Eighth Nerve Tumors Research Articles

Clinical outcomes of resecting scarpa's ganglion during vestibular schwannoma surgery

Vestibular schwannomas are slow-growing tumors arising from the Schwann cells of the vestibular nerve. Scarpa’s ganglion, the vestibular nerve ganglion, is located within the internal auditory meatus. Surgical treatment of vestibular schwannomas carries the potential of resecting Scarpa’s ganglion along with the tumor. No prior studies have evaluated outcomes based on the presence of Scarpa’s ganglion within tumor specimens. The neurosurgery patient records were queried for patients who underwent surgical resection of vestibular schwannomas at the University of Missouri Healthcare between January 1, 2008 and December 31, 2018. Inclusion criteria consisted of minimum age of 18, imaging demonstrating an eighth nerve tumor, surgical resection thereof, and a final pathological diagnosis of WHO grade I schwannoma. Data were collected retrospectively. The histological slides of the tumors were reviewed, and the presence or absence of the ganglion was noted. Outcomes analyzed included postoperative dizziness, hearing, and facial nerve function. Fifty-two patients met inclusion criteria. Ten (19%) resected tumors contained portions of the ganglion. No difference in risk of resection of ganglion occurred based on the surgical approach (p = 0.2454). Mean follow-up duration was 24.6 months ± 26.2 standard deviation. No differences in postoperative hearing or dizziness (p = 0.8483 and p = 0.3190 respectively) were present if Scarpa’s ganglion was resected. House-Brackmann classification of facial nerve function at last follow-up was similar (p = 0.9190). Resection of Scarpa’s ganglion with vestibular schwannomas does not increase risk of post-operative dizziness, facial nerve weakness, or hearing loss.

Electrophysiology of Cranial Nerve Testing: Auditory Nerve.

The electrocochleogram and brainstem auditory evoked potentials (BAEPs) are electrophysiologic signals used to assess the auditory nerve. The electrocohleogram includes the cochlear microphonic, the cochlear summating potential, and the eighth nerve compound action potential. It is used predominantly for hearing assessment and for diagnosis of Ménière disease and auditory neuropathy. Brainstem auditory evoked potentials are used for hearing assessment, diagnosis of dysfunction within the cochlea, the auditory nerve, and the brainstem auditory pathways up to the level of the mesencephalon, and intraoperative monitoring of these structures. The earliest BAEP component, wave I, and the eighth nerve compound action potential reflect the same process-the initial depolarization in the distal auditory nerve. Brainstem auditory evoked potential wave II receives contributions from the region of the cochlear nucleus and from the second depolarization in the distal auditory nerve. Wave III and later components are entirely generated rostral to the auditory nerve. Interpretation of BAEP studies is based on waves I, III, and V; auditory nerve dysfunction is manifested as prolongation of the I-III interpeak interval or absence of waves III and V. Eighth nerve tumors can cause a variety of BAEP abnormalities depending on which structures they affect. Adverse intraoperative BAEP changes can have many etiologies, including direct mechanical or thermal injury of tissue, ischemia (including cochlear ischemia or infarction due to compromise of the internal auditory artery), eighth nerve stretch, systemic or localized hypothermia, and artifactual BAEP changes due to technical factors.

Cochlear ablation effects on amino acid levels in the chinchilla cochlear nucleus

