Potential effects of managed care organizations in rural communities: a framework

OBJECTIVE: To define mechanisms accounting for transient deafness in three children (two siblings, ages 3 and 6, and an unrelated child, age 15) when they become febrile. DESIGN: Audiometric tests (pure-tone audiometry, speech and sentence comprehension), tympanometry, middle ear muscle reflex thresholds, otoacoustic emissions (OAEs), and electrophysiological methods (auditory brain stem responses [ABRs], sensory evoked potentials, peripheral nerve conduction velocities) were used to test the children when they were afebrile and febrile. RESULTS: ABRs, when afebrile, were abnormal with a profound delay of the IV-V and absence of waves I-III. The ABR in one of the children, tested when febrile, showed no ABR components. Measures of cochlear receptor function using OAEs were normal in both febrile and afebrile states. Cochlear microphonic potentials were present in the three children, and a summating potential was likely present in two. When afebrile, there was a mild threshold elevation for all frequencies in the 15-yr-old and a mild elevation of thresholds for just low frequencies in the two siblings. Speech comprehension in quiet was normal but impaired in noise. One of the siblings tested when febrile had a profound elevation (>80 dB) of pure-tone thresholds and speech comprehension was absent. Acoustic reflexes subserving middle ear muscles and olivocochlear bundle were absent when febrile and when afebrile. No other peripheral or cranial nerve abnormalities were found in any of the children. Sensory nerve action potentials from median nerve in one of the children showed no abnormalities on warming of the hand to 39 degrees C. CONCLUSION: These children have an auditory neuropathy manifested by a disorder of auditory nerve function in the presence of normal cochlear outer hair cell functions. They develop a conduction block of the auditory nerves when their core body temperature rises due, most likely, to a demyelinating disorder of the auditory nerve. The auditory neuropathy in the two affected siblings is likely to be inherited as a recessive disorder.     

Overexpressed phytochrome C has similar photosensory specificity to phytochrome B but a distinctive capacity to enhance primary leaf expansion

PURPOSE: Only few quantitative data are available on late effects in the healthy brain after radiosurgery. An animal model can contribute to systematically investigate such late effects. Therefore, a model applying radiosurgery at the rat brain was established. A long-term (19 months) follow up study with 66 animals after radiosurgery was carried out. METHODS AND MATERIALS: In 60 animals, an area in the frontal lobe of the brain was irradiated stereotactically with a 15 MV linac. Different doses of 20, 30, 40, 50, and 100 Gy with two field sizes (3.9 and 5.9 mm collimator) were selected, using the integrated logistic formula with input parameters from human brain. The induced alteration of the blood-brain barrier permeability was investigated by means of contrast enhanced magnetic resonance imaging. RESULTS: A first intracranial signal enhancement was observed in one animal 160 days after irradiation with 100 Gy. Beginning at 5 months all animals in the two 100 Gy groups homogeneously showed contrast enhancement, but none of the other groups. This remained until 13 months after irradiation. The volume of contrast enhancement as well as the increase of signal intensity were different between the two 100 Gy groups. After 19 months, the animals irradiated with lower doses also showed contrast enhancements, although not uniformly throughout one group. A maximum likelihood fit of the logistic formula P(D) = 1/[1 + (D50/D)k] to the incidence of late effects for the 5.9 mm collimator at 19 months after irradiation results in the parameters D50 = 37.4(-5.2,+6.1) Gy and k = 4.7 +/- 2.4. CONCLUSIONS: An animal model was established to study late normal brain tissue response. The observed late effects appeared very similar to the estimation of the integrated logistic formula for human brain. Based on these radiosurgery techniques, future experiments will focus on modifications in the irradiation modalities, i.e., irregular volumes, radiation quality or fractionation.     

Experimental arterial lesions after narrow-beam gamma irradiation used in stereotactic radiosurgery

An experimental study on rats showing the arterial lesions caused by the beams of gamma irradiation used in stereotactic radiosurgery is presented. The common carotid artery of the rat was irradiated with a single narrow-beam of gamma radiation with a maximum dose of 30 and 60 Gy. The results were evaluated at 4, 12 and 24 weeks after irradiation. Several focal changes in the arterial walls were observed in all periods of study with a predominance of hyperplastic (proliferative) alterations at doses of 30 Gy, and hyaline degeneration at doses of 60 Gy. Total occlusion was not observed in any case. In conclusion, vascular hyperplasia is more prominent with the lower doses evaluated.     

