Intraoperative monitoring of facial and cochlear nerves during acoustic neuroma surgery.

The likelihood of successful preservation of facial and cochlear nerve function during acoustic neuroma surgery has been improved by the advent of intraoperative monitoring techniques. The facial nerve is monitored by recording EMG from facial muscles, with no muscle relaxants used; mechanical irritation of the nerve during surgery causes increased EMG activity, which can be detected in real time using a loudspeaker. Brief episodes of activity associated with specific surgical maneuvers aid the surgeon in avoiding damage to the nerve, whereas prolonged tonic EMG activity may reflect significant neural injury. Electrical stimulation with a hand-held probe elicits evoked EMG responses, which can be used to locate and map the nerve in relation to the tumor. The threshold for eliciting evoked EMG responses provides a rough indicator of the functional status of the nerve. Different nerves in the posterior fossa (trigeminal, facial, spinal accessory) can be identified in multichannel recordings by the spatial distribution and latency of responses to electrical stimulation. The ability to elicit EMG responses from low amplitude stimulation of the facial nerve at the brain stem after tumor removal is a reasonable predictor of postoperative facial function. Cochlear nerve function is assessed by recording the ABR from ear canal and scalp electrodes or the CNAP with an electrode placed directly on the nerve at the brain stem root entry zone. The ABR is a well-known, noninvasive technique that can be adapted to intraoperative use relatively easily but is of limited utility owing to the delay inherent in signal averaging. Direct CNAP recordings require placement of an intracranial electrode in such a way as to contact the cochlear nerve without interfering with surgical access but have the distinct advantage of rapid feedback on changes in cochlear nerve status.

Multichannel cochlear implants. Channel interactions and processor design.

Multichannel electrical stimulation of the cochlear nerve can generate complex interactions between the individual channels. Two types of channel interactions have been investigated: those that occur when two or more electrode channels are simultaneously stimulated and those that occur when the stimuli from each channel are not temporally coincident. Experiments with three human subjects, implanted with scala tympani electrode arrays, indicate that responses are a strong function of the spacing between the channels, the subject, and the electrode geometry (ie, bipolar or monopolar geometries).

The use of kainic acid for studying the origins of scalp-recorded auditory brainstem responses in the guinea pig.

Kainic acid was injected into the medial nucleus of the trapezoid body (MNTB) of guinea pigs to evaluate its use in studying generator loci of the scalp-recorded auditory brain stem response (ABR). Sound-evoked near-field potentials from the MNTB and far-field ABRs were recorded before, during and up to 2 h after the injections. Two hours post-injection, small amounts of kainic acid (0.25 nmol in 0.1 microliters of Ringer solution) resulted in neuronal destruction which histologically appeared confined to the MNTB. Larger amounts (10 nmol in 1.0 microliters) produced more extensive lesions. Regardless of the dose of kainic acid, near-field activity evoked by contralateral ear stimulation was almost totally abolished and ABR wave III amplitude was reduced by as much as 60%. In future studies, the use of excitotoxic amino acids to produce lesions within complex nuclear subdivisions of the auditory pathway may yield valuable information as to the relative contributions that brainstem structures make to the various waves comprising the ABR and about the behavioral effects that axon sparing lesions produce.

Binaural interaction components. Their possible origins in guinea pig auditory brainstem response.

A two-step ablation technique under visual control was used to clarify the possible sites of origin of the binaural interaction component (BIC) of the auditory brainstem-evoked response (ABR). The BIC is obtained by summing ABRs elicited by monaural clicks to each ear and subtracting this sum from the ABR elicited by binaural clicks. Among the results were the following: first, the ABR and the BIC remained virtually unchanged after collicular aspiration; second, after midsagittal section of the brainstem, both wave III in the ABR and the BIC were abolished. These results suggest that although the inferior colliculus is a powerful integrative center for the processing of binaural signals, the neural structures in the guinea pig necessary for the primary generation of the BIC must be located caudal to the colliculus, yet rostral to the medial nucleus of the trapezoid body.