- Norton Neuroscience Institute, Norton Healthcare, 210 East Gray Street, Suite 1102, Louisville, KY 40202, USA
- Impulse Monitoring Inc., 10420 Little Patuxent Parkway, Suite 250, Columbia, MD 21044, USA
Christopher B. Shields
Norton Neuroscience Institute, Norton Healthcare, 210 East Gray Street, Suite 1102, Louisville, KY 40202, USA
DOI:10.4103/2152-7806.154775Copyright: © 2015 Shields CB. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
How to cite this article: Shields CB, E. Shields LB, Jiang YD, Yao T, Zhang YP, Sun DA. BAER suppression during posterior fossa dural opening. Surg Neurol Int 09-Apr-2015;6:57
How to cite this URL: Shields CB, E. Shields LB, Jiang YD, Yao T, Zhang YP, Sun DA. BAER suppression during posterior fossa dural opening. Surg Neurol Int 09-Apr-2015;6:57. Available from: http://sni.wpengine.com/surgicalint_articles/baer-suppression-posterior-fossa-dural-opening/
Background:Intraoperative monitoring with brainstem auditory evoked responses (BAER) provides an early warning signal of potential neurological injury and may avert tissue damage to the auditory pathway or brainstem. Unexplained loss of the BAER signal in the operating room may present a dilemma to the neurosurgeon.
Methods:This paper documents two patients who displayed a unique mechanism of suppression of the BAER apparent within minutes following dural opening for resection of a posterior fossa meningioma.
Results:In two patients with anterior cerebellopontine angle and clival meningiomas, there was a significant deterioration of the BAER soon after durotomy but prior to cerebellar retraction and tumor removal. Intracranial structures in the posterior fossa lying between the tumor and dural opening were shifted posteriorly after durotomy.
Conclusion:We hypothesized that the cochlear nerve and vessels entering the acoustic meatus were compressed or stretched when subjected to tissue shift. This movement caused cochlear nerve dysfunction that resulted in BAER suppression. BAER was partially restored after the tumor was decompressed, dura repaired, and bone replaced. BAER was not suppressed following durotomy for removal of a meningioma lying posterior to the cochlear complex. Insight into the mechanisms of durotomy-induced BAER inhibition would allay the neurosurgeon's anxiety during the operation.
Intraoperative monitoring provides early warning signals of potential danger that enable the surgeon to avoid neural damage. Brainstem auditory evoked responses (BAER) depicts the transmission of electrical activity of the auditory pathway from the cochlea to the inferior colliculus. BAER has been used to monitor neurological function in several neurosurgical operations such as microvascular decompression for trigeminal/glossopharyngeal neuralgia, hemifacial spasm, and resection of benign cerebellopontine angle or clival tumors.[
Accurate interpretation of electrophysiological changes intraoperatively is crucial, particularly with respect to differentiating false negative and false positive responses.[
We report two patients harboring posterior fossa meningiomas lying anterior to the cochlear nerve in whom the BAER was suppressed within a short time after durotomy and cerebrospinal fluid (CSF) drainage. We hypothesize that the durotomy induced structural shifts that resulted in cochlear ischemia caused by direct kinking or stretching of the cochlear nerve or its arterial supply. An additional patient is also reported in whom the tumor was located posterior to the cochlear nerve. This patient did not display BAER depression following durotomy. Thus, BAER inhibition was dependent on tumor location. Prompt exclusion of potential systemic and anesthetic factors that may cause BAER suppression and a clear recognition of the novel cause described in this report will minimize the neurosurgeon's concern that a serious neurological event has occurred and allow the operation to proceed with assurance.