Stereotactic radiosurgery for tentorial meningiomas

Radical microsurgical resection is the procedure of choice for tentorial meningiomas. Despite advances in microsurgery, tentorial meningiomas continue to challenge surgeons and patients. To evaluate the response of tentorial meningiomas, we evaluated 41 patients who had Gamma knife stereotactic radiosurgery during a 9 year period. Patient age varied from 32 to 79 years. Headache, trigeminal neuralgia, or facial paraesthesia were the most common presenting symptoms. Sensory deficits in the distribution of the trigeminal nerve were the most common finding. Eighteen patients (44%) had undergone between 1 and 5 (mean, 1.9) resections prior to radiosurgery; 23 had tumors diagnosed by neuroimaging. The average tumor diameter in this series was 20 mm. The maximum tumor dose varied from 24 to 40 Gy (mean, 30.5 Gy), and the tumor margin dose varied from 12 to 20 Gy (mean, 15.3 Gy). During the average follow-up interval of 3 years (range, 1-8 years), 19 patients had clinical improvement, 20 remained stable, and 2 patients deteriorated. Follow-up imaging showed a reduction in tumor size in 18 patients, no further tumor growth in 22, and an increase in tumor size in one (overall tumor control rate of 98%). Stereotactic radiosurgery using the Gamma Knife was a safe and effective primary or adjuvant treatment for patients with tentorial meningiomas.     

Analyses of neuro-otological complications after radiosurgery for acoustic neurinomas

PURPOSE: To find out the optimum treatment parameters and the proper indications for treatment of acoustic neurinomas, univariate and multivariate actuarial analyses of neuro-otological complications after stereotactic radiosurgery for acoustic neurinomas were performed. METHODS AND MATERIALS: The subjects were 46 patients with acoustic neurinomas who underwent unilateral radiosurgery between June 1990 and June 1994 and were followed up at the University of Tokyo. Age ranged from 13 to 77 years (median, 54 years). Tumor diameter ranged from 0 to 25 mm (mean, 12 mm) at the cerebellopontine angle and from 2 to 15 mm (mean, 8.3 mm) in the internal auditory meatus. Maximum tumor doses ranged from 20 to 40 Gy (mean, 31.4 Gy), and peripheral doses from 12 to 25 Gy (mean, 16.8 Gy). One to eight isocenters were used (mean, 3.2). Median follow-up was 39 months. Eight events concerning neuro-otological complications were chosen, and the potential risk factors for them were analyzed by the actuarial analyses (univariate and multivariate). The events examined include hearing loss, vestibular function loss, facial palsy, and trigeminal nerve dysfunction. In order to point out potential risk factors for neuro-otological complications, univariate analyses were performed using both the Wilcoxon test and the log rank test, and multivariate analyses were performed with the Cox proportional hazards model. Variables nominated as potential risk factors were 1) demographic variables such as patient age and sex, 2) tumor dimensions, 3) treatment variables such as tumor doses and number of isocenters, and 4) pretreatment hearing levels. A variable with significant p-values (p < 0.05) in two or more of the three actuarial analyses (two univariate and one multivariate) was considered a possible risk factor. RESULTS: The possible variables that increase the risk for each event analyzed were: neurofibromatosis type II (NF2) and the number of isocenters for total hearing loss; experience of prior operation, the tumor diameter in the internal auditory meatus, and NF2 for hearing threshold elevation; peripheral tumor dose for vestibular function loss; patient age or midporus transverse tumor diameter (the two variables were correlated), and the number of isocenters for facial palsy; and the number of isocenters for trigeminal neuropathy. CONCLUSION: NF2 and the tumor diameter were the common risk factors for hearing loss in previous studies and ours. For the 5th/7th nerve dysfunction, the tumor diameter was the common risk factor. The risk of using more isocenters remains controversial. The difference in risk factors for hearing impairment and vestibular function loss suggests different mechanisms for the two. Further studies with larger populations and longer follow-up periods are required in order to draw conclusions on the risk factors in radiosurgery.     