Inner ear damage can lead to hearing disorders, including tinnitus, hyperacusis, and hearing loss. We measured the effects of severe inner ear damage, produced by cochlear ablation, on the levels and distributions of amino acids in the first brain center of the auditory system, the cochlear nucleus. Measurements were also made for its projection pathways and the superior olivary nuclei. Cochlear ablation produces complete degeneration of the auditory nerve, which provides a baseline for interpreting the effects of partial damage to the inner ear, such as that from ototoxic drugs or intense sound. Amino acids play a critical role in neural function, including neurotransmission, neuromodulation, cellular metabolism, and protein construction. They include major neurotransmitters of the brain – glutamate, glycine, and γ-aminobutyrate (GABA) – as well as others closely related to their metabolism and/or functions – aspartate, glutamine, and taurine. Since the effects of inner ear damage develop over time, we measured the changes in amino acid levels at various survival times after cochlear ablation. Glutamate and aspartate levels decreased by 2weeks in the ipsilateral ventral cochlear nucleus and deep layer of the dorsal cochlear nucleus, with the largest decreases in the posteroventral cochlear nucleus (PVCN): 66% for glutamate and 63% for aspartate. Aspartate levels also decreased in the lateral part of the ipsilateral trapezoid body, by as much as 50%, suggesting a transneuronal effect. GABA and glycine levels showed some bilateral decreases, especially in the PVCN. These results may represent the state of amino acid metabolism in the cochlear nucleus of humans after removal of eighth nerve tumors, which may adversely result in destruction of the auditory nerve. Measurement of chemical changes following inner ear damage may increase understanding of the pathogenesis of hearing impairments and enable improvements in their diagnosis and treatment.

Clinical Observations with Evoked Otoacoustic Emissions at Mayo Clinic

Clinical applications of transient evoked otoacoustic emission (TEOAE) measures began in 1989 at Mayo Clinic Rochester. Normative data indicates: a) greater TEOAE level for women and right ears beginning in the teen years, b) mean TEOAE levels remain relatively stable through the first seven decades of life, c) between subject EOAE level variability for normal hearing adults is high (up to about 30 dB) while within subject variability is low (about 4 dB), suggesting EOAEs should not be used to estimate pure-tone thresholds but are valuable in monitoring hearing status. Good frequency specificity of TEOAEs is demonstrated across high frequency hearing loss. Examples of TEOAEs in differential diagnosis are provided for cases of sudden hearing loss, auditory neuropathy and eighth nerve tumors including pre and post operative testing assessing changes in auditory function related to surgical procedures. Finally, our experience with TEOAEs in newborn hearing screening is described.

Characteristics of patients with gaze-evoked tinnitus.

The authors describe symptoms and population characteristics in subjects who can modulate the loudness and/or pitch of their tinnitus by eye movements. Data were obtained by questionnaire. The study was conducted at a university center and a tertiary care center. Respondents had the self-reported ability to modulate their tinnitus with eye movements. Ninety-one subjects reported having gaze-evoked tinnitus after posterior fossa surgery involving the eighth nerve. Eighty-seven of them underwent removal of a vestibular schwannoma (acoustic neuroma), two had bilateral eighth nerve tumors (one underwent bilateral tumor removal; the other unilateral tumor removal), one underwent removal of a cholesteatoma, and one underwent removal of a glomus jugulare tumor. Seventeen subjects who had never had posterior fossa surgery reported gaze-evoked tinnitus. Of those with vestibular schwannomas, tumor size ranged from small (<2 cm) to large (>4 cm). The gender distribution was 48.3% male and 51.7% female. In 77% of patients, the gaze-evoked tinnitus was localized to the surgical ear or side of head; 21.8% had bilateral tinnitus that was louder in the surgical ear or side of head. In 86 of 87 subjects, loudness of tinnitus changed with eye movement. Eye movement away from the central (eyes centered) position increased the loudness of tinnitus in all 86 subjects who responded to this question. Seventy-three of 85 (85.9%) patients indicated that pitch changed with eye movement, with pitch increasing in 64/72 (88.9%) of them. Eighty-three of 87 (95.4%) patients reported total loss of hearing in the surgical ear. Seventy of 83 (84.3%) patients reported facial nerve problems immediately after surgery, 52 of 87 (60%) reported persistent facial weakness, and 16 of 87 (18.4%) patients reported persistent double vision. In those 17 subjects with gaze-evoked tinnitus and no posterior fossa surgery, the majority of respondents (14/17, 82.4%) were male. Gaze-evoked tinnitus after cerebellar pontine angle surgery is more common than was previously believed. In addition, posterior fossa surgery is not a prerequisite for the development of gaze-evoked tinnitus. It is likely that gaze-evoked tinnitus is a manifestation of functional reorganization. Gaze-evoked tinnitus could result from an unmasking of brain regions that respond to multiple stimulus/response modalities, and/or from anomalous cross-modality interactions, perhaps caused by collateral sprouting.

