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Laligam N. Sekhar and Aage R. Møller

✓ In the past, neurosurgeons have been reluctant to operate on tumors involving the cavernous sinus because of the possibility of bleeding from the venous plexus or injury to the internal carotid artery (ICA) or the third, fourth, or sixth cranial nerves. The authors describe techniques for a more aggressive surgical approach to neoplasms in this area that are either benign or locally confined malignant lesions.

During the last 2 years, seven tumors involving the cavernous sinus have been resected: six totally and one subtotally. The preoperative evaluation included axial and coronal computerized tomography, cerebral angiography, and a balloon-occlusion test of the ICA. Intraoperative monitoring of the third, fourth, sixth, and seventh cranial nerves was used to assist in locating the nerves and in avoiding injury to them. The first major step in the operative procedure was to obtain proximal control of the ICA at the petrous apex and distal control in the supraclinoid segment. The cavernous sinus was then opened by a lateral, superior, or inferior approach for tumor resection. Temporary clipping and suture of the ICA was necessary in one patient.

None of the patients died or suffered a stroke postoperatively. Permanent trigeminal nerve injury occurred in three patients; in two, this was the result of tumor invasion. One patient suffered temporary paralysis of the third, fourth, and sixth cranial nerves, and in another the sixth cranial nerve was temporarily paralyzed. Preoperative cranial nerve deficits were improved postoperatively in three patients. Radiation therapy was administered postoperatively to four patients. These seven patients have been followed for 6 to 18 months to date and none has shown evidence of recurrence of the intracavernous tumor.

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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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Cochlear nerve injuries caused by cerebellopontine angle manipulations

An electrophysiological and morphological study in dogs

Tetsuji Sekiya and Aage R. Møller

✓ Changes in the response from the cochlear nerve in dogs resulting from cerebellopontine angle (CPA) manipulations were correlated with histological changes in the nerve. The aim of this study was to determine the mechanisms underlying hearing deficits incurred as a result of manipulations in the CPA. Compound action potentials (CAP) were recorded from the cochlear nerve in response to click stimulation before, during, and after cerebellar and eighth nerve retractions were performed under anesthesia. The retractions were carried out to elicit different degrees of change in the latency and waveform of the CAP. About 30 minutes after completion of the manipulations, the dogs were perfused with a fixative and their cochlear nerves and brain stems were prepared for histological studies. The results showed that retraction of the eighth nerve caused a disintegration of the myelin sheath, and there were multiple and extensive foci of petechial hemorrhage and thromboses of the vasa nervorum of the cochlear nerve.

In two dogs in which retraction was carried to a point at which the N2 peak of the CAP was abruptly obliterated, there was a separation of the central and peripheral myelin junction (Obersteiner-Redlich (OR) zone) and bleeding from the vasa nervorum at the OR zone. In the dogs in which the changes in the CAP had almost recovered before fixative perfusion, there were petechial hemorrhages within the cochlear nerve trunk, thus showing that improvement of electrophysiological responses may not always correlate with the absence of morphological changes.

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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

✓ 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

✓ 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

✓ 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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Margareta B. Møller and Aage R. Møller

✓ Auditory function was studied before and after surgery in 143 consecutive patients who were operated on for hemifacial spasm by microvascular decompression of the intracranial portion of the facial nerve. The acoustic middle ear reflex was abnormal preoperatively in 41% of the patients, indicating that the vascular abnormalities that caused the hemifacial spasm also affected the auditory nerve. Three patients suffered a profound hearing loss in the ear on the operated side, and one lost hearing function totally. In addition, 24 patients had a moderate elevation in the pure-tone threshold at one or more octave frequencies. Of these, 16 patients experienced a hearing loss at only one frequency (8000 Hz), while eight had a threshold evaluation of no more than 20 dB in the speech frequency range (500, 1000, and 2000 Hz). Two patients were deaf on the side of the spasm before the operation. Three patients were not tested postoperatively, and one patient was tested only after surgery. Thus, in this series of 143 patients, only 2.8% suffered a significant hearing loss as a complication of facial nerve decompression to relieve hemifacial spasm.

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Akira Kuroki and Aage R. Møller

✓ Three cadaveric heads were dissected to investigate the microsurgical anatomy around the foramen of Luschka. It was found possible to place a recording electrode in proximity to the cochlear nuclei by inserting it in the lateral recess of the fourth ventricle through the foramen of Luschka. In operations of the cerebellopontine angle using the retromastoid approach, access to the foramen of Luschka and the lateral recess is obtained by retracting the biventral lobule of the cerebellum in a caudal—rostral direction under a caudal—rostral/medial field of vision. The craniectomy might need to be enlarged a few millimeters in the caudal direction. A wick electrode can be inserted in the lateral recess beneath the choroid plexus in a rostromedial direction and to a depth of approximately 3 to 5 mm from the foramen of Luschka without excessive retraction of the cerebellum. The optimum position for the recording electrode is in the triangle formed by the axis of the cochlear nerve and the glossopharyngeal nerve and by the lip of the foramen of Luschka. The caudal retromastoid approach is more suitable than the translabyrinthine technique for recording from the cochlear nuclei as well as for implantation of stimulating electrodes into the cochlear nuclei for use as hearing prostheses.