Predictive value of Blink reflex and facial corticobulbar motor evoked potential in cerebellopontine angle tumor surgery

OBJECTIVE

The current study examined the efficacy of the facial corticobulbar motor evoked potentials (FCoMEPs) and blink reflex (BR) on predicting postoperative facial nerve function during cerebellopontine angle (CPA) tumor surgery.

METHODS

Data from 110 patients who underwent CPA tumor resection with intraoperative FCoMEPs and BR monitoring were retrospectively reviewed. The association between the amplitude reduction ratios of FCoMEPs and BR at the end of surgery and postoperative facial nerve function was determined. Subsequently, the optimal threshold of FCoMEPs and BR for predicting postoperative facial nerve dysfunction were determined by receiver operating characteristic curve analysis.

RESULTS

Valid BR was record in 103 of 110 patients, whereas only 43 patients successfully recorded FCoMEP in orbicularis oculi muscle. A reduction over 50.3% in FCoMEP (O. oris) amplitude was identified as a predictor of postoperative facial nerve dysfunction (sensitivity, 77.1%; specificity, 83.6%). BR was another independent predictor of postoperative facial nerve deficit with excellent predictive performance, especially eyelid closure function. Its optimal cut-off value for predicting long-term postoperative eyelid closure dysfunction was was 51.0% (sensitivity, 94.4%; specificity, 94.4%).

CONCLUSIONS

BR can compensate for the deficiencies of the FCoMEPs. The combination of BR and FCoMEPs can be used in CPA tumor surgery.

SIGNIFICANCE

The study first proposed an optimal cut-off value of BR amplitude deterioration (50.0%) for predicting postoperative eyelid closure deficits in patients undergoing CPA tumor surgery.

Hyperventilation-induced nystagmus in peripheral vestibulopathy and cerebellopontine angle tumor

OBJECTIVE

To determine the incidence and characteristics of hyperventilation-induced nystagmus (HIN) in cerebellopontine angle (CPA) tumors and unilateral peripheral vestibulopathy (UPV), and to elucidate differential contribution of hyperventilation to bring out vestibular asymmetry between acute and chronic phases of UPV.

METHODS

We recorded horizontal HIN in 33 patients with CPA tumors and 145 with UPV. The UPV included patients of either acute (7 days or less from symptom onset, n = 47) or chronic (more than 7 days from symptom onset, n = 98) phases.

RESULTS

The incidence of HIN was higher in the CPA tumor than in the UPV group (82 vs 34%, p < 0.01) and was also higher in the acute than in the chronic UPV group (60 vs 21%, p < 0.01). Furthermore, HIN was more commonly ipsilesional (i-HIN) in the CPA tumor than in the UPV group (52 vs 8%, p < 0.01) and more commonly ipsilesional in the acute than in the chronic UPV group (21 vs 1%, p < 0.01). The patients with i-HIN and acoustic neuroma had a tendency to harbor smaller tumors and to have less severe caloric asymmetry.

CONCLUSIONS

The contribution of hyperventilation on vestibular nystagmus differs depending on the disease phase or underlying pathologies. Our study demonstrates that hyperventilation-induced nystagmus (HIN) beating to the side of reduced caloric response, hearing impairment, or abnormal auditory brainstem response responses may be a valuable sign for bedside detection of cerebellopontine angle (CPA) tumors. CPA tumor should be a prime suspicion in patients with acute vertigo and ipsilesional HIN, especially when the vertigo accompanies hearing impairments.

Unusual exophytic subependymoma in the bulbo-cerebellar angle. Case report

Subependymoma was first described by Scheinker in 1945; it frequently occurs in the ventricles and rarely in the spinal canal representing 0.7% of all central nervous system tumours. Most of these intraventricular tumours are subclinical entities, remaining of small size and discovered at autopsy with 0.4%incidence. We report a case of subependymoma with a completely exophytic growth from the foramen of Luscka: only a similar one has been described in the literature but with a lesser cysternal involvement. Neuroradiological and anatomopathological features of subependymoma are discussed.

Meningioma in the Posterior Fossa Without Dural Attachment -Case Report-

A 14-year-old boy presented with a very rare meningioma in the posterior cranial fossa without dural attachment. Magnetic resonance imaging revealed a 3-cm, well-circumscribed, heterogeneously enhanced, round mass without dural tail sign in the right side of the posterior fossa. Right vertebral angiography revealed very faint tumor staining supplied by the right posterior inferior cerebellar artery. Total removal of the tumor was performed. Intraoperatively, the mass exhibited no attachment to the dura mater, cerebellar parenchyma, or choroid plexus, but was firmly attached to the arachnoid tissue near the foramen of Luschka. Histological and immunohistochemical studies established the diagnosis of meningothelial meningioma (World Health Organization grade I).

The role of endoscopic third ventriculostomy in the management of hydrocephalus associated with cerebellopontine angle tumours

BACKGROUND

Progressive hydrocephalus secondary to cerebellopontine angle tumours has been traditionally managed with ventriculo-peritoneal shunting. Endoscopic third ventriculostomy provides an alternative treatment option and the success rate in this patient group has not previously been reported.

