Myogenic influences on the electrical auditory brainstem response (EABR) in humans

The polarity sensitivity of the electrically stimulated human auditory nerve measured at the level of the brainstem

Recent behavioral studies have suggested that the human auditory nerve of cochlear implant (CI) users is mainly excited by the positive (anodic) polarity. Those findings were only obtained using asymmetric pseudomonophasic (PS) pulses where the effect of one phase was measured in the presence of a counteracting phase of opposite polarity, longer duration, and lower amplitude than the former phase. It was assumed that only the short high-amplitude phase was responsible for the excitation. Similarly, it has been shown that electrically evoked compound action potentials could only be obtained in response to the anodic phases of asymmetric pulses. Here, experiment 1 measured electrically evoked auditory brainstem responses to standard symmetric, PS, reversed pseudomonophasic, and reversed pseudomonophasic with inter-phase gap (6 ms) pulses presented for both polarities. Responses were time locked to the short high-amplitude phase of asymmetric pulses and were smaller, but still measurable, when that phase was cathodic than when it was anodic. This provides the first evidence that cathodic stimulation can excite the auditory system of human CI listeners and confirms that this stimulation is nevertheless less effective than for the anodic polarity. A second experiment studied the polarity sensitivity at different intensities by means of a loudness balancing task between pseudomonophasic anodic (PSA) and pseudomonophasic cathodic (PSC) stimuli. Previous studies had demonstrated greater sensitivity to anodic stimulation only for stimuli producing loud percepts. The results showed that PSC stimuli required higher amplitudes than PSA stimuli to reach the same loudness and that this held for current levels ranging from 10 to 100% of the dynamic range.

Nondepolarizing neuromuscular blockade does not alter sensory evoked potentials

OBJECTIVE

The objective of this study was to determine whether changes in nondepolarizing neuromuscular blockade (NMB) alter the latency or amplitude of visual, brain stem auditory, or median nerve somatosensory evoked potentials.

METHODS

Ten adult cynomologous Macaca fascicularis monkeys were studied during continuous ketamine anesthesia infusion. The NMB was incrementally adjusted between no block and complete block using infusions of vecuronium or atracurium. The NMB was measured by observing the mechanical response of the hand muscles to train-of-four stimulation of the median nerve and by quantification of the peak-to-peak electromyographic activity of the thenar muscles to supramaximal median nerve stimulation. The following variables were measured: the latencies of the three major peaks of the brain stem auditory evoked potential, the latency and amplitude of the cortical visual evoked potential, the latency of the spinal cord response, and the latency and amplitude of the cortical median nerve somatosensory evoked response.

RESULTS

No statistically significant changes were observed for any evoked response variable measured at any degree of NMB when compared with values obtained in the unblocked control state.

CONCLUSIONS

The results suggest that fluctuating degrees of NMB may contribute little to changes in these evoked potentials during intraoperative monitoring, given the testing parameters used.