ABR Indices

ABR absolute latencies and interpeak intervals were reviewed for 684 non-tumor ears and 75 eighth-nerve tumor ears having various degrees of high frequency hearing loss. For non-tumor ears, the percentage of abnormal absolute latencies for waves I, III, and V increased rather systematically as hearing loss increased, whereas absolute latencies for eighth-nerve tumor ears were largely abnormal regardless of hearing loss. Interpeak intervals were normal for most of the non-tumor ears, but the I–III and I–V intervals were abnormal for most of the tumor ears. The most sensitive index for otoneurologic assessment was a combination of abnormal wave V interaural latency differences or I–V interpeak interval, and the most specific criterion was the I–V interpeak interval. Tumor size influenced sensitivity of most ABR indices.

Preserved Hearing Following Vestibular Schwannoma Surgery

Audiologic test results were reviewed for 30 patients whose hearing was preserved following eighth nerve tumor surgery. Speech reception thresholds, 4,000-Hz thresholds, acoustic reflexes, and ABR wave I absolute latencies were generally unchanged or poorer following surgery; word recognition scores were essentially unchanged. By contrast, ABR waves III and V, and the I–III and I–V interpeak intervals, were generally improved postoperatively, including several patients who had clear waves following surgery despite absent preoperative responses. The improved ABR tracings likely indicate increased neural synchrony due to reduced pressure on the cochlear nerve following surgery, whereas the reduced hearing sensitivity probably reflects cochlear damage coincident to the surgery.

Four-Channel ABR Recordings

Simultaneous four-channel (ipsilateral, contralateral, horizontal, and noncephalic) ABR recordings were reviewed by three audiologists for 187 ears of nontumor patients, and 13 ears of patients having eighth nerve tumors. Waveforms from a subgroup of 63 ears were reexamined by the same audiologists to assess inter- and intra-judge consistency. For nontumor ears, the ipsilateral tracings provided the best identification of Waves I and III; the ipsilateral and noncephalic recordings allowed equal identification of Wave V. Waves I, III, and V generally were more robust from the ipsilateral electrode array than from any of the other electrode montages for the tumor ears. Three-way inter-judge agreement ranged from 82% to 87% for ipsilateral Waves I, III, and V and for noncephalic Wave V. Agreement between pairs of judges ranged from 86% to 100%. Intra-judge agreement ranged from 87% to 94% for ipsilateral Waves I, III, and V, and from 92% to 95% for noncephalic Wave V.

Neurofibromatosis type 2: Report of a family and review of current evaluation and treatment

Significant advances during the past decade have greatly improved our understanding of neurofibromatosis type 2, a genetic disease which results in bilateral acoustic neuromas. The emergence of gadolinium-enhanced magnetic resonance imaging has allowed early detection of minute intracanalicular eighth-nerve tumors, less than 1 cm in diameter. Recombinant DNA studies have clarified the genetics that underlie neurofibromatosis type 2 and separate it from a variety of related conditions, such as von Recklinghausen's neurofibromatosis. Early diagnosis and surgical removal of these tumors may offer the only hope of preserving hearing and facial nerve function. A report of the evaluation and treatment of a family with multiple affected individuals will exemplify these conclusions.

Preoperative and Postoperative Auditory Brain-Stem Response Results for Patients With Eighth-Nerve Tumors

Preoperative and postoperative auditory brain-stem response results and audiologic data were reviewed for seven patients whose hearing was preserved after eighth-nerve tumor surgery. Auditory brain-stem response absolute latencies (I, III, and V) and interpeak intervals (I-III, III-V, and I-V) were identified more often postoperatively than preoperatively. Even though postoperative absolute latencies were usually abnormal, the interpeak intervals were usually normal following surgery. Postoperative hearing sensitivity was unchanged or slightly poorer for five of the seven patients, but postoperative speech discrimination was the same or better for six of the seven patients. All three patients having acoustic reflex testing showed improvement postoperatively.

