Prominent expression of Narp in central vestibular pathways: selective effect of labyrinth ablation.

Recent in vitro studies demonstrated that Narp, a secreted immediate early gene (IEG) product, induces AMPA receptor clustering. Accordingly, Narp has been implicated in mediating activity-dependent changes in synaptic efficacy. To help define the role of Narp in vivo, we conducted immunohistochemical studies of Narp in rat brain. Unexpectedly, we found robust Narp expression in several discrete areas linked to the vestibular system: the anterodorsal nucleus (ADN) of the thalamus, which relays head orientation information to the cortex, the lateral vestibulospinal (Deiters') nucleus and Purkinje cells in the flocculonodular lobe of the cerebellum. Although strong Narp expression in Deiters' nucleus and the cerebellum was present consistently, Narp expression in the ADN displayed a high degree of variability among animals. To check if this variability in ADN Narp expression reflects its dependence on fluctuating levels of vestibular input, we monitored Narp immunostaining following bilateral labyrinth ablation. This procedure significantly suppressed Narp immunostaining in the ADN, indicating that it is stimulated by naturally occurring vestibular input. In contrast, labyrinth ablation did not affect Narp staining in Deiters' nucleus or the flocculonodular lobe of the cerebellum, presumably because these areas are driven by inputs from multiple systems. As previous studies implicate Narp in synaptic plasticity, these findings suggest that this IEG may mediate ongoing adjustments in synaptic strength or connectivity in several pathways linked to the vestibular system.

Superior canal dehiscence syndrome.

OBJECTIVE: To present the symptoms, signs, and findings on diagnostic tests of patients with the superior canal dehiscence syndrome and to describe the surgical procedures used to treat the dehiscence in five patients. DESIGN AND SETTING: Prospective study of a series of patients identified as having this syndrome at a tertiary care referral center. PATIENTS AND RESULTS: Seventeen patients with vertigo, oscillopsia, or both evoked by intense sounds or stimuli that caused changes in middle ear and/or intracranial pressure were identified over a 4-year period. The evoked eye movements had vertical and torsional components, with the direction corresponding to the effect of the stimuli in causing excitation (Valsalva against pinched nostrils, tragal compression, sounds) or inhibition (Valsalva against a closed glottis or jugular venous compression) of the affected superior semicircular canal. Thirteen (76%) of these patients also experienced chronic dysequilibrium that was often the most debilitating symptom. Dehiscence of bone overlying the superior semicircular canal on the affected side was confirmed with computed tomographic scans in each case. Surgical procedures through the middle fossa approach to plug or resurface the superior canal were performed in five patients (canal plugging in three cases and resurfacing of the dehiscence without plugging in two). The debilitating symptoms resolved or improved after the procedures. Signs of vestibular hypofunction, without loss of hearing, were noted in one patient after plugging of the superior canal and in one other patient after resurfacing of the canal. CONCLUSIONS: The superior canal dehiscence syndrome is identified based on characteristic symptoms, signs, and computed tomographic findings. The clinical presentation and findings can be understood in terms of the effect of the dehiscence on the physiology of the labyrinth. The syndrome is a treatable cause of vestibular disturbance.

A comparison of inter-aural attenuation with the Etymotic ER-3A insert earphone and the Telephonics TDH-39 supra-aural earphone.