Prognostic factors for hearing preservation following observation-only versus stereotactic irradiation (STI) in patients with acoustic neuroma

Prognostic factors for hearing preservation following observation-only versus STI were compared in patients with acoustic neuroma. There were 48 patients with acoustic neuroma treated by fractionated STI. Between 1991 and 1997, patients were given 36 Gy in 20 fractions over 5 weeks (36 Gy/20 Fr/5 wks) to 44 Gy/22 Fr/6 wks followed by a 4 Gy boost. A linear accelerator was used for irradiation. Twenty-three patients with acoustic neuroma who were followed without any treatment (observation-only group) were selected as matched controls. The largest hearing loss was observed at the frequency of 2 kHz in the observation-only group and at 1-2 kHz in the STI group. Hearing loss at 1 kHz was more frequent in the STI group (p < 0.01). There were no significant prognostic factors which predicted hearing preservation in the observation-only group. Stereotactic irradiation has been suggested to damage to cochlear nerve function in patients whose cochlear nerve had been impaired already. Tumor control rate of STI appeared to be as good as single fraction radiosurgery rates in the literature and better than in the observation-only group. Hearing preservation rate in the STI group was as good as in the observation-only group and appeared to be better than single fraction radiosurgery. In conclusion, because there were no factors predictive of hearing preservation in the observation-only group, it is difficult to select patients for observation only. Fractionated STI is potentially the treatment of choice, resulting in the same hearing preservation rate as achieved with observation only, although longer follow-up periods are needed.     

Thyroxine affects physiological and morphological development of the ear

The onset and development of distortion product otoacoustic emissions (DPE) representing cochlear amplifier activity were studied in neonatal hyperthyroid (n = 10) and control (n = 10) rat pups. These were compared to the onset and development of auditory nerve-brainstem evoked responses (ABR) representing overall cochlear function, and to morphological development of the ear. DPEs were recorded at an earlier postnatal age to high (8 kHz) frequencies and progressed to lower (3 kHz) frequencies with age. ABRs to high-intensity clicks were recorded at least 2 days before DPEs, although DPE onset at 8 kHz preceded adult-like ABR thresholds. Both ABR and DPEs appeared earlier in the hyperthyroid rats. Histological evidence showed earlier morphological development of the ear in these animals. ABR thresholds and DPE amplitudes matured at a slower rate in the experimental group despite their earlier onset. There was no difference in ABR and DPE thresholds between adult hyperthyroid and control rats. However, in the experimental group, DPEs had smaller amplitudes to high (70 dB SPL) and to low (50 dB SPL) stimulus intensities at low frequencies. Hence, despite thyroxine-injected rat pups having earlier onset of auditory structure and function (lower ABR thresholds and earlier functioning active cochlear amplifier), it appeared that neonatal hyperthyroidism affected the later state of the cochlea, such that DPEs, especially to low-frequency stimuli, were depressed during and after maturation.     

Analysis of evoked otoacoustic emissions in patients with high frequency cochlear hearing loss

Evoked otoacoustic emissions (EOAE) and auditory brainstem evoked response (ABR) were tested in patients with high frequency cochlear hearing loss and subjects with normal hearing. Results showed that: 1. In the normal group, the main component of EOAE varied between 0.5-5kHz. 2. There was a close correlation between the pure-tone audiogram figure and EOAE spectrum. 3. The I/O function curves of EOAE displayed non-linear characteristics and a trend of saturation with higher stimulus levels in the normal group. In the patient group the growth rates of EOAE and ABR were remarkably higher than those in the normal group with high stimulus levels. It suggests that the abnormal EOAE are probably relevant to the recruitment phenomenon in cochlear hearing loss.     

Sudden deafness and facial palsy from metastatic bronchogenic carcinoma

A 52-year-old man developed sudden total bilateral deafness, and unilateral facial palsy, without other symptoms and findings. He died two months later of of bronchogenic carcinoma metastatic to dura, brainstem, pons, carebellopontine angle, cerebellum and cranial nerves III, VI, VII and VIII. There was bilateral internal auditory canal erosion. Tumour replaced right facial, acoustic and vestibular nerves. Tumour infiltrated spiral ganglion, cochlear nerve, cochlear aqueduct, and destroyed nearly all facial nerve fibres to the level of the stapedius muscle. No tumour cells were found on the left, but few fibres of facial, acoustic and vestibular nerves survived. Both ears showed some cochlear outer hair cell destruction. Metastatic tumour to temporal bone or dura should be considered when loss of peripheral VIIth or VIIIth nerve function occurs.     