Modulation detection by patients with eighth-nerve tumors.

Detection thresholds for sinusoidally amplitude-modulated broad-band noise were measured as a function of modulation frequency for 4 normally hearing listeners and for 6 patients suffering eighth-nerve tumors. Measurements were obtained using a method-of-adjustment (MOA) procedure. On average, the threshold values revealed that more modulation was needed across all modulation frequencies for the patients' affected ears relative to either the normally hearing listeners or the patients' better ears. The cutoff frequency derived from the average modulation-threshhold function (MTF) for the affected ears was about half the normal value. For a simple lowpass-filter model of the process, the latter result suggested a doubling of the auditory time constant in the affected ears. These patients, on average, exhibited the characteristic high-frequency audiometric hearing loss most often associated with eighth-nerve tumors. Their MTFs closely resembled MTFs described previously for high-frequency audiometric hearing loss. The results of statistical analyses suggested that high frequency audiometric hearing loss, irrespective of other influences, is the most parsimonious explanation for the increased modulation thresholds measured for the eighth-nerve tumor patients.

Auditory brain-stem response in patients with acoustic neuromas. Wave presence and absence.

Auditory brain-stem response (ABR) waveforms from 61 patients with eighth nerve or cerebellopontine angle tumors were analyzed for wave presence or absence. Eighteen patients yielded ABRs lacking any readable waves; 27 patients revealed ABRs in which waves were partially present (ie, one or two waves present); and 16 patients demonstrated ABRs with all waves (I, III, and V) present. In the partially present ABR group, waves I and V were most prevalent. The group lacking waves (absent ABR group) had significantly poorer hearing sensitivity and larger tumors than the partial ABR group. Various additional group comparisons for hearing sensitivity and tumor size did not yield any significant differences among the three groups.

Acoustic Neurinomas

Eight nerve tumors account for about 8 to 10% of all intracranial tumors, and for about 71 to 75% of all cerebello-pontine angle tumors. They occur particularly in the middle decades of life and are twice as frequent in females as in males. They originate from the distal neurolemmal portion of the nerve, in most instances, from the vestibular division, and represent a neoplasia of the cells of Schwann. Two prominent types of tissue "A" and "B" of Antoni can be distinguished histologically. Regressive changes are frequent and lend them a colorful appearance. Bilateral eighth nerve tumors occur in about 4%. These tumors reveal certain characteristic differences. The differential diagnosis must include: meningiomas, epidermoids and gliomas, as well as a number of other space-occupying lesions.

Latency of the Acoustic Reflex in Eighth-Nerve Tumor

To the Editor .—We agree with Jerger and Hayes 1 that the latency characteristics of the acoustic reflex are not a helpful indicator of an eighth-nerve tumor. We have found other indicators, eg, reflex decay and abnormalities of the auditory-evoked potential, to be much more relevant in the diagnosis of acoustic neuromas. In our series of patients, with eighth-nerve tumors we also have not noted an increase in latency. We have noted, however, a decrease in the rate of rise of the acoustic reflex in the ear contralateral to an acoustic tumor (only crossed reflexes were used). This decrease in slope would be consistent with a decrease in conductivity in the cochlear nerve on the side of the tumor. We have had a considerable experience (with more than 1,000 patients) with acoustic reflex latency, amplitude, and slope characteristics in a population of patients with vertigo with normal hearing. 2 None

Brainstem evoked response audiometry.

Brainstem evoked response audiometry is useful in the identification of threshold levels as well as in the diagnosis of eighth nerve tumors. The instrumentation is a modification of the electroencephalograph and an averaging computer; 3,000 clicks, 16/s, are presented and averaged for each printed response. Evoked responses can be obtained from a patient who is unable or unwilling to respond to conventional behavioral testing; best results are obtained from quiet or asleep patients.

Latency of the acoustic reflex in eighth-nerve tumor.