One of the many reported advantages of the insert earphone over the supra-aural earphone is increased inter-aural attenuation (IA). Minimum values of IA determine the need for masking of the non-test ear in air-conduction audiometry. The aim of the present study was to measure inter-aural attenuation for the Etymotic Research ER-3A insert earphone (with deep and shallow insertion of the ear plug within the ear canal) and compare this with the supra-aural Telephonics TDH-39/MX41-AR earphone/cushion combination. Subjects were 18 adults ranging in age from 38 to 68 years (mean 50 years). Each subject had no hearing in one ear following translabyrinthine surgery for removal of an acoustic neuroma. The opposite ear had hearing thresholds better than 40 dB HL and an air-bone gap of less than 10 dB at any audiometric frequency. Pure tone air-conduction thresholds were obtained in the range 0.25-8 kHz. Deep insertion of the insert earphone was deemed to occur when the outside edge of the ear plug was flush with the entrance of the ear canal. Shallow insertion was deemed to occur when half of the ear plug (6 mm) was inside the entrance of the ear canal. IA was defined operationally as the difference between the good-ear and poor-ear not-masked air conduction threshold for a given audiometric frequency and earphone. The results show that the TDH-39/MX41-AR combination provides a median IA of approximately 60 dB with a lower limit of approximately 45 dB. Greater IA was obtained with the ER-3A insert earphone but this depended on the depth of insertion. With a deep insertion, the 1A values were some 15-20 dB greater than with the supra-aural earphone. Although frequency-specific IA values are provided, a simple rule of thumb is to apply masking to the non-test ear when the pure tone airconduction signal from the ER-3A insert earphone exceeds the bone conduction threshold of the non-test ear by 55 dB HL or more. If it is not possible to obtain a deep insertion depth this value should be reduced by 5 dB.

Evidence that the ginkgo biloba extract, EGb 761, neither accelerates nor enhances the rapid compensation of the static symptoms of unilateral vestibular deafferentation in guinea pig.

The concentrated Ginkgo biloba extract, EGb 761, has previously been reported to enhance and accelerate vestibular compensation following unilateral vestibular deafferentation (UVD), in particular, compensation of the dynamic postural symptoms such as locomotor dysequilibrium. However, many of these studies have not included a complete analysis of the static symptoms of UVD, such as spontaneous nystagmus (SN), yaw head tilt (YHT), and roll head tilt (RHT), nor have they included a dose-response analysis or vehicle controls for EGb 761. The aim of the present study was to examine the effects of the EGb 761 extract on static vestibular compensation in guinea pig, using a dose-response analysis and both vehicle and saline controls. Analysis of variance showed that there was a significant decrease in SN frequency (P < 0.05) and a significant change in the rate of SN compensation (P < 0.05), using 3 i.p. injections of EGb 761 (25, 50, or 100 mg/kg), or vehicle, or saline, at 0, 25, and 40 h post-UVD. However, post-hoc testing revealed that this was due entirely to significant differences between the saline and vehicle groups at 35, 40, and 50 h post-UVD (P < 0.05 in all cases) and between the saline and the 100 mg/kg and 25 mg/kg EGb 761 groups at 35 and 50 h post-UVD, respectively (P < 0.05 for both comparisons); there were no significant differences between the vehicle and drug groups at any time. YHT and RHT were not significantly different between the drug, saline, and vehicle groups. In a second set of experiments, the 50 and 100 mg/kg EGb 761 i.p. injection frequencies were doubled. However, once again, neither SN nor YHT were significantly different between the EGb 761 groups and the vehicle controls. These results suggest that 1) EGb 761 does not significantly enhance or accelerate compensation of the static symptoms of UVD in guinea pig and 2) the EGb 761 vehicle may exert some effects on its own. Therefore, EGb 761 may be of limited use in the treatment of acute vestibular dysfunction in humans.

Evaluation of surgical approaches to endoscopic auditory brainstem implantation.