Effects of gamma knife radiosurgery for brain tumors: clinical evaluation

BACKGROUND: Gamma knife radiosurgery is a safe and effective alternative to microsurgery in the management of selected intracranial lesions. In our initial three-year experience with gamma knife radiosurgery, 431 patients were treated using this method. This report presents the treatment results for three different types of brain tumors: benign meningiomas, malignant metastases and gliomas. METHODS: A retrospective study was performed to analyze a consecutive series of 71 meningiomas, 31 metastatic tumors and 21 gliomas treated by gamma knife radiosurgery between March 1993 and May 1996. The treatment results were investigated using regular magnetic resonance examinations and tumor volume measurement at six-month intervals to observe sequential changes of the tumors. Patients with meningiomas were further divided into three groups according to the peripheral radiation doses: high-dose (20-17 Gy, n = 18), medium-dose (16-15 Gy, n = 33) and low-dose (14-12 Gy, n = 20). The Generalized Estimation Equation was applied to compare treatment results in these three groups with different doses and tumor volumes. RESULTS: Volume measurements of the 71 meningiomas showed that 76% decreased in size, 16% stabilized and 8% increased in size. The volumes increased most frequently in the early stage (6-12 months) after treatment and subsequently regressed after the twelfth month. The tumor control rate for meningiomas in our three-year follow-up was over 90%. For meningiomas, the statistical analysis showed that both the radiation dose and tumor volume were significantly related to the development of adverse radiation effects (p < 0.05). In metastatic tumors, rapid tumor regression after radiosurgery was found in 87% of the patients. In gliomas, radiosurgery effectively inhibited tumor growth in selected patients with small, circumscribed, less infiltrative tumors. Ependymomas and low-grade astrocytomas had more favorable outcomes than other gliomas. CONCLUSIONS: Gamma knife radiosurgery is effective for controlling tumor growth in benign meningiomas for up to three years after surgery. In selected cases of malignant metastasis and gliomas, most patients appeared to benefit from the treatment with symptomatic improvement and prolonged survival. Treatment strategy and dose selection in radiosurgery should be adjusted to optimize tumor control and avoid adverse radiation effects.     

Significant reduction in circuit pressure with modified plasma separation chamber for a heparin removal device

OBJECTIVE: This study aimed to determine the maximum dose of radiation the CLARION 1.2 cochlear implant can withstand safely. INTRODUCTION: Cochlear implants restore functional hearing to patients with sensorineural deafness. Because some patients may need radiation therapy, it is important to investigate the influence of ionizing radiation on cochlear implant function. METHODS: This study tested the function of four CLARION 1.2 implants (Advanced Bionics, Sylmar, CA, U.S.A.) after varying radiation treatments with gamma rays. The first implant received a cumulative dosage of 69 Gy over nine treatments (single doses between 0.1-30 Gy). The second was irradiated with a total of 90 Gy, receiving three treatments of 30 Gy each. The third and fourth received doses more typical of patient therapy (i.e., 2 Gy) approximately 30 times, for a cumulative dosage of approximately 60 Gy. Implant function was tested after every treatment; the CLARION implant incorporates a back-telemetry system, allowing impedance and current output testing. RESULTS: Despite the type of treatment, the results were quite consistent: difficulties in function occurred when the cumulative dosage inside the implant was approximately 60 Gy. The first implant recovered completely and the second recovered partially. DISCUSSION: The CLARION 1.2 cochlear implant seems to safely withstand approximately 60 Gy of radiation before experiencing functional difficulties. In a clinical situation, the implant would not likely be in the target volume irradiated, and thus the patient's therapeutic cumulative dosage might be higher.     