METHODS

We report a retrospective series of 11 patients with cerebello-pontine angle tumours who presented with symptomatic hydrocephalus, or developed hydrocephalus following radiosurgery, who underwent endoscopic third ventriculostomy.

RESULTS

Seven patients (63.6%) remain shunt free. There were no complications following endoscopy in any patient. Where the ventriculostomy failed there was no additional morbidity.

CONCLUSIONS

Endoscopic third ventriculostomy is a low morbidity procedure, which avoids the inherent problems of shunts, particularly infection and should be considered for patients with hydrocephalus and cerebello-pontine angle tumours.

Case of microarteriovenous malformation-induced trigeminal neuralgia treated with radiosurgery

Radiosurgery to the right fifth cranial nerve was performed with the Gamma Knife on a 39- year-old patient who presented with classic symptoms of trigeminal neuralgia (TN), but was found on imaging studies to harbour a small intrinsic vascular malformation within the nerve. Based on size and drainage, the arteriovenous malformation (AVM) was Spetzler-Martin Grade III and no previous history of bleeding was reported. The patient had failed a trial of carbamazepine, and no surgical procedures had been performed. A decrease in symptoms was reported by the 6-month follow-up. A review of the literature on microAVM-induced TN is provided as well as a discussion of management.

[Multimodal intraoperative electrophysiological monitoring during cerebellopontine angle tumor surgery. Benefit or loss?]

In the Neurosurgery Department, Silesian University School of Medicine, continuous monitoring of selected neurophysiological functions of the central and peripheral nervous system was introduced in 1998 as a routine procedure in cerebellopontine angle surgery and some other operations performed in the petroclival region. Such benefits from this method as increased patient safety, availability of information about dynamic changes in the monitored structures, and the possibility of cranial nerves localization using stimulation in the operating area, are quite obvious. The paper presents results of a detailed statistical analysis of the amount of time required for preparation and for operating in 174 cerebellopontine angle tumor surgeries performed in the years 1986-2002 with (group M) and without (group BM, before the year 1998) intraoperative monitoring. Subgroups distinguished according to the histological type of tumor were evaluated. Out of 95 procedures performed in group M, 57 were operations of acoustic neurinoma cases, 15 meningiomas, 8 cases of epidermal cyst, and 15 other growth processes in the cerebellopontine angle region. Among 79 operations in group BM, there were 57 cases of acoustic neurinoma, 4 cases of meningioma, 8 cases of epidermal cyst, and 10 of other types of neoplastic growth. In group M as compared to group BM the pre-op. preparation time was found to be significantly longer in cases of the VIII-th nerve neurinoma, and of other tumors. No statistically significant differences in the amount of operating time in were found between any of the subgroups. Both radicality of tumor removal and facial nerve status have clearly improved since intraoperative monitoring was introduced.

[Nystagmographic findings in pontocerebellar angle tumors]

OBJECTIVE

To determine the diagnostic and prognostic value of vestibular function tests in cerebellopontine angle tumors

METHODS

Analysis of preoperative nystagmographic findings in 29 patients affected of cerebellopontine angle tumors and its possible correlation with anatomical and progressive patterns

RESULTS

Tumor size and growth pattern were statistically significantly related to auditory and vestibular functions and to central findings on nystagmography. Age, gender, tumor size and central findings on nystagmography did not show influence upon postoperative dysbalance. However, alterations related with adaptation of the vestibular system (positional nystagmus and asymmetry on rotational tests) correlated significantly with postsurgical dysbalance.

CONCLUSIONS

The tests for vestibular function reflect a the characteristics of growth of the cerebellopontine angle tumors and can help to identify the patients susceptible of prolonged postoperative dysbalance.

[Unilateral tinnitus--diagnostics and treatment]

900 patients with tinnitus were treated between January 1998 and August 2000 in ENT Rehabilitation Center in Poznan. Unilateral tinnitus was observed in more than 50% of the patients. In all the patients a history was taken paying a special attention to the loudness estimation by the patient and to the influence of the tinnitus on the everyday life. Laryngological, audiological and neurological examinations and laboratory tests were performed. In some cases endocrinological examination was performed. In the group of patients with unilateral tinnitus a special attention was paid to the possibility of ponto-cerebellar angle tumour. After audiological diagnostics CT or MRI examinations were performed. Authors emphasize, that in patients with unilateral tinnitus a proper observation is required to exclude expansive process within the acoustic nerve.

Nausea as a complication of low-frequency repetitive transcranial magnetic stimulation of the posterior fossa

BACKGROUND

Transcranial magnetic stimulation (TMS) can non-invasively investigate the function of human brain. However, it can induce a focal pain at the stimulated site on the scalp or seizures when applied with high frequency (>1 Hz). Here we report an induction of nausea as a complication of low-frequency repetitive TMS (rTMS) of the cerebellum.