OBJECTIVES: This study investigates the use of endoscopy for the placement of an auditory brainstem implant by translabyrinthine, retrosigmoid (suboccipital), and middle cranial fossa approaches. STUDY DESIGN: Cadaver dissection and endoscope-assisted placement of the auditory brainstem implant. METHODS: Translabyrinthine, retrosigmoid, and middle cranial fossa dissections were performed bilaterally in five cadaveric heads. An auditory brainstem implant was placed within the lateral recess of the fourth ventricle under endoscopic visualization. The implantation was performed with all approaches and documented by digital image capture followed by production of dye-sublimation photographic prints. RESULTS: The lateral recess was visualized with the endoscope in all three approaches to the brainstem. The 30 degrees endoscope provided the best visualization by translabyrinthine and retrosigmoid dissection and was essential for the middle cranial fossa approach. Refinement of implant position was readily achieved, as even the deepest portion of the recess could be seen with all three approaches. CONCLUSIONS: This study finds that endoscopy provides superior visualization of the lateral recess of the fourth ventricle than the operating microscope with all approaches. The retrosigmoid approach is recommended, as it provides the best view of the implantation site and the easiest angle for placement of the prosthesis. The use of the endoscope may allow for a smaller craniotomy than with conventional microscopic techniques, depending on tumor size. The translabyrinthine approach provides a good view of the lateral recess but had no advantage over other approaches. The middle cranial fossa approach is only possible with angled endoscopes; however, it is technically the most difficult and places the facial nerve at greatest risk.

State of the art on the development of the implantable hearing device for partial hearing loss.

This article summarizes the state of the art of the implantable hearing devices for partial hearing loss. Devices such as temporal bone stimulators and piezoelectric and electromagnetic middle and inner ear transducers are discussed. Cochlear implants are excluded. (See "Update on Cochlear Implantation" in this issue.) Those readers interested in a more in-depth study of implantable hearing devices should consult the February 1995 issue of The Otolaryngologic Clinics of North America which is dedicated to this matter (Guest Editor: Anthony J. Maniglia, MD).

Carbon dioxide laser occlusion of the guinea pig posterior semicircular canal.

Preservation of hearing is possible with selective ablation of the vestibular system and mechanical occlusion of the semicircular canals. Complete ablation of all three canals would improve exposure of the internal auditory canal fundus (e.g., for acoustic tumor exposure), but mechanical packing of the vestibule would disrupt normal sound transduction. This study was designed to assess the feasibility of preserving hearing with CO2 laser occlusion, without mechanical packing of the posterior semicircular canal membranous labyrinth. Twenty adult Hartley guinea pigs underwent occlusion of the right posterior semicircular canal with one of three techniques: mechanical packing, laser coagulation, or laser coagulation with mechanical packing. Electrocochleographic thresholds to clicks and 1-kHz and 8-kHz tone bursts did not change significantly 6 weeks after posterior semicircular canal occlusion with any of these techniques. Histopathologic examination revealed complete canal occlusion with all methods. These findings suggest that mechanical occlusion and CO2 laser occlusion of the posterior semicircular canal do not significantly affect cochlear function in the guinea pig. CO2 laser occlusion of the membranous labyrinth may prove useful for more extensive selective vestibular ablation by obviating the need for mechanical packing of the labyrinth.

Intraoperative electrocochleographic monitoring of inner ear surgery for endolymphatic hydrops. A review of cases.

The purpose of this review of cases was to determine whether or not the SP/AP amplitude ratio changes during unidirectional inner ear valved shunt surgery for the decompression of the hydropic labyrinth in Meniere's disease. A series of 62 patients underwent shunt surgery over a 20 month period. In 43 cases (43 ears), ECoG responses were successfully recorded throughout the duration of surgery. A pair of two-tailed paired t-tests were computed for SP/AP amplitude ratios acquired during intraoperative ECoG monitoring at the baseline and closing stage of surgery. The first t-test compared the mean SP/AP amplitude ratios at baseline and closing for those cases in which the SP/AP amplitude ratio at baseline was abnormal (baseline > 35%). The t was significant, t(19) = 4.63, p < 0.01). The second t-test compared the mean SP/AP amplitude ratios at baseline and closing for those cases in which the SP/AP amplitude ratio at baseline was WNL (baseline < or = 35%). The t was insignificant, t(20) = 0.31, p > 0.05. Variations between baseline and closing measurements were categorized as either a reduction, no change (very stable over time), or an increase in the SP/AP amplitude ratio. A change in the SP/AP amplitude ratio from baseline to closing of > or = 7% (x = 2%, SD = 2%) was considered statistically significant. Overall (43 cases), 49% of the variations fell into the no change category, with 42% and 9% of the cases placed in the reduction and increase categories, respectively. Of the cases (22) in which the baseline measurement was outside the limits of normal, 64% showed a reduction, 32% showed no change, and 4% showed an increase in the SP/AP amplitude ratio at closing. Changes in the SP/AP amplitude were observed at various stages of the surgical procedure. Reductions were found at each of the five surgical steps identified, with the majority of the changes almost evenly divided between the stages of mastoid drilling and opening sac. Increases in the SP/AP amplitude ratio were found in four cases. The increases were observed during drilling of the mastoid bone in two cases and sac decompression and opening the sac in the remaining cases.