Radiosurgery of cavernous sinus meningiomas with gamma-knife

The treatment results of cavernous sinus meningioma with gamma-radiosurgery are reported. There were 18 cases of cavernous sinus meningioma, including 2 males and 16 females, whose age ranged from 39 to 63 with an average of 51.0 years. As prior treatments, operative tumor resection or biopsy had been carried out in 14 cases, and the pathology was verified. The other 4 cases were diagnosed clinically with radiological studies. The mean tumor diameter was 28.3mm (17.7-35.0) during the radiosurgery. The maximum dose ranged from 22 to 36Gy (mean 28.0Gy), with the marginal tumor dose ranging from 11 to 18Gy (mean 13.9Gy). Irradiation to the near-by optic nerves was less than 10Gy. Follow-up period ranged from 12 to 50 months with a mean of 25.5 months. MRI showed a minor tumor shrinkage in 9 (50.0%) and no obvious change in 8 (44.4%), and tumor progression in 1 (5.6%), which required a 2nd radiosurgery. Neurologically facial pain and facial dysesthesia were well improved (7/13). However the ophthalmoparesis was usually unchanged and only 1 out of 11 (9.1%) improved after radiosurgery. Deterioration of neurological signs was rare. Symptomatic edema presenting neurological signs was not seen. In conclusion, radiosurgery with a gamma-knife is one of the useful alternatives to operative intervention in the treatment of cavernous sinus meningiomas, not only for tumor control, but also for relief from the symptoms.     

CD1 hearing-impaired mice. I: Distortion product otoacoustic emission levels, cochlear function and morphology

The levels of distortion product otoacoustic emissions (DPOAEs) were measured in a strain of hearing-impaired mutant mice (CD1) at various stages of outer hair cell impairment and compared to those of a control inbred strain (CBA/J). Parallel measurements of cochlear potentials and auditory brainstem evoked responses (ABRs) were performed and surface preparations of organs of Corti were observed using phalloidin staining of filamentous actin. Comparison of DPOAEs (elicited by stimulus levels of 60 and 70 dB SPL) with standard functional tests allowed the categorization of CD1 ears into two groups on the basis of the presence or absence of DPOAE, which corresponded to mean ABR thresholds greater or less than 40 dB nHL respectively. When adopting ABR threshold as the gold standard, this procedure yielded rates of false-positives and -negatives ranging from 5 to 16%. However, individual predictions of electrophysiological function from DPOAE levels were not accurate, owing to their large variance, and attempts to optimize stimulus levels did not reduce this variance. In contrast, the profiles of DPOAE level vs. f2 exhibited large correlations with ABR threshold profiles as a function of f2. It was also noteworthy that the mean levels of DPOAEs in CD1 mice recorded in frequency intervals with normal ABR thresholds were significantly smaller than those of CBA/J mice. Although hearing loss was revealed early both by DPOAEs and by other functional tests, surface preparations often remained normal until about 3-4 months of age.     

The effects of cochlear hearing loss, age and sex on the auditory brainstem response

This study analyzed the effects of auditory impairment, age and sex on the auditory brainstem response (ABR) wave latencies. ABR wave I, wave V and I-V interval measures were extracted from the clinical records of 201 patients with cochlear hearing loss. Females had consistently earlier wave V latencies and shorter I-V intervals than males. No age effects were observed. Degree of impairment had a systematic effect on ABR wave latencies and I-V intervals. Wave I displayed latency extension with increasing levels of high-frequency hearing loss, whilst for wave V increases in latency were dependent upon both degree and slope of the hearing loss. Present results suggest that many of the previously reported sex differences and variable interactions seen for the ABR can be accounted for by differences in the underlying distribution of audiogram shapes within and between study populations. Different audiometric configurations were found to produce consistent differential effects on both wave I and wave V latency and thus influence the I-V interval. This study underlines the need to develop a more detailed model of impairment effects if correction factors are to be employed more effectively in ABR testing for retrocochlear pathology.     

Radiation optic neuropathy after stereotactic radiosurgery

PURPOSE: The purpose of the study is to report the occurrence of optic neuropathy after stereotactic radiosurgery for perichiasmal tumors. METHODS: Records of four patients with visual deterioration after stereotactic radiosurgery were reviewed, including clinical findings, neuroimaging results, and treatment methods. RESULTS: Optic neuropathy developed 7 to 30 months after gamma knife radiosurgery. All patients experienced an abrupt change in visual function. Clinical findings indicated anterior visual pathway involvement. Patterns of field loss included nerve fiber bundle and homonymous hemianopic defects. Gadolinium-enhanced magnetic resonance imaging (MRI) showed swelling and enhancement of the affected portion of the visual apparatus in three patients. Systemic corticosteroids were administered in all patients and one partially recovered. One patient also received hyperbaric oxygen without improvement. CONCLUSIONS: Although rare, optic neuropathy may follow radiosurgery to lesions near the visual pathways. Careful dose planning guided by MRI with restriction of the maximal dose to the visual pathways to less than 8 Gy will likely reduce the incidence of this complication.     

