Direct brainstem recording of auditory evoked potentials during vestibular schwannoma resection: nuclear BAEP recording

Technical note and preliminary results

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✓ 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.

Article Information

Address reprint requests to: Cordula Matthies, M.D., Department of Neurosurgery, Nordstadt Hospital, Haltenhoffstrasse 41, D-30167 Hannover, Germany.

© AANS, except where prohibited by US copyright law.

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Figures

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    Photographs showing two small monopolar tag electrodes each consisting of a 1- to 2-mm-diameter tag made of silver or copper coated by silver with silicone-insulated wires (upper). These are attached to the retraction side of the cerebellar retractor using sterile drapes with the strings hidden at the back of the blade, and the uninsulated tags are placed at the distal end of the retractor blade (lower).

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    Intraoperative photographs. After suboccipital craniotomy, dura incision, and CSF drainage from the pontomedullary cistern, the retractor is inserted for cerebellar retraction. The two electrode tips are placed near the medullocerebellar junction (upper left); the upper electrode (4) is placed at the cerebellar surface. The lower electrode (3) is located in the vicinity of the cochlear nerve (1) at its entrance to the brainstem just anterior to the anterior inferior cerebellar artery (2). A small portion of the tumor (5) is situated between the nerve and the electrode. The tumor is enucleated (upper right) with the retractor electrodes in place. The lower electrode (3) comes into close contact with the cochlear nerve entry zone and the tumor is removed completely (lower) with preservation of the nerves and arteries.

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    Traces showing correlation between conventional (A) and direct (B and C) BAEP recordings. In this 48-year-old man with a left-sided vestibular schwannoma, the conventional BAEP at 100-dB rarefaction stimulation consists of component waves P-I through P-V; the downward negative peaks N-I, N-III, and NV are also obtained (A). Direct brainstem recording (brainstem—vertex) shows corresponding responses; only P-I is inverted on direct recording, whereas P-II, P-III, P-IV, and P-V show the same polarity as conventional components, are synchronous, and 10 (B) to 100 times larger up to the end of surgery (C). Also note that component waves II and III are the largest.

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    Traces showing the improved visibility of BAEPs using direct monitoring. In a 53-year-old woman with an intrameatal—extrameatal tumor, on 100-dB rarefaction stimulation a conventional BAEP is recorded with component waves P-I, P-II, P-III, and P-V (as well as N-I, N-III, and N-V) during CSF drainage and retractor positioning (A). At cerebellar retraction (and positioning of the direct electrodes), the BAEP is slightly delayed and the direct BAEP provides 10-fold larger responses (B) with all components being present; component waves P-I and N-I are inverted. During tumor dissection and up to closure of the internal auditory canal, the direct BAEP response is preserved, occurs slightly earlier again, and stays significantly larger (C) than the conventional BAEP response (D). Hearing is preserved at the preoperative level (preoperative loss of 45 dB and postoperative loss of 40 dB at 1–3 kHz).

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    Traces showing the sensitivity of direct and conventional BAEP monitoring sequences. The monitoring sequence in a 56-year-old patient with right-sided deafness and left-sided vestibular schwannoma is presented by direct recording on traces A1, B1, C1, D1, and E1 and by conventional recording on traces A2, B2, C2, D2, and E2. Upward BAEP component waves P-I, P-II, P-III, and P-V are stable at retractor fixation (A). After some drilling, P-III and P-V appear later (B). As the tumor is dissected off the nerve, direct recording shows some amplitude reduction in P-III (C1), whereas the conventional recording is stable (C2). At the time the tumor is pulled upward and dissected at the tumor—nerve border, the P-III latency appears later (D). After a short break recovery is observed. During muscle closure of the internal auditory canal, the direct recording trace signals an amplitude increase and an earlier onset of P-III (E1).

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