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Makoto Nakamura, Florian Roser, Mehdi Dormiani, Madjid Samii and Cordula Matthies

Object. Meningiomas of the cerebellopontine angle (CPA) can either arise from or secondarily grow into the inner auditory canal (IAC). This location may have a great impact on hearing function following surgery to remove these lesions. The aim of this retrospective study was to investigate the reliability and predictive importance of auditory brainstem responses (ABRs) for the determination of postoperative auditory function in patients with CPA meningiomas in comparison with results obtained in patients who undergo surgery for vestibular schwannomas.

Methods. In a consecutive series of 1800 meningiomas surgically treated between 1978 and 2002, 421 lesions were located in the CPA. In 38 patients with CPA meningiomas involving the IAC, the findings of intraoperative ABR monitoring and the hearing status of each patient before and after surgery were retrospectively analyzed.

On analysis, ABR monitoring demonstrated stable findings in 24 patients throughout tumor resection and fluctuating signals in 10 patients. Among the 24 patients with stable ABRs, postoperative hearing function improved in three patients, remained the same in 15, and worsened in six patients, including one patient who displayed postoperative deafness. There was even one patient recovering from preoperative deafness. Among the 10 patients with unstable ABRs, intermittent decreases in amplitude and deformations of variable duration in the ABR wave were noted. The risk of deafness was considerably higher in patients with prolonged phases of intermittent ABR deterioration.

Conclusions. The presence and absence of ABRs during surgery for CPA meningiomas reliably predicted the presence and absence of postoperative auditory function. Intermittent deterioration of ABRs may result in postoperative deafness, depending on the duration of these events during surgery. Improvements in hearing are only seen when the ABRs are stable for amplitudes and latencies throughout surgery.

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Florian Roser, Makoto Nakamura, Mehdi Dormiani, Cordula Matthies, Peter Vorkapic and Madjid Samii

Object. Only some meningiomas of the cerebellopontine angle (CPA) extend into the internal auditory canal (IAC) or arise from its dural lining. The authors investigated cases of CPA tumors in which the meningioma was inserted in the dura mater in or at the ICA or infiltrated a cranial nerve.

Methods. The authors reviewed patient charts including surgical and clinical records, intraoperative recordings of auditory evoked potentials, records of postoperative auditory examinations, and imaging studies.

In a series of 421 patients harboring CPA meningiomas, 72 patients in whom there was dural involvement of the IAC were investigated. Total tumor resection was achieved in 86.1%. In 34 patients, opening of the IAC was required for total tumor removal; this procedure did not influence the patient functional outcome. Among patients with secondary involvement of the IAC, anatomical preservation of the facial and cochlear nerves was obtained in 94%, whereas among patients in whom the lesion arose from the dura in or at the IAC these values were 80 and 75%, respectively. Functional preservation of the seventh and eighth cranial nerves in cases of tumor extension within the IAC was 86 and 77%, respectively, whereas in cases in which the IAC was involved it was only 60%. In four of five patients in whom the tumor had its origin in the dura mater within the IAC, the seventh or eighth cranial nerve had to be sacrificed to achieve tumor removal because of the lesion's infiltrative behavior. Facial nerve reconstruction by sural grafting was performed in the same operative procedure.

Conclusions. Meningiomas of the CPA involving the IAC require special surgical management. Dural involvement of the IAC requires opening by using a diamond drill, a procedure that does not influence cranial nerve outcome. The increased rate of cranial nerve morbidity is attributed to the infiltrative behavior of these meningiomas. If affected nerve segments have to be sacrificed, immediate reconstruction enables satisfactory long-term results.

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Steffen K. Rosahl, Gerhard Mark, Martin Herzog, Christos Pantazis, Farnaz Gharabaghi, Cordula Matthies, Thomas Brinker and Madjid Samii

Object. A new generation of penetrating electrodes for auditory brainstem implants is on the verge of being introduced into clinical practice. This study was designed to compare electrically evoked auditory brainstem responses (EABRs) to stimulation of the cochlear nucleus (CN) by microsurgically implanted surface electrodes and insertion electrodes (INSELs) with stimulation areas of identical size.

Methods. Via a lateral suboccipital approach, arrays of surface and penetrating microelectrodes with geometric stimulation areas measuring 4417 µm2 (diameter 75 µm) were placed over and inserted into the CN in 10 adult cats. After recording the auditory brainstem response (ABR) at the mastoid process, the CN, and the level of the inferior colliculus, EABRs to stimulation of the CN were recorded using biphasic, charge-balanced stimuli with phase durations of 80 µsec, 160 µsec, and 240 µsec at a repetition rate of 22.3 Hz. Waveform, threshold, maximum amplitude, and the dynamic range of the responses were compared for surface and penetrating electrodes.

The EABR waveforms that appeared for both types of stimulation resembled each other closely. The mean impedance was slightly lower (30 ± 3.4 kΩ compared with 31.7 ± 4.5 kΩ, at 10 kHz), but the mean EABR threshold was significantly higher (51.8 µA compared with 40.5 µA, t = 3.5, p = 0.002) for surface electrode arrays as opposed to penetrating electrode arrays. Due to lower saturation levels of the INSEL array, dynamic ranges were almost identical between the two types of stimulation. Sectioning of the eighth cranial nerve did not abolish EABRs.