SUBJECTS AND METHODS

Eight right-handed normal volunteers underwent low-frequency (0.9 Hz) rTMS of the right cerebellum. The stimulus intensity was set at 90% of the resting motor threshold determined by TMS to motor cortex.

RESULTS

Nausea lasted as long as 10 min after the end of rTMS without apparent neurological deficit in two subjects. This symptom was replicated when the same protocol was applied on a different day in the same subjects.

CONCLUSIONS

Low-frequency rTMS of cerebellum is still a safe procedure, but the experimenters should keep in mind the possibility of inducing nausea.

Click- and short tone burst-evoked myogenic potentials in cerebellopontine angle tumors

We report results of vestibular-evoked myogenic potentials (VEMPs) in patients with cerebellopontine angle tumors and compare results obtained using clicks with those obtained using 500 Hz short tone bursts (STB). We reviewed the records of 87 patients with cerebellopontine angle tumors. Clicks (0.1 ms, 95 dB nHL) were presented to all patients and STB (500 Hz, rise/fall time 1 ms, plateau time 2 ms, 95 dB nHL) were presented to 27 patients. Click-evoked VEMPs were abnormal in 69/87 patients (79%; no response in 55 patients, decreased response in 14 patients, normal response in 18 patients). STB-evoked VEMPs were abnormal in 22/27 patients (82%; no response in 18 patients, decreased response in 4 patients, normal response in 5 patients). Click- and STB-evoked VEMPs were identical in 23/27 patients (85%). Two patients showed normal STB-evoked VEMPs and decreased click- evoked VEMPs, and 2 patients showed decreased STB-evoked VEMPs and absent click- evoked VEMPs. These results confirm our previous study in a small number of patients. Vestibular afferents seem to respond better to 500 Hz STBs than to clicks.

Contralateral suppression of transient evoked otoacoustic emissions in patients with cerebello-pontine angle tumor

OBJECTIVE

During measurement of transient evoked otoacoustic emissions (TEOAEs), acoustic stimulation of the contralateral ear reduces or suppresses TEOAE amplitude. This is thought to be due to the inhibitory control that the medial efferent auditory nerve exerts on outer hair cell (OHC) function. The main aim of this study was to investigate the effect of cerebello-pontine angle (CPA) tumor on the medial efferent nerve pathways to both tumor and non-tumor ears by examining alterations in TEOAE amplitude that result from contralateral acoustic stimulation.

DESIGN

Contralateral suppression of TEOAEs using broadband noise was measured preoperatively in 17 patients with unilateral CPA tumor and 17 normally hearing controls, matched for age and gender.

RESULTS

The control ears demonstrated significantly more suppression than the tumor and non-tumor ears in the patient group. There was, however, no significant difference in suppression between the tumor and non-tumor ears, and the statistical correlation for suppression between them was high. There was no effect of gender, hearing threshold levels, or size and type of tumor on suppression, although there was an effect of age on suppression in both the control and patient groups where suppression reduced as age increased. Four of the 17 patients had TEOAEs, which were clearly present in the tumor ear despite substantial hearing loss, three of which had no measurable hearing.

CONCLUSIONS

It is hypothesized that neural compression by CPA tumor disrupts the medial efferent nerve control mechanism to the OHCs of tumor ears. It also is hypothesized that neural compression reduces transmission of afferent nerve impulses from the tumor ear, which cross over to the medial olivo-cochlear complex and reduce the inhibitory control of OHC function in the non-tumor cochlea.

[Monitoring of neurophysiologic modalities during surgery for cerebellopontine angle tumors. Personal experiments]

The purpose of intraoperative monitoring of many modalities is to save some structures of the nervous system being at risk of damaging during surgical procedures. In cerebellopontine angle (cpa) tumour cases these nervous system structures can include cranial nerves (trigeminal, facial, cochlear, accessory), motor and sensory tracts localised within brainstem, and other. Continuous registration of somatosensory and auditory evoked potentials as well as electromyography of masseter muscle, orbicular muscle of eye and trapezius muscle during procedure is the method of brainstem, cochlear tract and cranial nerves status evaluation. Direct stimulation of cranial nerves within posterior fossa using bipolar electrode is the method of facial, trigeminal and accessory nerves localisation, especially in patients with large tumours. In Department of Neurosurgery Silesian University School of Medicine for intraoperative monitoring of many modalities in cpa lesion cases Nocolet Viking IV D unit with special IOM software is employed. Authors presented own experience in such method and effect in treatment of group of 15 patients operated with electrophysiological intraoperative monitoring.

Brainstem response in cerebellopontine angle tumors

OBJECTIVE

To evaluate the auditory brainstem response (ABR) findings in cerebellopontine angle (CPA) tumors and focus on those with normal ABR results.

STUDY DESIGN

This was a retrospective evaluation of ABR findings. All subjects with diagnosed CPA tumors who were referred for treatment were included.