Response of the primary auditory cortex to electrical stimulation of the auditory nerve in the congenitally deaf white cat

Neural activity plays an important role in the development and maintenance of sensory pathways. However, while there is considerable experience using cochlear implants in both congenitally deaf adults and children, little is known of the effects of a hearing loss on the development of the auditory cortex. In the present study, cortical evoked potentials, field potentials, and multi- and single-unit activity evoked by electrical stimulation of the auditory nerve were used to study the functional organisation of the auditory cortex in the adult congenitally deaf white cat. The absence of click-evoked auditory brainstem responses during the first weeks of life demonstrated that these animals had no auditory experience. Under barbiturate anaesthesia, cortical potentials could be recorded from the contralateral auditory cortex in response to bipolar electrical stimulation of the cochlea in spite of total auditory deprivation. Threshold, morphology and latency of the evoked potentials varied with the location of the recording electrode, with response latency varying from 10 to 20 ms. There was evidence of threshold shifts with site of the cochlear stimulation in accordance with the known cochleotopic organisation of AI. Thresholds also varied with the configuration of the stimulating electrodes in accordance with changes previously observed in normal hearing animals. Single-unit recordings exhibited properties similar to the evoked potentials. Increasing stimulus intensity resulted in an increase in spike rate and a decrease in latency to a minimum of approximately 8 ms, consistent with latencies recorded in AI of previously normal animals (Raggio and Schreiner, 1994). Single-unit thresholds also varied with the configuration of the stimulating electrodes. Strongly driven responses were followed by a suppression of spontaneous activity. Even at saturation intensities the degree of synchronisation was less than observed when recording from auditory brainstem nuclei. Taken together, in these auditory deprived animals basic response properties of the auditory cortex of the congenitally deaf white cat appear similar to those reported in normal hearing animals in response to electrical stimulation of the auditory nerve. In addition, it seems that the auditory cortex retains at least some rudimentary level of cochleotopic organisation.     

Pharmacokinetics of the PDGF-antagonist trapidil in patients with and without renal impairment

BACKGROUND: From July 1989 to February 1996, 130 patients underwent stereotactic radiosurgery. We report the results of the first 50 patients eligible for a minimum of three years of follow-up. METHODS: Twenty women and 30 men, (mean age: 37.5 years) were treated by dynamic rotation on a 6 MV linear accelerator. Prior treatment was embolization in seventeen, surgery in three and embolization and surgery in six. All had DSA and enhanced CT scanning, while some had MRI. Forty-seven treatments used a single isodose. Restricting eloquent normal tissue to 15 Gy, margin doses (at 50-90% isodose) were 12 Gy (one patient); 15 Gy (sixteen patients); 20 Gy (31 patients); 25 Gy (two patients). Maximum diameters were: < 1.5 cm (12 patients); < 2.0 cm (nine patients); < 2.5 cm (twelve patients); < 3.0 cm (thirteen patients; 3.0 cm (four patients). RESULTS: Forty-five patients were evaluable at three years, with thirty-nine having angiography. Twenty-five had angiographically confirmed obliterations; two had parenchymal AVMs obliterated but with residual dural components; four had MRI evidence of obliteration (refused angiography). One patient acutely had a seizure; one patient (with hemorrhages, resection, and embolizations preceding two applications of radiosurgery, separated by 3.5 years) had worsening of memory. CONCLUSIONS: Our uncorrected (five patients unevaluable at three years) and corrected angiographically confirmed obliteration rates are 54% and 60% respectively. Our follow-up (98% accounting of cohort; 78% angiographic rate) and explicit derivation of denominators help delineate the efficacy of radiosurgery at these doses.     

Otoacoustic emissions in full-term newborns at risk for hearing loss

Distortion product otoacoustic emissions (DPOEs) and click-evoked otoacoustic emissions (CEOEs) characteristics of the normal newborn population have been previously reported in the literature. There is little information about DPOE evaluations in the newborn population at risk for hearing loss. The authors now report the DPOE and/or CEOE data from six full-term subjects at risk for hearing loss or with highly suspected hearing loss. These subjects were less than 1 year of age and at risk for hearing loss secondary to a history of hereditary hearing loss, meningitis, hyperbilirubinemia, and ototoxic drug exposure. Audiometric evaluation included auditory brainstem responses (ABR), behavioral observation audiometry, and tympanometry. The CEOEs and DPOEs were found to be decreased or absent in the subjects with suspected hearing loss secondary to cochlear pathology; they were found to be normal in a subject with a suspected central hearing loss. This study's data suggest that otoacoustic emissions when combined with ABR can provide a frequency-specific evaluation of cochlear function and help determine the anatomic site of a pathologic lesion.     

