEP is used for several indications including transverse myelitis, MS, thoracic outlet syndrome, and intraoperative monitoring during TABLE 16.3 Somatosensory-evoked potential normal data Wave Latency Interside difference Median nerve N9/EP 11.80 0.87 P14 16.10 0.70 N20 21.50 1.20 P14–N20 7.10 1.20 Peroneal nerve LP 13.50 0.50 P27 31.80 2.24 N33 40.70 5.96 LP–P27 interval 19.38 2.30 Tibial nerve LP 2.10 1.20 P37 41.70 1.40 LP–P37 interval 20.50 1.50 224 EVOKED POTENTIALS Misulis-16 11/5/02 1:59 PM Page 225 TABLE 16.4 Somatosensory-evoked potential waveform origins Wave Origin Median N9/EP Afferent volley in plexus N13 Dorsal horn neurons P14 Caudal medial lemniscus N18 Brainstem and thalamus? N20 Thalamocortical radiations Tibial LP Dorsal roots and entry zone N34 Brainstem and thalamus? P37 Primary sensory cortex carotid endarterectomy. Interpretation of abnormalities is as follows: • Delayed N9 with normal NP–P14 and P14–N20 intervals: Lesion in the somatosensory nerves at or distal to the brachial plexus. • Increased N9–P14 interval with normal P14–N20 interval: Lesion between Erb’s point and the lower medulla. • Increased P14–N20 with normal N9–P14 interval: Lesion between the lower medulla and the cerebral cortex. Amplitude abnormalities should be interpreted with caution. A marked asym- metry can be caused by a lesion affecting some but not all of the afferent fibers. If the lesion is not sufficiently severe to produce a latency change; however, the study is probably normal. An absent N20 is abnormal when N9 and P14 are present. If P14 is absent but N9 and N20 are normal and the N9–N20 interval is normal, the study indicates a lesion between the brachial plexus and medulla, but no statement can be made of brain conduction. Tibial SEP Methods Stimulus settings are similar to those used for median SEP, and details of the stimulus and recording parameters are presented in Table 16.5. The proxi- mal stimulating electrode (cathode) is placed at the ankle between the medial malleolus and the Achilles tendon. The anode is placed 3 cm distal to the cathode. A ground is placed proximal to the stimulus electrodes, usually on the calf. Stimulus intensity is set so that each stimulus produces a small amount of plantar flexion of the toes. Recording electrodes are placed as follows: • CPi = Ipsilateral cortex between C3 and P3 or C4 and P4 • CPz = Midline between Cz and Pz • FPz = Fpz position of the 10–20 electrode system • C5S = Over the C5 spinous process Somatosensory-Evoked Potentials 225 Misulis-16 11/5/02 1:59 PM Page 226 TABLE 16.5 Tibial nerve somatosensory-evoked potential stimulus and recording parameters Parameter Value Number of trials 1,000–4,000 Stimulating
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