PATIENTS

All 309 patients with radiographically confirmed (computed tomography or magnetic resonance imaging [MRI]) CPA tumors (153 before 1993 and 156 after 1993).

INTERVENTION

All patients underwent complete audiologic examination, electronystagmography, ABR testing, and ipsilateral transtympanic electrocochleography. All tumors were histologically confirmed.

MAIN OUTCOME MEASURES

The patients were categorized according to ABR results (normal or pathologic findings or no response); according to histology (vestibular schwannomas or meningiomas); and according to tumor size by MRI (small, up to 15 mm; medium, 16-25 mm; large, 26-40 mm; and very large, over 40 mm).

RESULTS

Normal ABR results were found in 18.4%; pathologic ABR results in 31.4%; and no response in 50.2%. Small vestibular schwannomas (under 15 mm) showed a higher incidence of normal ABR results (41.7%). CPA tumors with normal ABR results (n = 57) caused no hearing loss (n = 12), symmetrical hearing loss (n = 11), or ipsilateral hearing loss with Short Increment Sensitivity Index 100% and normal acoustic reflex (cochlear deafness, n = 34). Caloric response was normal in 59.7% of these tumors.

CONCLUSION

Because validation of ABR as screening for acoustic tumors is based on diagnosed tumors, and because MRI allows detection of very small tumors, the incidence of normal ABR increases and its sensitivity would be expected to decrease. ABR is not sufficient for early detection of small CPA tumors.

[Electronystagmographic and videonystagmographic studies in cerebello-pontine angle tumors]

Otoneurological investigations were carried out in 24 patients with cerebello-pontine angle tumors demonstrated in computed tomography. Standardized history taking was followed by electronystagmographic and videonystagmographical tests. In no case correct electrophysiological results were obtained. Eye-tracking test and gaze nystagmus were most frequently abnormal and together with other data these abnormalities indicated the location of the process. A more extensive use of videonystagmography was postulated.

[The identification and functional tracking of the facial nerve during the removal of tumors of the cerebellopontine angle]

Intracranial identification by electrostimulation and monitoring of the status of the facial nerve was intraoperatively used in 21 patients with cerebellopontine angle tumors of varying histological structure. Monopolar and bipolar electrostimulation, as well as electromyography and mechanography for recording the function of the facial nerve were compared. During removal of cerebellopontine angle tumors, identification and monitoring of the function of the facial nerve provide anatomic retention of this nerve when the tumor is radically eliminated. This is of essential significance for acoustic neurinomas. A combination of monopolar and bipolar stimulation for identification of the facial nerve simplifies removal of these tumors. As compared with electromyography, mechanography of facial nerve function during these operations is a more convenient technique due to the simplicity and absence of false operation for electric interferences. The study provided the optimum procedure for intracranial stimulation to identify the facial nerve within the cerebellopontine angle.

MR imaged neurovascular relationships at the cerebellopontine angle

Improvements in MR imaging techniques allow visualisation of the anatomy of the cerebellopontine angle (CPA) in increasingly accurate detail, revealing the complex interrelationship of the neurovascular structures in this region. We wished to assess whether vessels and vascular loops intimately associated with cranial nerves VII and VIII, corresponded to any abnormality or symptom pattern, and thus had any clinical significance. The MR scans of 108 patients were retrospectively reviewed and the imaging status of VII, VIII, the vessels, presence of vascular loops and their relationship to the nerves, coded and recorded. The patients' records were independently reviewed and the presence and "sidedness' of asymmetrical hearing loss, tinnitus, vertigo, and the results of caloric and brain stem evoked responses recorded. The vessels were closely associated with VII and VIII in over 30% of this sample with vascular loops imaged in 21% of patients, and clearly imaged entering the IAM in 7% of both right and left CPAs studied. There was no statistically significant relationship demonstrated between the proximity of the vessels, or vascular loops, to the nerves and a symptom, or symptom pattern. These findings should be considered a normal variant on MR scanning.

F-wave recordings from nasal muscle for intraoperative monitoring of facial nerve function

The generation of the F-wave in muscle after peripheral motor nerve stimulation depends upon the functional integrity of the entire motoaxon between its axon hillock and the motor endplates. Cerebellopontine angle (CPA) surgery bears the risk of damaging the facial nerve in its root exit zone; functional degradation should therefore be reflected by changes of the nasal muscle F-wave. Constant current pulses supraliminal for direct (M) muscular responses were applied subcutaneously to the zygomatic branch of the facial nerve ipsilateral to the operated side, EEG needle electrodes were placed in the nasal muscle and referred to glabella for recording of myoelectric activity. Anesthesia was achieved by application of fentanyl, midazolame and N2O and low doses of short acting muscular relaxants; the above-mentioned narcotics did not influence F-wave recordings, whereas the administration of muscle relax ants was often deleterious to F-wave monitoring. 22 patients with different tumors in the CPA, the cerebellum or the vicinity of the brainstem were subject to this monitoring procedure up to now. F-wave monitoring was effective in 12/22 cases. In 7/12 patients the signal was stable without any resulting dysfunction postoperatively. In 3/12 cases changes in amplitude and/or latency were observed indicating a subsequent facial palsy of a moderate degree. In 2 cases the F-wave was lost-these suffered from a facial paralysis afterwards. As an illustrative case the course of one of these is pointed out. In 10/22 procedures F-wave monitoring was ineffective due to the prolonged administration of muscle relaxants. The F-wave appears to be a valid tool for intraoperative prognosis of postoperative facial nerve function in CPA surgery. The borderlines of F-wave changes indicating damage to neural function by surgical manipulation are to be defined next.

