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  • Author or Editor: Peter J. Jannetta x
  • By Author: Møller, Aage R. x
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Aage R. Møller and Peter J. Jannetta

✓ A method is described for recording sound-evoked responses directly from the eighth nerve during operations in the cerebellopontine angle, particularly during microvascular decompression operations for cranial nerve dysfunction. This method provides important information to the surgeon during the operation that cannot be provided by recording brain-stem auditory evoked potentials (BAEP) using surface electrodes. By recording sound-evoked responses intracranially and recording BAEP with scalp electrodes, the risk of hearing loss in these operations is decreased.

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Aage R. Møller and Peter J. Jannetta

✓ Intracranial responses from the auditory nerve and the cochlear nucleus were recorded from patients undergoing neurosurgical operations during which these structures were exposed. Responses to stimulation of the ipsilateral ear with short tonebursts from the vicinity of the cochlear nucleus show a large surface-negative peak, the latency of which is close to that of peak III in the auditory brain-stem evoked potentials recorded from scalp electrodes. There was also a response to contralateral stimulation, smaller in amplitude and with a longer latency. It is concluded that the cochlear nucleus is the main generator of peak III responses, and that structures of the ascending auditory pathway that are more central than the cochlear nucleus are unlikely to contribute to wave III of the auditory brain-stem evoked potentials.

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Aage R. Møller and Peter J. Jannetta

✓ Facial electromyographic (EMG) responses were monitored intraoperatively in 67 patients with hemifacial spasm who were operated on consecutively by microvascular decompression of the facial nerve near its exit from the brain stem. At the beginning of the operation, electrical stimulation of the temporal or the zygomatic branch of the facial nerve gave rise to a burst of EMG activity (autoexcitation) and spontaneous EMG activity (spasm) that could be recorded from the mentalis muscle in all patients. In some patients, the spontaneous activity and the autoexcitation disappeared after the dura was incised or when the arachnoid was opened, but stimulation of the temporal branch of the facial nerve caused electrically recordable activity in the mentalis muscle (lateral spread) with a latency of about 10 msec that lasted until the facial nerve was decompressed in all but one patient, in whom it disappeared when the arachnoidal membrane was opened.

When the facial nerve was decompressed, this lateral spread of antidromic activity disappeared totally in 44 cases, in 16 it was much reduced, and in seven it was present at the end of the operation at about the same strength as before craniectomy. In four of these last seven patients there was still very little improvement of the spasm 2 to 6 months after the operation; these four patients underwent reoperation. In two of the remaining three patients, the spasm was absent at the 3- and 7-month follow-up examination, respectively, and one had mild spasm. Of the 16 patients in whom the lateral spread response was decreased as a result of the decompression but was still present at the end of the operation, 14 had no spasm and two underwent reoperation and had mild spasm at the last examination. Of the 44 patients in whom the lateral spread response disappeared totally, 42 were free from spasm and two had occasional mild spasm at 6 and 13 months, respectively, after the operation.

Monitoring of facial EMG responses is now used routinely by the authors during operations to relieve hemifacial spasm, and is performed simultaneously with monitoring of auditory function for the purpose of preserving hearing. The usefulness of monitoring both brain-stem auditory evoked potentials recorded from electrodes placed on the scalp and compound action potentials recorded directly from the eighth cranial nerve is evaluated.

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Aage R. Møller and Peter J. Jannetta

✓ Recordings were made from facial muscles and the facial nerve near its entrance into the brain stem in patients with hemifacial spasm (HFS). The purpose of this study was to determine if the synkinesis commonly seen in patients with HFS could be linked to ephaptic transmission at the presumed site of the lesion (at the root entry zone (REZ) of the facial nerve). When the mandibular branch of the facial nerve was electrically stimulated, a response could be recorded from the orbicularis oculi muscles during the operation. The latency of the earliest response was 11.03 ± 0.66 msec (mean response of seven patients ± standard deviation (SD)). With equivalent stimulation a response could also be recorded from the facial nerve near the REZ; the latency of this response was 3.87 ± 0.36 msec. Stimulation of the facial nerve at the same location yielded a response from the orbicularis oculi muscle, with a latency of 4.65 ± 0.25 msec. The latency of the earliest response from the orbicularis oculi muscle to stimulation of the marginal mandibular branch of the facial nerve (11.3 msec) is thus larger than the sum of the conduction times from the points of stimulation of the marginal mandibular branch to the REZ of the facial nerve and from the REZ of the facial nerve to the orbicularis oculi muscle (8.52 ± 0.38 msec). It is therefore regarded as unlikely that the earliest response of the orbicularis oculi muscle to stimulation of the mandibular branch of the facial nerve is a result of “crosstalk” in the facial nerve at a location near the REZ, and it seems more likely that HFS caused by injury of the facial nerve is a result of reverberant activity in the facial motonucleus, possibly caused by mechanisms that are similar to kindling.

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Preservation of facial function during removal of acoustic neuromas

Use of monopolar constant-voltage stimulation and EMG

Aage R. Møller and Peter J. Jannetta

✓ The authors describe a modification in the way the facial nerve is stimulated electrically during operations to remove medium and large-sized (> 2 cm) acoustic tumors. This consists of monopolar stimulation with low internal impedance. Proper use of this modified stimulation technique together with acoustic monitoring of the electromyographic responses of facial muscles helps to preserve facial nerve function in patients undergoing these operations, and also decreases the duration of the operation.

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Margareta B. Møller, Aage R. Møller, Peter J. Jannetta and Laligam Sekhar

✓ This report reviews the characteristic symptoms of disabling positional vertigo (DPV), and the tests used to reach a differential diagnosis of this disorder. Twenty-one patients were operated on consecutively for management of DPV between March, 1983, and September, 1984. In all patients one or more arteries or veins was found to be compressing the eighth cranial nerve when the nerve was exposed for microvascular decompression to relieve the symptoms of DPV. After the operation, 16 of the 21 patients were free of symptoms, or symptoms were so much improved that the patients returned to normal work or social life. Two patients had no improvement and three had limited relief of symptoms postoperatively. None of the patients experienced hearing loss as a result of the operation to relieve DPV, but one patient suffered a cerebellar contusion during the operation.