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Betty L. Grundy, Peter J. Jannetta, Phyllis T. Procopio, Agnes Lina, J. Robert Boston and Earl Doyle

✓ Brain-stem auditory evoked potentials (BAEP) were monitored during 54 neurosurgical operations in the cerebellopontine angle. The BAEP were irreversibly obliterated in five patients who required deliberate section of the auditory nerve. Technical difficulties interfered with monitoring in three cases, and three patients had deafness and absent BAEP preoperatively.

Reversible alterations in BAEP were seen during 32 operations, with recovery after as long as 177 minutes of virtually complete obliteration. Changes in BAEP were associated with surgical retraction, operative manipulation, positioning of the head and neck for retromastoid craniectomy, and the combination of hypocarbia and moderate hypotension. In 19 cases, waveforms improved after specific interventions made by the surgeon or anesthesiologist because of deteriorating BAEP. In 13 other cases, BAEP recovered after maneuvers not specifically related to the electrophysiological monitoring, most often completion of operative manipulation. Whenever BAEP returned toward normal by the end of anesthesia, even after transient obliteration, hearing was preserved. Irreversible loss of BAEP occurred only when the auditory nerve was deliberately sacrificed. The authors conclude that monitoring of BAEP may help prevent injury to the auditory nerve and brain stem during operations in the cerebellopontine angle.

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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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Cerebellopontine angle meningiomas

Microsurgical excision and follow-up results

Laligam N. Sekhar and Peter J. Jannetta

✓ During the years 1974 through 1981, 22 patients with cerebellopontine angle meningiomas underwent surgery: 14 tumors were excised completely, and eight subtotally. A retromastoid approach was used in 19 cases and a subtemporal approach in three cases. There was no operative mortality and the quality of survival was good. Five patients suffered new cranial nerve deficits as a result of the operation. The average follow-up period was 5 years. One tumor thought to be completely removed has recurred, but has not required another operation so far. One subtotally excised tumor required reoperation. Computerized tomography and arteriography were important in preoperative evaluation. Good neuroanesthesia, the use of the surgical microscope and microtechnique, and an understanding of the pathological relationships were factors contributing to good results.

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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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Wishwa N. Kapoor and Peter J. Jannetta

✓ A patient with trigeminal neuralgia experienced a generalized seizure and a prolonged syncopal episode. He was found to be asystolic during the syncopal episode. There was no recurrence of loss of consciousness after implantation of a pacemaker. Mechanical stimulation of the trigeminal nerve during craniotomy for microvascular decompression of the trigeminal nerve resulted in bradycardia. Since vascular decompression of the trigeminal nerve, there has been no recurrent facial pain, and no further syncope, seizures, or bradycardia. Syncope and seizures have not been previously reported in association with trigeminal neuralgia, although they are well described with glossopharyngeal neuralgia.

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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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Bruce R. Cook and Peter J. Jannetta

✓ The syndrome of tic convulsif consists of ipsilateral concurrent trigeminal neuralgia and hemifacial spasm. Since Cushing's 1920 description of this syndrome in three patients, 37 additional cases have been reported in the world literature. Of the 15 with adequate operative descriptions, 10 had vascular abnormalities and five had tumors. The authors report 11 cases of tic convulsif treated by microvascular decompression of both the fifth and seventh cranial nerves. At operation, 21 of 22 nerves were found to have root entry zone vascular compression. One trigeminal nerve was considered normal. One seventh nerve had a tumor displacing the anterior inferior cerebellar artery into its root entry zone. The average follow-up period in this series was 6 years 2 months (range 1 to 8½ years). Eight patients (73%) were pain-free, two (18%) had frank recurrences, and one (9%) had mild discomfort. Eight patients (73%) were totally free of facial spasm, and two others (18%) had only a trace of residual spasm. These results are comparable to those achieved by treating the individual syndromes with microvascular decompression. Therefore, microvascular decompression of both the fifth and seventh cranial nerves is recommended as the treatment of choice in tic convulsif.

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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.