[Petronasal fistula caused by the epidermal cyst of the cerebellopontine angle region]

The authors report a rare case of the occurrence of the paradoxical CFN rhinerrhea in a three months after epidermal cyst of the cerebellopontine region removal. The good result of the surgical fistula obliteration was reached by using muscle graft and tissue glue.

MRI of the inner ear

The recent advances in MRI of the temporal bone brought about by the availability of intravenous contrast agents sensitive to the disruption of blood-brain barrier have further expanded the role of MRI in the evaluation of patients with vestibulocochlear symptoms. Not only can MRI be used to diagnose vestibular schwannomas (and other intracanalicular and cerebellopontine angle lesions mimicking it) with a high degree of accuracy, but it can now identify a variety of inflammatory and neoplastic processes of the membranous labyrinth which, in the past, were only made presumptively and occasionally confirmed by surgery and pathology. This enhanced diagnostic capacity of diseases of the vestibular cochlear system may help in the future to improve the management of these patients.

Contralateral suppression of transiently evoked otoacoustic emissions and neuro-otology

Transiently evoked otoacoustic emissions can be suppressed with simultaneous contralateral sound stimulation. This is considered to be effected via the efferent pathway from the superior olivary complex (SOC) to the contralateral cochlea. This study examined this effect in patients with extrinsic and intrinsic lesions of the brainstem which may affect the efferent pathway either within the vestibular nerve which carries the efferent bundle to the cochlea or within the brainstem at the level of the SOC. Suppression is reduced or absent in these patients and the site and size of the lesion determines whether the suppression is affected unilaterally or bilaterally. Lesions affecting the auditory afferent pathway without significant alteration in hearing appear to affect the efferent pathway too.

Facial nerve function following cerebellopontine angle surgery: prognostic value of intraoperative thresholds

Intraoperative stimulation of the facial nerve during surgery of the cerebellopontine angle greatly aids the surgeon in identification of the nerve. The preservation of the ability to stimulate the facial nerve, at the brain stem following tumor removal, has been shown to correlate with good postoperative facial outcome. Thresholds of facial nerve stimulation recorded in our series, using constant voltage stimulation, showed statistically significant relevance to facial nerve outcome. When the difference between thresholds before and after tumor removal was 0.2 V or less or when the threshold after tumor removal was less than or equal to 0.2 V, a good postoperative facial outcome could be expected. This paper discusses the relevance of stimulation threshold prior to tumor removal to the size of tumor.

Inverted Bruns' nystagmus in arachnoid cysts of the cerebellopontine angle

A 9-year-old girl with huge, double arachnoid cysts at the cerebellopontine angle is reported. After a cyst-peritoneal shunt operation, she developed fine, high-frequency gaze nystagmus towards the lesion side combined with coarse, large amplitude nystagmus in the opposite direction, which is the reverse of the Bruns' nystagmus. We call this reverse phenomenon 'inverted' Bruns' nystagmus. As the nystagmus disappeared after a second shunt operation, the nystagmus was presumed to be caused by the cystic lesion.

Effectiveness of acoustic reflex threshold criteria in the diagnosis of retrocochlear pathology

This study examines the acoustic reflex threshold criteria, derived from a large group of patients with cochlear hearing loss proposed by Cohen & Prasher (1992), in order to evaluate their effectiveness in differentiating between cochlear and retro-cochlear lesions. This criterion was tested on 63 patients with surgically confirmed cerebello-pontine angle (CPA) lesions. The false results obtained with this and other criteria were compared. The best balance between the false positive and negative results in the cochlear and retrocochlear group is provided by the criterion of any two adjacent test frequencies (proposed in the companion paper) having an upper limit of 105 dB for hearing below 60 dB and 110 dB for hearing over 60 dB. This study has also singled out the interaural difference criterion as the best diagnostic indicator with the lowest false results with only 1 false negative from 63 CPA cases and 3 false positive cases from 61 cochlear lesions with hearing thresholds over 55 dB.