Promontory stimulation following labyrinthectomy: implications for cochlear implantation.

Transtympanic electric promontory stimulation is a psychoacoustic test used to assess residual acoustic neurons in profound sensorineural hearing loss. The test was performed in six patients who had previously undergone transmastoid (N = 5) or transcanal-oval window (N = 1) labyrinthectomy as a means of determining the feasibility of future cochlear implantation. Four patients had unilateral Meniere's disease, one had labyrinthitis, and one had delayed onset vertigo. All patients perceived a definite auditory sensation in the labyrinthectomized ear during stimulation. The results of threshold, dynamic range, and difference limen testing were similar to those obtained during preoperative stimulation of cochlear implant candidates (N = 12) who subsequently became successful users. There was no evidence of response degradations as the time following labyrinthectomy increased. The results of this study suggest the possibility of successful cochlear implantation following labyrinthectomy. Supporting histologic data are reviewed.

An intrasubject comparison of electric and acoustic middle latency responses.

The middle components of the auditory evoked response (middle latency response, MLR) were evoked by acoustic clicks from the normal-hearing ear and by charge-balanced biphasic current pulses from the severe-to-profoundly hearing-impaired ears of patients undergoing labyrinthectomy for the management of intractable vertigo. A vertex-positive peak with a latency ranging from 27 msec to 38 msec (Pa) was characteristic of both the electric and the acoustic MLR. In subjects, the electric Pa always preceded the acoustic Pa in latency. In addition, the electric Pa had a sharper appearance than did the acoustic Pa. The electric MLRs were elicited by a range of stimulus intensities and persisted after the completion of labyrinthectomy.

Evidence for an alteration in brainstem cholinergic pathways following unilateral labyrinthectomy in the frog.

Following VIIIth nerve stimulation, field potentials recorded in the contralateral vestibular nuclei of isolated medullae are larger in amplitude and more sensitive to atropine in chronically hemilabyrinthectomized frogs than those of controls. The atropine-sensitive component occurs at a latency which precludes involvement of the monosynaptic commissural projection between second order vestibular neurons. Therefore, in addition to this commissural projection, the contributions of more indirect pathways have to be considered in an attempt to understand the neuronal basis of behavioral improvement following vestibular lesions.

Exercise and recovery from vestibular injury.

Exercise after vestibular injury is potentially an important modality in the recovery process. In this study, the effects of injury on the vestibular system were examined experimentally in cats. Data from healthy normal cats, labyrinthectomized cats, and labyrinthectomized cats treated with exercise were examined. Evaluation of performance was accomplished with caloric, optokinetic, and sinusoidal acceleration testing. Results in normal cats indicated stable, symmetrical vestibular responses on repetitive testing, without evidence for habituation. Comparison of the experimental groups showed that the animals treated with exercise had a significant difference in terms of recovery. The exercised group demonstrated a diminished directional preponderance, a shorter period of complete asymmetry, and a shorter recovery time, which was 58 to 70 per cent faster than that of the non-exercised group. The authors conclude that a general type of exercise will markedly affect and shorten the recovery pattern in the labyrinthectomized cat.