A 49-year-old female (height: 5′11″; weight: 230 pounds; body mass index [BMI]: 32.1) presented with right facial numbness and slight difficulty with mastication and swallowing. Examination demonstrated decreased sensation to touch in her right V1 and V2 distributions as well as decreased hearing from the right ear. The magnetic resonance imaging (MRI) scan revealed a 1.4 × 2.7 × 1.7 cm right cerebellopontine angle (CPA) tumor with clival extension and anterior indentation of the pons that lay anterior to the internal auditory meatus [
(a) Case #1: MRI showing a 1.4 × 2.7 × 1.7 cm meningioma in the right CPA extending anteriorly into Meckel's cave. The arrow depicts the internal auditory meatus, and the asterisk denotes the tumor site. (b) Case #2: MRI demonstrating a 2.8 × 1.3 × 2.8 cm meningioma arising from the right side of the clivus. In each case the internal acoustic meatus was located posterior to the tumor, with the cochlear nerve and vessels located adjacent to the tumor. The arrow depicts the internal auditory meatus, and the asterisk denotes the tumor site. (c) Case #3: MRI showing a 2.9 × 3.2 × 3.6 cm meningioma lying adjacent to the right sigmoid sinus, posterior to the internal auditory meatus. The arrow depicts the internal auditory meatus, and the asterisk denotes the tumor site
Anesthesia was induced utilizing sufentanil and rocuronium, maintained with sevoflurane, and supplemented with fentanyl and propofol as needed. The patient was placed in a park bench position, her head was fixed in a Mayfield clamp, and her face was directed at an angle of 45° to the floor. BAER and electromyography (EMG) were used to monitor facial, cochlear, glossopharyngeal, vagus, accessory, and hypoglossal nerve functions continuously using a Caldwell Cascade device (Caldwell Laboratories, Kennewick, WA). Monitoring was performed by a certified neurophysiologist from the neuromonitoring company Impulse Monitoring, Inc. (Columbia, MD). An individual with a doctoral degree at Impulse Monitoring, Inc. supervised the technologist. The protocols of this company included quality control and routine techniques used to determine noise from signal.
EMG recordings were unaffected throughout the operation. Auditory stimuli (Caldwell Laboratories, Kennewick, WA), delivered through foam earphones (Elymotic Research Inc, Elk Grove Village, IL) were inserted into the ear canals, sealed with bone wax to hold the earphones in position, and covered with Tegaderm film (3M Healthcare, St. Paul, MN). BAER recordings used conventional averaging techniques that were induced by rectangular rarefaction sound stimulation of 11.1 Hz at 90 dB. Masking noise was applied on the contralateral side to that being tested to eliminate a “crossover” response. The active electrode was placed on the ipsilateral mastoid, reference electrode on the vertex, with the forehead serving as ground. The bandpass was set within 50–2000 Hz. Impedance of the recording electrodes was maintained at <5 kΩ. A total of 2000 signals administered at 11.2 Hz were averaged per recording to reduce interference from electric noise in the operation room. BAERs were alternatively obtained from each side with each response recorded for 3 min. The intact side served as control. Continuous monitoring was performed throughout the operation with no interval between samples. Noise was differentiated from physiologic responses by repetitive recordings obtained during continuous monitoring.
Temporal resolution of the recorded responses was 2400 samples per second using the Cadwell machine (sampling rate;
Baseline bilateral BAER was recorded following induction of anesthesia [
Consecutive recordings showing BAER suppression for 14 minutes after dural opening and before placement of cerebellar retraction (traces 2-4). Waves I and the IV-V complex were lost during dural opening in Cases #1 and #2. Partial return of BAER occurred during wound closure. (a) Case #1: Diminished amplitudes of peaks I, II, and III with delayed latency of the wave IV-V complex became apparent after dural opening (traces 2-4). The wave IV-V complex improved during dural closure and bone flap replacement (traces 6-10). There was no return of wave I. (b) Case #2: Soon after dural opening a loss of wave III became apparent, and the latency of peaks I and V were prolonged (traces 2-4). Wave I was restored after tumor removal, dural closure, and mesh replacement (traces 6-8). There was no return of wave V. Left BAER remained intact throughout the operations in Cases #1 and #2. (c) Case #3: There was no increase in the latency and no amplitude suppression of wave I during or following the durotomy (traces 2-4)
The surgical pathology demonstrated a World Health Organization (WHO) grade I meningioma. A postoperative MRI scan revealed a small amount of residual tumor in the anterior aspect of Meckel's cave that was not recognized intraoperatively. At the patient's 3-week postoperative visit, she complained of headaches, nausea, and vomiting. There was also increased numbness and tingling of the right side of her face. When hearing was tested several weeks postoperatively, it was unchanged compared with preoperatively. A computed tomography (CT) scan revealed ventriculomegaly and transependymal absorption of CSF and, therefore, the patient underwent placement of a ventriculoperitoneal shunt.