Contralateral effects of cerebello-pontine angle exposure on human auditory brain-stem evoked potentials

Auditory brain-stem evoked potentials (ABEPs) were recorded during surgical procedures which exposed the cerebello-pontine angle (CPA) in humans. Recordings made with the CPA contralateral to stimulus exposed were compared with those obtained with the skin sutured at the end of surgery. Single-channel as well as 3-channel Lissajous' trajectory (3-CLT) analyses were used to evaluate the effect of the surgical exposure on ABEP. The results suggest that exposure of the CPA contralateral to the stimulated ear did not affect dipole equivalent orientation nor magnitude, but did affect timing of the recorded activity being more pronounced for segments 'd'-'e' (corresponding to waves IV-V) than for 'a'-'b' (waves I-II). The results imply that the effects of disrupting the volume conductor may have been overwhelmed by other effects, such as local temperature changes. These changes, although not associated with clinical sequella, should be accounted for when analyzing subtle quantitative changes involving surgical exposures.

Hearing preservation in acoustic neuroma surgery: a continuing study

Hearing preservation in acoustic neuroma surgery is possible in a limited number of cases. Although there have been many articles published about hearing preservation, there have been few studies of long-term hearing results, nor is it known if there is an increased rate of tumor recurrence when hearing preservation is attempted. Twenty-two patients who underwent a hearing preservation procedure via the retrosigmoid approach were selected from 80 consecutive patients with cerebellopontine angle tumors operated on from February 1984 to November 1987. Useful hearing was retained in 11 cases as reported in a previously published study. Seven patients continue to have useful hearing after 3 to 5 years; 3 have shown a gradual but slight decline. There has been no tumor recurrence in these patients, but 2 patients, operated on early in the series and who had lost hearing, had recurrent tumor.

Effect of hearing loss of cochlear origin on the auditory brain stem response

Auditory brain stem response (ABR) testing is widely used to detect lesions of the auditory neural pathways. The ABR waves depend not only on the integrity of the neural pathways, but also on the condition of the cochlea. To properly interpret the ABR response, it is necessary to understand the effects of cochlear hearing loss on the ABR wave latencies. We studied two populations of subjects with cochlear hearing loss: one with varying degrees of high-frequency hearing loss and the other with varying degrees of flat configuration hearing loss. The degree of cochlear hearing loss was quantified in several different ways and subjected to one linear and three nonlinear regression analyses to test for accuracy in predicting ABR wave latencies and interpeak intervals (waves I, III, V, I-V, I-III, and III-V) for three click intensities. Hearing loss levels from 2 to 6 kHz, in particular 4 kHz, were superior to other audiometric test frequencies as predictors of ABR wave latencies for the group with the high-frequency losses. No particular characterization was found to be superior for the flat hearing loss configurations. From these results, modeled predictions of wave latencies as a function of degree and configuration of hearing loss were made. The modeled predictions are then used to suggest guidelines for interpretations of ABR results where hearing impaired patients are involved.

Malignant meningioma metastasizing through the cerebrospinal pathway

A case of malignant meningioma metastasizing through the cerebrospinal pathway is presented. The primary tumor was a parasagittal malignant meningioma invading into the brain. The tumor seeded to the cerebellopontine angle cistern and thoracic spine after multiple operations. Although this type of tumor borders the CSF, metastasis through the cerebrospinal pathway is rare, and only 18 such cases have been reported (2, 3, 10, 12).

Phase audiometry in neuroaudiological practice

Six groups of subjects, altogether 107 in number, were studied with phase audiometry. Three control groups were studied (normal-hearing healthy controls, persons with cochlear hearing losses, and patients with neurological disease not affecting the auditory system), and three patient groups were studied (patients with cerebellopontine angle tumours, with brainstem lesions, and with temporal lobe lesions). Two phase audiometers were used. The sensitivity was 85% for patients with cerebellopontine angle tumours, 71% for patients with brainstem lesions, and 69% for patients with temporal lobe lesions. There was good agreement between the phase audiometers, though one of them (BIAB Phase Audiometer) had better results for two of the patient groups. Phase audiometry can be recommended in neuroaudiological practice.

Late onset cerebello-pontomesencephalic degeneration

Two siblings are presented with late onset, rapidly progressive truncal ataxia, paralysis of down-gaze and loss of up-gaze saccades in association with other oculomotor dysfunctions as well as dementia. Electron microscopic muscle studies revealed abnormal distribution and form of the mitochondria, probably being the ultrastructural basis of the pathologic changes. A neurological syndrome as that described here has not been reported before.

Cerebellopontine angle epidermoid in a noise-exposed patient

A cerebellopontine angle epidermoid was identified in a noise-exposed patient as part of an occupational hearing conservation program. The patient is a 24-year-old employee of a major national newspaper who, because of workplace noise levels, was required to be part of a hearing conservation program. A shift in thresholds in 1 year was identified by the audiologic reviewer and referral for audiologic and otologic services was recommended and carried out. The steps leading from the initial air-conduction, pure-tone audiogram to final neurosurgical removal of the epidermoid CPA lesion was described. Such referral is essential if potentially health and life threatening conditions are to be identified and managed, although the primary objectives of the hearing conservation program remains the prevention of sensorineural hearing loss from workplace noise exposure.

