Guideline 11B: RECOMMENDED STANDARDS FOR INTRAOPERATIVE MONITORING OF SOMATOSENSORY EVOKED POTENTIALS

Guideline 11B: RECOMMENDED STANDARDS FOR INTRAOPERATIVE MONITORING OF SOMATOSENSORY EVOKED POTENTIALS Stimulus and Safety: A constant current stimulator is recommended for use in the operating room. Care should be exercised to prevent blood or other fluid from contaminating the stimulating site. Either standard disk electroencephalography (EEG) electrodes or sterile subdermal needle electrodes may be used. Disk EEG electrodes should be applied to the scalp with collodion and sealed with plastic tape or sheet to prevent drying and to protect them from blood or other fluids. Contact impedance for disk electrodes should be less than ___ Kohms. Subdermal needle electrodes should be similarly secured; it is important that OR personnel be made aware of the use of locations of needle electrodes, so that they may observe necessary caution to avoid needle sticks. 5 Stimulus Isolation and Subject Grounding: The stimulation unit must be isolated from the main portion of the stimulator circuitry to avoid a large _______ flow to the patient in the case of stimulator malfunction. Commercial somatosensory stimulators designed for human use contain appropriate isolation circuitry. The ground may be placed on the limb that is stimulated to minimize the stimulus artifact. current Stimulus Parameters: Monophasic rectangular pulses of _____-300 µs duration and __-40 mA intensity are recommended for stimulation of peripheral nerves. Failure of stimulation may occur when there is a significant increase in contact impedance or due to the development of a salt bridge, such as when excessive electrode paste short circuits the two stimulating electrodes. However, at times stimulation may fail due to patient related factors such as limb edema, peripheral neuropathy, or variant anatomy. Before increasing current levels to intensity above 30-40 mA, stimulating electrodes should be carefully evaluated. 100 30 B. Neurophysiologic Intraoperative Monitoring of the Spinal Cord: The risk of neurologic deficit resulting from spinal cord damage is 0.5-1.6% in cases of instrumentation for scoliosis.(MacEwen, Bunnell et al. 1975; Nuwer, Dawson et al. 1995; Coe, Arlet et al. 2006) In cases of surgical decompression for spinal cord tumors or trauma, the risk increases to about 20%. Surgery on the descending thoracic aorta exposes patients to the highest risk of injury to the ____ ____ with the incidence of paraplegia approaching 40%.(Husain, Ashton et al. 2008) spinal cord B. Neurophysiologic Intraoperative Monitoring of the Spinal Cord: Monitoring of SSEPs directly assesses the function of the _______ columns and may serve as a surrogate marker for “global” spinal cord function. Although there is good correlation between preservation of SSEPs and normal motor function, there are reported cases of postoperative paraplegia with preserved intraoperative SSEPs. (Ben-David, Haller et al. 1987; Nuwer, Dawson et al. 1995; Minahan, Sepkuty et al. 2001) Preservation of SSEPs does not guarantee preservation of motor function. For this reason, motor evoked potential (MEP) monitoring, which assesses the motor pathways in the ventral aspect of the spinal cord, may be conducted simultaneously with SSEP monitoring. dorsal B. Neurophysiologic Intraoperative Monitoring of the Spinal Cord: The selection of the nerve to be stimulated to obtain the SSEP is determined by the segmental level of the surgical procedure. Spinal cord surgery above the C6 level can be monitored by SSEPs to ________ nerve stimulation. _______nerve SSEP monitoring can be used when the surgery involves the lower cervical segments (above C8). Surgery involving levels below the C8 segment requires monitoring of SSEPs to stimulation of the posterior tibial or common peroneal nerve. Other smaller nerves are used less often as their SSEPs are smaller in amplitude and harder to reproduce. median ulna