A 33-year-old male (height: 5′9″; weight: 219 pounds; BMI: 32.3) presented with unstable gait and the sensation of heaviness and numbness of his feet. He also complained of paresthesias of the right occipital scalp. Examination revealed gait ataxia with a tendency to lean to the right and a positive Romberg sign. Hearing was intact bilaterally. MRI revealed a 2.8 × 1.3 × 2.8 cm anteriorly situated CPA and clival meningioma with displacement of the anterior pons [
Anesthesia, surgical preparation, and BAER and EMG monitoring were the same as described in the previous patient. The baseline BAER from each side was nearly identical. Measurements on the right (tumor side) were: I =1.65 ms, III = 4 ms, V = 6.18 ms, I-III = 2.35 ms, III-V = 2.18 ms, I-V = 4.53 ms versus I = 1.5 ms, III = 4 ms, V = 5.71 ms, I-III = 2.5 ms, III-V = 1.71 ms, I-V = 4.21 ms on the left (nontumor side) [
The tumor was a WHO grade I meningioma. A postoperative MRI scan demonstrated resection of >50% of the tumor that decreased the mass effect on the pons. The patient developed a right 6th nerve palsy postoperatively that was treated with ocular therapy. Facial nerve function remained intact, and hearing was unchanged compared with preoperatively. However, there was minimally decreased hearing from the right ear.
A 60-year-old female (height: 5′3″; weight: 120 pounds; BMI: 23.23) with right CPA meningioma was compared with the two previous patients. This patient described a several month history of episodic posterior occipital headaches as well as neck pain. The MRI scan revealed a 2.9 × 3.2 × 3.6 cm tumor lying adjacent to the right sigmoid sinus, posterior to the internal auditory meatus (IAM) [
Anesthesia, surgical approach, durotomy, and exposure of the tumor were identical to Cases #1 and #2. The baseline BAER from each side was recorded with measurements on the right (tumor side) were: I = 1.51 ms, III = 4 ms, V = 5.9 ms, I-III = 2.49 ms, III-V = 1.90 ms, I-V = 4.39 ms versus I = 1.48 ms, III = 3.75 ms, V = 5.71 ms, I-III = 2.27 ms, III-V = 1.86 ms, I-V = 4.23 ms on the left (nontumor side) [
The tumor arose from the sigmoid sinus. The surgeon considered the surgery to be a gross total removal.
The pathology was a WHO grade I meningioma. A postoperative MRI scan demonstrated a near complete resection, with only a minimal degree of enhancement observed along the sigmoid sinus. There was also evidence of a heterogeneous attenuation of the right cerebellum indicating a cerebellar infarct and hydrocephalus. The patient was managed with a ventricular drain and rehabilitation. There was no hearing loss postoperatively.
BAER is used to monitor auditory pathways intraoperatively from the cochlear nerve to the inferior colliculus.[
(a) Illustration of the mechanism of BAER inhibition with a tumor situated anterior to the cochlear nerve. The cochlear nerve and adjacent blood vessels (internal auditory artery and loop of the anterior inferior cerebellar artery) are fixed at the internal auditory meatus. The cochlear nerve and adjacent blood vessels serve as a potential vulnerable site (arrowhead) following the posterior shift of the cochlear nerve after dural opening. The arrow depicts the shift of the structures away from the tumor and toward the direction of the dural opening. (b) Tumors situated at or posterior to the internal auditory meatus will shift posteriorly following dural opening and fail to cause kinking of the cochlear nerve and arteries. The arrow depicts the tumor displacement posteriorly towards the dural opening
BAER is used to monitor auditory nerve and brainstem function clinically[
We have described two patients with meningiomas located anterior to the IAM who demonstrated BAER suppression soon after durotomy and prior to direct surgical manipulation of the cochlear nerve complex. Instead of being alarmed or aborting surgery prior to tumor resection, neurosurgeons must be aware of this mechanism of “unexplained” BAER suppression and should continue the operation without delay.
The authors acknowledge Norton Healthcare, Louisville, KY for their continued support.
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