Preservation of hearing in the surgical removal of cerebellopontine angle tumors

It is claimed that postsurgical hearing preservation is possible in about 5 to 8 per cent of cases of acoustic neuroma. Even with small tumors, hearing preservation can be accomplished in about half of the patients who are fortunate to have anatomic integrity of facial and cochlear nerves, as well as intact inner ear blood supply at the end of the surgical procedure. Monitoring of seventh and eighth nerve function through evoked potentials may be important. On the other hand, even if evoked potentials are preserved during surgery with wave V latency and amplitude similar to preoperative recording, hearing may still be completely lost during the immediate postoperative period. There is no predictable pattern that assures postoperative preservation of hearing. In Case 2, the seventh and eighth nerves as well as the inner ear blood supply were carefully dissected and anatomically and electrophysiologically preserved during the intraoperative period. At the end of the surgical procedure, the evoked potentials were the same as preoperatively. Nevertheless, the patient ended up with no detectable hearing postoperatively. We feel that the suboccipital-retrosigmoid transcanal approach can be safely used for the removal of cerebellopontine angle tumors of all sizes. We believe the argument that "only the translabyrinthine approach can accomplish total tumor removal" is not valid. No complications have been attributed by having the patient in the semisitting position. Older patients, who cannot tolerate the semisitting position, are operated on while in the park bench position. Planned subtotal removal of a CPA tumor is done with the patient in the supine position (transmastoid-retrolabyrinthine or retrosigmoid). The relationship between the otologic surgeon and neurosurgeon is very important. The surgical approach used should be selected on the basis of the combined experience of the surgical team. Finally, an attempt should be made to preserve facial nerve function and hearing in all suitable patients. The postoperative course using the suboccipital-retrosigmoid approach is usually benign, and the patients are discharged from the hospital between 7 and 10 days following surgery. The translabyrinthine approach, in our opinion, should be reserved for smaller lesions in patients with anacusis or with residual hearing that is not worth saving.

Phase audiometry--a rapid method for detecting cerebello-pontine angle tumours?

Four groups of subjects have been investigated, using a manual phase audiometer. Group 1 consisted of 20 normal-hearing subjects. Group 2 of 22 subjects with unilateral or asymmetric cochlear hearing loss. Group 3 of 14 subjects with surgically verified CPA tumours, and Group 4 of 15 subjects with unilateral conductive impairment. In this report we suggest an upper borderline value for detecting interaural time differences of 120 microseconds. If the subject is able to detect only a value exceeding 120 microseconds, there is likely to be a retrocochlear lesion.

Orthodromic (intra/extracranial) neurography to monitor facial nerve function intraoperatively

This report introduces the technique of orthodromic neurography for monitoring of facial nerve function during operation in the cerebellopontine angle. By stimulation of the intracisternal segment of the facial nerve, a compound nerve action potential with amplitudes of 15 to 480 microV can be recorded extracranially from the nerve near the stylomastoid foramen after 0.95 to 2.27 ms. Usually there is no need for signal averaging, and the method is independent of the effect of muscle relaxants. With the use of the same electrophysiological equipment as for evoked potential neuromonitoring, immediate and repeated localization of the facial nerve and its discrimination from the trigeminal and the lower cranial nerves during nerve preparation within the tumor capsule is possible.

Bruns bidirectional nystagmus in cerebellopontine angle tumours

Bruns nystagmus is an uncommon, bidirectional optokinetic disturbance associated with advanced cerebellopontine angle tumours. In a retrospective analysis of 115 such tumours, Bruns nystagmus was observed in 18 patients (16%). All tumours in this group were 3 cm or greater in diameter. Fourth ventricle displacement as assessed by computerized cranial tomography was present in all cases where the fourth ventricle was visualized (89%). Seven of 15 cases (47%) had significant contralateral N5 delay on brainstem auditory evoked potential testing, 5 (33%) were normal and 3 (20%) gave no consistent result. It was concluded that Bruns nystagmus occurs in large tumours associated with significant brainstem distortion. The neurophysiological abnormalities demonstrated by brainstem audiometry appear to be a less sensitive measure of brainstem compression than neuro-anatomical assessment using CT.

Limits of ABR and contribution of transtympanic electrocochleography in the assessment of cerebellopontine angle tumours

Auditory brainstem responses (ABR) and transtympanic electrocochleography (ECochG) were analysed for 63 patients with tumours of the cerebellopontine angle (CPA) and/or internal auditory meatus (IAM). ABR recordings indicated a clearly prolonged wave I-V interval (above 4.3 ms) in half of the patients, hence suggesting a retrocochlear disorder. For the remaining patients ABR alone was not sufficient for clear diagnosis and ECochG was therefore used in addition. When the latency of wave V was uncertain due to the absence of wave I, the latency of wave N1 was always measurable by ECochG. The NI-V interval evaluated in this way was always significantly prolonged (above 4.5 ms) relative to a control group of patients with a sensorineural hearing loss. In 9 of the 13 subjects without any discernible ABR, an ECochG response could be recorded and its threshold was often better than the mean pure tone audiogram. This study confirms the validity of transtympanic ECochG in the diagnosis of retrocochlear disorders. The time difference observed between wave N1 (ECochG) and wave I (ABR) in CPA tumours is discussed and a possible explanation is proposed.

