Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Monitoring somatosensory evoked potentials
3.1
Introduction
Intraoperative somatosensory evoked potential (SEP) monitoring selectively helps reduce the risk of injuring the dorsal somatosensory system for proprioception, vibration sense, and discriminative touch . It also complements motor evoked potential (MEP) monitoring of surgeries mainly risking corticospinal motor injury . In addition, when MEPs are not done SEPs can still indirectly help avoid motor injury because of sensory and motor system proximity , although selective pathway damage may sometimes cause motor deficit without SEP deterioration or SEP deterioration without motor deficit . Finally, SEP mapping can localize the sensorimotor cortex for peri-Rolandic brain surgery and the dorsal column midline for spinal cord surgery .
This chapter reviews the history, methodology, and criteria for SEP monitoring and cortical mapping, and presents possible future advances. It assumes that readers know the relevant anatomy and neurophysiology. Chapter 6 , Intraoperative neurophysiological monitoring of the sacral nervous system, handles dorsal column mapping.
3.2
History
The first SEP monitoring attempts made in the 1960s involved neurosurgical and orthopedic spine surgeries risking spinal cord injury . Efforts gradually expanded to neurosurgery risking brain, brainstem, or peripheral nerve injury; descending aortic procedures risking spinal cord infarction; and vascular procedures risking cerebral infarction, such as intracranial aneurysm repair or carotid endarterectomy.
Early results were disappointing due to experimental techniques and primitive evoked potential instruments. However, methodological advances and specialized monitoring devices improved reliability and culminated in effective traditional methods and guidelines established in the 1990s . A series of subsequent investigations developed optimal methods that further improve SEP monitoring .
3.3
Methodology
This section describes basic techniques for intraoperative SEP monitoring and compares the anesthesia and derivations of traditional and optimal methods.
3.3.1
Basic techniques
Basic techniques apply to both traditional and optimal methods, including electrodes, stimulation, recording, potentials and sites, and averaging.
3.3.1.1
Electrodes
Surface electrodes provide safe and reliable intraoperative recording and have <2 kΩ impedance after gentle epidermal abrasion . These electrodes provide time for accurate preoperative application at the bedside, which shortens operating room setup and enables early postinduction recording and optimization. Standard ECG or other flexible adhesive electrodes self-secure on smooth skin at peripheral stimulation and recording sites, whereas rigid bar stimulating electrodes are inadvisable because they risk sustained-pressure skin necrosis . Reusable prebraided and labeled EEG cup electrode sets provide secure scalp recording when fixed with collodion. The peripheral and scalp SEPs shown in this chapter were made with surface stimulating and recording electrodes.
Programs choosing operating room setup face time constraints and commonly adopt single-use needle electrodes for quickness. Occasionally needles are required for tibial nerve stimulation through thick or edematous ankles. Needle electrodes have <5 kΩ impedance and similar efficacy, but risk needlestick or other infections, trauma, and burns due to their small surface areas that generate high energy density and heat when electrosurgery current accidentally passes through them . Tape secures straight needles at peripheral sites and corkscrew needles self-secure in the scalp.
Invasive single-use subdural or epidural electrodes for cortical or spinal recording have small but potentially serious risks of hemorrhage, trauma, or infection . Thus it seems prudent to limit their use to special indications such as cortical or dorsal column mapping.
3.3.1.2
Stimulation
Stimulation employs a proximal cathode and distal anode about 3 cm apart and typically placed over the median nerve at the wrist and posterior tibial nerve at the ankle. Alternative sites include the ulnar nerve at the wrist and the peroneal or tibial nerve at the knee, which may be necessary with peripheral neuropathy or other distal obstacles. Dermatomal stimulation produces less reliable results .
The stimuli are 0.2 ms constant-current rectangular pulses. Supramaximal intensity is safe at distal sites, and it is advisable to avoid spurious SEP amplitude changes from stimulus fluctuations. However, motor threshold intensity or neuromuscular blockade would be safer for stimulation at the knee where supramaximal intensity risks anterior compartment syndrome from strong repetitive tibialis anterior muscle contractions .
The stimulus rate must not be evenly divisible into 50 or 60 Hz to prevent time-locked interference at power line frequency. Faster stimulation speeds acquisition but reduces cortical SEP amplitudes ( Fig. 3.1 ). Around 4.7–5.1 Hz is generally a satisfactory rate, but adjustments may help optimize results .
Left–right stimulation interleaving doubles acquisition speed by enabling concurrent bilateral recording. Four-limb interleaving also doubles but may not further accelerate acquisition because stimulus frequency must be reduced (e.g., from 4.7 to 2.3 Hz) to accommodate four staggered sweeps. Nevertheless, this technique advantageously increases cortical SEP amplitudes because of slower stimuli ( Fig. 3.1 ) and enables concurrent four-limb acquisition. Simultaneous bilateral tibial nerve stimulation to increase scalp SEP amplitude through summation is inadvisable because it could mask unilateral pathological decrements.
3.3.1.3
Recording
Low impedance and tight lead braiding or twisting are essential for reducing extrinsic electromagnetic interference ( Fig. 3.2 ). The notch filter is off to prevent ringing artifacts that can distort or simulate biological signals. Suitable low- to high-frequency filters are 30–300 Hz for scalp and 0.2–1000 Hz for cubital and popliteal fossa recording ( Fig. 3.3 ) .
Sixteen-bit digital resolution is adequate. The sampling rate must be more than twice the highest high-frequency filter to prevent aliasing; 3000–4000 Hz is sufficient with the above settings. Amplifier gains should utilize dynamic range to contain unclipped biologic signals. Rejection levels should pass biological signals and exclude sweeps containing higher amplitude artifacts while avoiding excessive rejections that delay acquisition. Recording sweeps of 50 and 100 ms are appropriate for upper and lower limb scalp SEPs; pathologically delayed potentials may need longer sweeps and shorter sweeps may be suitable for short-latency peripheral responses.
3.3.1.4
Potentials and sites
3.3.1.4.1
Peripheral controls
Peripheral cubital and popliteal fossa SEPs recorded at the elbow and knee serve as important controls . Concurrent deterioration of peripheral and rostral SEPs indicates stimulus failure or distal nerve conduction failure due to limb ischemia or pressure, and fixing the problem restores signals without a false surgical alarm ( Fig. 3.4 ). Conversely, peripheral SEP preservation immediately excludes these conditions. One can also determine supramaximal stimulus intensity with unaveraged peripheral sweeps.
Erb’s point recording above the clavicle can additionally detect brachial plexus conduction failure due to compression or stretch from shoulder malposition . However, one can deduce this without Erb’s point by noticing upper limb rostral SEP deterioration out of the surgical context, cubital fossa signal preservation, shoulder malposition, and signal restoration after repositioning . Also, Erb’s point recording alone cannot differentiate brachial plexus from distal conduction failure.
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here