Conclusions. Microsurgical insertion of electrodes into the CN complex may be guided and monitored using techniques similar to those applied for implantation of surface electrodes. Lower thresholds and almost equivalent dynamic ranges indicate that a more direct access to secondary auditory neurons is achieved using penetrating electrodes.

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Madjid Samii, Rama Eghbal, Gustavo Adolpho Carvalho and Cordula Matthies

Object. A careful retrospective analysis of 36 cases was performed to evaluate the pre- and postoperative rates of morbidity that occur in patients with brainstem cavernous angiomas.

Methods. The authors evaluated immediate postoperative and follow-up outcomes with regard to clinical findings, the incidence of preoperative hemorrhage(s), location and size of the lesions, and the timing of the surgical procedure after the last hemorrhagic event. Specifically, the following parameters were analyzed: 1) number of hemorrhages; 2) the precise brainstem location (pontomesencephalic, pons, and medulla oblongata); 3) pre- and postoperative cranial nerve status; 4) pre- and postoperative motor and sensory deficits; 5) size (volume) of the lesions; and 6) pre- and postoperative Karnofsky Performance Scale (KPS) scores. Multiple hemorrhages were observed in 16 patients, particularly in those with pontomesencephalic cavernous angiomas (75%). The mean preoperative KPS score was 70.3 ± 16.3 (± standard deviation). Twenty-six patients (72.2%) presented with cranial nerve impairment, 13 (36.1%) with motor deficits, and 17 (47.2%) with sensory disturbance. Volume of the lesions ranged from 0.18 to 18.18 cm3 (mean 4.75 cm3). Postoperative complications included new cranial nerve deficits in 17 patients, motor deficits in three, and new sensory disturbances in 12 patients. In a mean follow-up period of 21.5 months, KPS scores were 80 to 100 in 22 patients. Timing of surgery (posthemorrhage) and multiple hemorrhages did not influence the long-term results. Higher preoperative KPS scores and smaller-volume lesions, however, were factors associated with a better final outcome (p < 0.05). Major morbidity was related mainly to preoperative status and less to surgical treatment. The incidence of new postoperative cranial nerve deficits was clearly lower than that demonstrated preoperatively because of the brainstem hemorrhages.

Conclusions. Based on these findings, resection of brainstem cavernomas is the treatment of choice in the majority of these cases because of the high incidence of morbidity related to one or often several brainstem hemorrhages.

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Cordula Matthies and Madjid Samii

✓ The usefulness of intraoperative monitoring in cerebellopontine angle surgery should be improved by obtaining faster and stronger brainstem auditory evoked potential (BAEP) responses. A new technique of direct recording at the brainstem has been developed, which is applicable to all tumor sizes. By placing a retractor with electrodes attached to its tip at the cerebellomedullary junction, the authors have recorded BAEP amplitudes that are 10 times greater than those recorded using the conventional technique. Only small sampling numbers (64–256 recordings) are required and are obtained in 5 to 15 seconds. The technique has been applied successfully in 34 patients who underwent vestibular schwannoma resections. It has also been tested in patients with intrameatal—extrameatal meningiomas and in those with vascular compressive disorders; there have been no false results. The advantages of this new technique are: 1) identification of BAEP components is easier and faster; 2) reliable BAEP responses are obtained in some cases in which conventional BAEP responses are lost or severely deformed; and 3) BAEP response deterioration and improvement are recognized earlier than would occur using the conventional technique. This last advantage provides the surgeon with a useful warning at a stage of surgery at which BAEP changes are still temporary and can be reversed. This method is different from other trials of intradural BAEP recordings in three respects: its use is not limited to particular tumor sizes; there is no interference with the surgical process; and, most important, the obtained responses correlate well with those of conventional BAEP responses, probably because the recording site is in the vicinity of the anterior cochlear nucleus. In conclusion, the chances of useful monitoring feedback with adequate adaptation of the microsurgical strategy are improved considerably.

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Gustavo Adolpho Carvalho, Guido Nikkhah, Cordula Matthies, Götz Penkert and Madjid Samii

✓ Surgical management and prognosis of traction injuries of the brachial plexus depend on the accurate diagnosis of root avulsion from the spinal cord. Myelography, computerized tomography (CT) myelography, and recently magnetic resonance (MR) imaging have become the main radiological methods for preoperative diagnosis of cervical root avulsions. Most of the previous studies on the accuracy of CT myelography and MR imaging studies have correlated the radiological findings with the extraspinal surgical findings at brachial plexus surgery. Surgical experience shows that in many cases extraspinal findings diverge from intradural determinations. Consequently, only correlation with the intradural surgical findings will allow assessment of the factual accuracy of CT myelography and MR imaging studies.

In a prospective study, 135 cervical roots (C5–8) were evaluated by CT myelography and/or MR imaging and further explored intradurally via a hemilaminectomy. The accuracy of the preoperative CT myelography—based diagnosis in relation to the intraoperative findings was 85%. On the other hand, MR imaging demonstrated an accuracy of only 52%. The most common reasons for false-positive or false-negative findings were: 1) partial rootlet avulsion; 2) intradural fibrosis; and 3) dural cystic lesions. Computerized tomography myelography scans using 1- to 3-mm axial slices prove to be the most reliable method to evaluate preoperatively the presence of complete or partial root avulsion in traumatic brachial plexus injuries.

Because extradural judgment of cervical root avulsion can be unreliable, accurate assessment of intraspinal root avulsion enormously simplifies the decision concerning the choice of donor nerves for transplantation and/or neurotization during brachial plexus surgery.