Intraoperative Neurophysiologic Monitoring: Some Technical Considerations

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Abstract
This paper contains technical information and recommendations that may be used to to facilitate intraoperative neurophysiologic monitoring (INM). Electrodes should be attached securely, with low and balanced impedances. Leakage and drying of electrode gel should be averted. Backup electrodes are advisable. Optimal connecting wire techniques can reduce noise pickup and electrical hazards. Preoperative studies may be used to identify preexisting abnormalities and plan optimum recording techniques. Anesthesia and neuromuscular blockade can interfere with INM; electrical stimuli used for INM can also interfere with the anesthesiologist's assessment of the patient. Baseline data measurements should be repeated when there are substantial changes in the anesthetic regimen or INM recording techniques. The absence of paralysis must be verified prior to any INM procedure that utilizes EMG or motor responses; these responses may also be absent when technical factors prevent stimulating current from reaching the target neural tissue. When rostral SSEPs change, proximal recordings over stimulated nerves can differentiate CNS dysfunction from technical problems and limb ischemia. Stimuli should be larger than the just-suprathreshold values used extraoperatively, in order to provide a safety margin. Both cortical and subcortical components should be recorded when SSEPs are used to monitor the spinal cord. Salt bridges between paired stimulating electrodes and protective circuitry erroneously triggered by monopolar cautery can interfere with nerve stimulation, producing “false positive” SSEP changes. Electrocautery can temporarily saturate the input stages of physiologic amplifiers; personnel performing INM should manually pause data acquisition when this occurs. Line frequency notch filters may be useful for EEG and BAEP recordings, but should not be used for SSEPs. If substantial AC line frequency artifact is present, the stimulus repetition rate can be adjusted to avoid harmonic relationships with the line frequency.