ACNS Guidelines Exam Questions With Verified Answers

ACNS Guidelines Exam Questions With Verified Answers 1. Terminology (General Requirements) - Answer Waveforms should be as described in Guideline 9D 2. Stimulus and Safety (General Requirements) - Answer 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 electroencpehalography (EEG) electrodes or sterile subdermal needle electrodes may be used. Disk EEG electrodes should be applied to scalp w/ collodion and sealed w/ plastic tape or sheet to prevent drying and protect them from blood or other fluids. Contact impedance for disk electrodes should be less than 5 Kohms. Subdermal needle electrodes should be similarly secured; it is imp. that OR personnel be made aware of use locations of needle electrodes so they will use necessary caution to avoid needle sticks. 2a. Stimulus Isolation and Subject Grounding (General Requirements) - Answer The stimulation unit must be isolated from the main portion of stimulator circuitry to avoid a large current flow to patient in case of stimulator malfunction. Commercial somatosensory stimulators designed for human use contains appropriate isolation circuitry. The ground may be placed on limb that is stimulated to minimize stimulus artifact. 2b. Stimulus Parameters (General Requirements) - Answer Monophasic rectangular pulses of 100-300 microsedcond duration and 30-40 mA intensity are recommended for stimulation of peripheral nerve. Failure of stimulation may occur when there is a significant increase in contact impedance or due to development of salt bridge (when excessive electrode past short circuits 2 stimulating electrodes). Stimulation may fail due to patient related factors like limb edema, peripheral neuropathy, or variant anatomy. Before increasing current levels to intensity above 30-40 mA, stimulating electrodes should be carefully evaluated. B. Neurophysiologic Intraoperative Monitoring of the Spinal Cord - Answer The risk of neurologic deficit from SC damage is 0.5-1.6% for instrumented scoliosis cases. Surgical decompression of SC tumors/trauma risk increases to about 20%. Surgery on thoracic aorta exposes patient's to highest risk for SC, paraplegia approaching 40%. Monitoring SSEPs directly assess function of dorsal columns and serve as surrogate marker for "global" spinal cord function. Preservation of SSEPs does not guarantee preservation of motor function. This is why MEPs are conducted simultaneously with SSEP monitoring. Selection of nerve stimulated is determined by segmental level of surgical procedure. Surgery above C6 level can be monitored by median nerve. ulnar nerve can be used for lower cervical segments (above C8). Surgery below C8 requires posterior tibial nerve of common peroneal nerve. Other smaller nerves used less often since SSEPs are smaller in amplitude and harder to reproduce. 1. Monitoring of Cervical Spinal Cord (Neurophysiologic Intraoperative Monitoring of the Spinal Cord) - Answer NIOM for surgeries when the cervical SC is at risk involve SSEP monitoring w/ stim of ulnar or median nerve. Median nerve is utilized if surgery is above level C6. Surgeries below C6 and above C8 use ulnar nerve. 1a. Stimulation - Answer 1. Placement of stimulating electrodes 2. Subject grounding 3. Stimulation rate 4. Side of Stimulation i. Placement of stimulating electrodes (1a. Stimulation) - Answer Median nerve: cathode placed between tendon of palmaris longus and flexor carpi radialis muscles, 2 cm proximal to wrist crease. Anode placed 2-3 cm distal to cathode or on dorsal surface of wrist. Either disk electrodes or subdermal needle electrodes may be used. ii. Subject grounding (1a. Stimulation) - Answer A plate electrode on the palmar surface of the forearm or band electrode around the forearm should be used as the ground electrode. iii. Stimulation rate (1a. Stimulation) - Answer A rep rate of 2-9/s is suggested for obtaining SSEP for NIOM. A higher stim rate, up to 20/s may be useful in certain instances to increase the speed of monitoring. But the high stim rate can result in lower amplitudes of responses, increasing amount of time needed to obtain a reproducible response. Stim rates must be optimized to obtain reliable responses in shortest time possible. Stim rates that are multiples of line current frequency (60 Hz for North America) should be avoided, and fine adjustments of stim fequency often help eliminate line noise artifact from recordings. iv. Side of stimulation (1a. Stimulation) - Answer SSEPs obtained following unilateral median or ulnar nerve stimulation. Most current equipment permits right and left stimulation to be interleaved, w/ independent R and L SSEP recording being obtained currently. This helps obtain responses quicker.