Cochlear nerve monitoring during cerebellopontine angle operations

The authors present their experience with intraoperative monitoring of cochlear nerve action potentials (AP) in 30 adult patients. Operative procedures were acoustic neuroma excision with attempted hearing preservation and selective vestibular neurectomy in patients with incapacitating Meniere's disease and serviceable hearing (SRT less than 50 db, discrimination greater than 60%). Loss of AP is detected rapidly and has been demonstrated after manipulation of the cochlear nerve and after coagulation of small arteries on the tumour capsule. Presence of an AP at the end of the procedure usually correlates with postoperative preservation of hearing. AP monitoring appears to be a reliable means of detecting potentially reversible changes in cochlear nerve function intraoperatively.

Auditory brain stem response--interwave measurements in acoustic neuromas

Auditory brain stem responses (ABR) of 61 patients with tumors of the VIIIth nerve or cerebellopontine angle were analyzed for latency of the I-III, III-V, and I-V interwave intervals. Of the 61 patients, 16 yielded ABR records with repeated waves I, III, and V so that interwave intervals could be measured. Results indicate that not only is the I-III interval often abnormal, but in almost 50% of the cases the III-V interval is also extended. Additional analysis included comparing a group of subjects with cochlear lesions with the 16 patients with VIIIth nerve tumors. These findings indicated a higher degree of specificity than sensitivity for the ABR interwave measurement. These findings, as well as additional analyses, are discussed.

Changes in contralateral brain-stem auditory evoked responses to cerebellopontine angle compression in rats

Eleven rats were subjected to graded compression of the brain stem at the cerebellopontine angle (CPA) following craniectomy and cerebellectomy. Clicks were delivered to the ear contralateral to the compression site, and brain-stem auditory evoked responses (BAER's) were collected before and during compression. With increasing compression, there was an increase in the latency and a decrease in the amplitude of the peaks of the BAER. The later peaks of the BAER (V and VI) demonstrated changes with minimal compression that progressed as compression increased, while changes in the amplitude and latency of the earlier peaks (II, III, and IV) occurred at the higher grades of compression. Following decompression of the brain stem, there was a decrease in the latencies of most of the BAER peaks. Peak amplitude remained depressed following brain-stem decompression. These experimental findings corroborate clinical reports of abnormal BAER's elicited from stimulation of the ear contralateral to a CPA tumor. The relative sensitivity of the later peaks of the BAER to even minimal brain-stem compression explains the prolonged III to V interpeak latency reported in the literature in patients with brain-stem compression from a CPA lesion. The possible mechanisms for the BAER changes that were observed contralateral to compression are discussed.

Auditory evoked cortical responses to frequency glides in subjects with retrocochlear hearing impairment

Auditory cortical responses evoked by stimulation with frequency glides of a continuous tone have been recorded from 44 subjects with intracranial tumours affecting auditory function. Response latencies obtained when stimulating the ear on the side of the tumour were different from the non-tumour ear as well as from groups with cochlear impairment and normal hearing. Tumours caused a reduction of a specific sensitivity to the frequency glide stimulation, affecting further processing of the dynamic change in tone frequency. The test appears to have diagnostic potential.

The differential diagnosis of cerebello-pontine angle lesions. A multidisciplinary approach with special emphasis on the brainstem auditory evoked potential

92 patients thought to have a cerebello-pontine angle tumour on initial presentation were studied prospectively using standard neuro-otological techniques, brainstem evoked potential recording with a variety of montages, middle latency auditory evoked potentials and CT scanning. 64 patients had a tumour, 40 of which were acoustic neuromas. By analysing the data from all disciplines it has been possible to delineate clusters of variables that are of value in the differential diagnosis of cerebello-pontine angle lesions.

Diagnostic features of the auditory brainstem response in identifying cerebellopontine angle tumours

This report concerns the diagnostic features of the ABR which are most effective in identifying patients with cerebellopontine angle tumours. The conclusions are based on the results of 40 normal subjects, 35 patients without a cerebellopontine angle tumour and 68 patients without a tumour. One hundred per cent of the tumour patients and ten per cent of the non-tumour patients had abnormal ABR results.

[The clinical value of the orbicularis oculi reflex in combination with auditory evoked brain stem potentials in the diagnosis of disease processes in the cerebellopontile angle and brain stem]

The results of the Orbicularis oculi reflex (OOR) and the anditory evoked brain stem potential (a. e. BSP) supplement each other in the furnished information in lesions of the central and lower brain stem. The a. e. BSP permits quite a reliable localisation in the brain stem and in the cerebellopontile angle. the OOR moreover helps to discover disturbances in the N. facialis and the N. trigeminus. In intracanalicularly situated acousticus neurinomas lacking a. e. BSP the pathologically changed reflex response of the OOR indicates the process in the internal auditory canal.