We evaluated acoustic reflex morphologic features in four subjects with confirmed, unilateral acoustic neuroma. All four subjects showed marked reduction in absolute reflex amplitude and alteration in the reflex amplitude-intensity function in the ear with the eighth-nerve disorder. The early, fast-rising component of the normal reflex was also typically absent in the ears with tumor. Interaural latency comparisons were made in three ways. At equal reflex sensation levels and equal reflex sound pressure levels, latency ws substantially delayed in the ear with the eighth-nerve disorder. At equivalent reflex amplitudes, however, latency was equivalent in normal ears and ears with eighth-nerve disorder. Results suggest that delayed onset of the acoustic reflex in subjects with eighth-nerve disorder may reflect amplitude and wave-form morphologic effects rather than a latency prolongation per se.

Vestibular nerve compression in eighth-nerve tumors.

The superior vestibular nerve was studied histologically in 25 patients who underwent removal of inferior vestibular nerve schwannoma. In most cases, the nerve fibers were structurally normal but the endoneurial space showed various degrees of capillary stasis and/or extravasation of red blood cells. Increased fibrosis of the endoneurial space was seen in about two-thirds of the cases; it was mild in all except two cases. The endoneurial space appeared edematic in about one-third of the cases. Wallerian degeneration of individual fibers was occasionally observed in most cases and was severe in the two cases with moderate or severe endoneurial fibrosis. In about one-third of the cases there were large numbers of myelinated nerve fibers with thinned myelin sheaths. The observed structural changes are compatible with those seen in human and experimentally induced chronic compressive or entrapment neuropathies. Thus, early symptoms of eighth-nerve tumors might develop due to direct compression of the eighth-nerve trunk within the internal auditory canal.

Electrocochleography and auditory brainstem electric responses in patients with pontine angle tumors.

In 45 patients with surgically proven pontine angle tumors, compound action potential (AP) and summating potential (SP) were recorded with transtympanic electrocochleography (ECochG) together with brainstem electric responses (BSER). The aims were to quantify the mechanism by which tumors cause hearing loss and evaluate the diagnostic potentials of ECochG and BSER for detecting eighth nerve and brainstem tumors. Except for AP latency and narrow band AP waveform, response parameters recorded by ECochG are uncorrelated. Four uncorrelated parameters were abnormal in only 10% of the cases, three in 25%, two in 40%, and one in 90%. The BSER criterion was the latency delay between waves I and IV and resulted in about 90% detection, improving to 95% when used in combination with ther interaural wave V delay criterion. ECochG results provide evidence that, for hearing losses up to 60 dB HL, the origin is cochlear, resembling that caused by Meniere's disease. Evidence is presented that the increase in I-V delay in the BSERs is caused by differential action of the tumor upon low and high frequency fibers in the auditory nerve and that desynchronization of the firings of the nerve fibers is of more importance than an increase in neural conduction time. ECochG as the sole test for detection of pontine angle tumors appears to be of limited value. Brainstem response on its own has great merits; however, it should be emphasized that no wave I was detected in about 30% of the cases. The 95% detection score obtained with BSER depends on specifying the latency of wave I. For these cases, we substituted the latency of the AP recorded by ECochG.

Brainstem evoked response audiometry in confirmed eighth nerve tumors

Brainstem evoked response audiometry has been described as an effective predictor of eighth nerve disease. This study reports audiologic findings in 52 cases of surgically confirmed eighth nerve and cerebellopontine angle tumors. The results of brainstem audiometry were consistent with an eighth nerve lesion in 51 cases. Its use in site of lesion tests is highly recommended.

Neuroglial tissue in human eighth nerve specimens

One hundred eighty nerve biopsy specimens from patients with eighth nerve tumors or other vertiginous diseases requiring vestibular neurectomy were studied. In many specimens light microscopy revealed pale areas among the myelinated nerve fibers in the neurolemmal portion of the eighth nerve. Electron microscopy showed that these were ectopic areas of glial tissue consisting of bundles of numerous cytoplasmic processes of fibrous astrocytes. The abundance of filaments within each process and the occurrence of microtubuli suggest that these processes are reactive astrocytes and accordingly are present as a consequence of the reparative potential of the astroglial tissue after a neuronal lesion. The possible functional significance of the findings is discussed.