Endoscope-assisted vestibular neurectomy

OBJECTIVE/HYPOTHESIS: In some instances endoscopes offer better visualization than the microscope and frequently allow less invasive surgery. This study was undertaken to determine whether endoscopy is safe and effective during neurectomy of the vestibular nerve. METHOD: Ten patients with intractable unilateral Meniere's disease underwent a retrosigmoid craniotomy for neurectomy of the vestibular nerve. Endoscopy with a Hopkins telescope was used during each procedure to study posterior fossa anatomic relationships and to assist the neurectomy. Preoperative and postoperative audiometric evaluation was performed in all patients undergoing vestibular neurectomy. Nine of these patients had preoperative electronystagmography, and four patients completed postoperative electronystagmography. The 1995 American Academy of Otolaryngology-Head and Neck Surgery's Committee on Hearing and Equilibrium guidelines for the diagnosis and evaluation of therapy in Meniere's disease were used. RESULTS: Complete neurectomy was achieved in all 10 patients. Endoscopy allowed improved identification of the nervus intermedius and the facial, cochlear, and vestibular nerves and adjacent neurovascular relationships without the need for significant retraction of the cerebellum or brainstem. In addition, endoscopic identification of the cleavage plane between the cochlear and vestibular nerves medial to or within the internal auditory canal (n = 3) was not made with the 0-degree endoscope; however, identification was made with the 30- or 70-degree endoscope in all cases. In all patients with Meniere's disease, elimination of the recurrent episodes of vertigo (n = 10) or otolithic crisis of Tumarkin (n = 1) was achieved. CONCLUSIONS: Posterior fossa endoscopy can be performed safely. Endoscope-assisted neurectomy of the vestibular nerve may offer some advantages over standard microsurgery including increased visualization, more complete neurectomy, minimal cerebellar retraction, and a lowered risk of cerebrospinal fluid leakage.     

Electrical stimulation of the auditory nerve. I. Correlation of physiological responses with cochlear status

The purpose of the present study was to evaluate evoked potential and single fibre responses to biphasic current pulses in animals with varying degrees of cochlear pathology, and to correlate any differences in the physiological response with status of the auditory nerve. Six cats, whose cochleae ranged from normal to a severe neural loss (< 5% spiral ganglion survival), were used. Morphology of the electrically evoked auditory brainstem response (EABR) was similar across all animals, although electrophonic responses were only observed from the normal animal. In animals with extensive neural pathology, EABR thresholds were elevated and response amplitudes throughout the dynamic range were moderately reduced. Analysis of single VIIIth nerve fibre responses were based on 207 neurons. Spontaneous discharge rates among fibres depended on hearing status, with the majority of fibres recorded from deafened animals exhibiting little or no spontaneous activity. Electrical stimulation produced a monotonic increase in discharge rate, and a systematic reduction in response latency and temporal jitter as a function of stimulus intensity for all fibres examined. Short-duration current pulses elicited a highly synchronous response (latency < 0.7 ms), with a less well synchronized response sometimes present (0.7-1.1 ms). There were, however, a number of significant differences between responses from normal and deafened cochleae. Electrophonic activity was only present in recordings from the normal animal, while mean threshold, dynamic range and latency of the direct electrical response varied with cochlear pathology. Differences in the ability of fibres to follow high stimulation rates were also observed; while neurons from the normal cochlea were capable of 100% entrainment at high rates (600-800 pulses per second (pps)), fibres recorded from deafened animals were often not capable of such entrainment at rates above 400 pps. Finally, a number of fibres in deafened animals showed evidence of 'bursting', in which responses rapidly alternated between high entrainment and periods of complete inactivity. This bursting pattern was presumably associated with degenerating auditory nerve fibres, since it was not recorded from the normal animal. The present study has shown that the pathological response of the cochlea following a sensorineural hearing loss can lead to a number of significant changes in the patterns of neural activity evoked via electrical stimulation. Knowledge of the extent of these changes have important implications for the clinical application of cochlear implants.