Postoperative vestibular-evoked myogenic potentials in cases with vestibular schwannomas

Although still the subject of discussion, vestibular-evoked myogenic potentials (VEMPs) have been considered to reflect the function of the saccular and, more recently, the cochlear tracts. To accurately determine the precise afferent pathway carrying VEMPs, we studied the outcomes of VEMPs and other examinations in patients with unilateral vestibular schwannomas. Eleven patients with unilateral vestibular schwannomas resected using a middle cranial fossa approach were included in the study. Patients underwent pure-tone threshold audiometry, caloric tests and analysis of auditory brainstem responses (ABRs) and VEMPs pre- and postoperatively. The results were compared with those obtained in patients with intact superior or inferior vestibular and cochlear nerves. Among the 11 patients studied, 4 retained their VEMPs postoperatively. Three of the 10 patients with inferior vestibular schwannomas exhibited normal VEMPs, preserved hearing levels (20 dB HL) and anatomically intact superior vestibular nerves. In all of these cases, ABRs more closely correlated with VEMPs than with caloric responses. In one of the cases with inferior vestibular schwannomas, VEMPs were preserved postoperatively and VEMP latencies were shortened, which indicates the preoperative presence of a conduction block in either the cochlear or superior vestibular nerve. VEMPs may be conducted in both the superior vestibular and cochlear nerves, as well as in the inferior vestibular nerve. Thus, evaluation of saccular nerve function should be performed carefully, especially in cases where hearing is preserved. It appears that cochlear conduction may proceed along two pathways, one direct and the other via the brainstem, but this remains to be verified.     

The otolithic organ as a receptor of vestibular hearing revealed by vestibular-evoked myogenic potentials in patients with inner ear anomalies

The human vestibule has preserved an ancestral sound sensitivity and it has been suggested that a reflex could originate from this property underlying cervical muscle micro-contractions secondary to strong acoustic stimulation. Previous studies have established that an early component of loud sound-evoked myogenic potentials from the sternocleidomastoid muscle originate in the vestibule. This is based on findings that the response can still be obtained from patients with complete loss of cochlear and vestibular (semi-circular canal) function. Our data confirm, in a more direct way, a saccular origin of this short-latency acoustic response and verifies that a saccular acoustic response persists in the human ear. The contribution of this response to the perception of loud sounds is discussed. It is concluded that vestibular response to sound might be used to assist in the rehabilitation of deafness.     

大鼠前庭内侧核神经元的内在膜特性及其对前庭模拟传入信号的反应

目的 观察大鼠前庭内侧核神经元的膜特性及其对前庭外周模拟传入信号的放电反应,探讨前庭系统生理功能的可能机制.方法 运用红外微分干涉相差技术,可视状态下对前庭内侧核神经元进行全细胞记录,按平均动作电位形状对神经元分型,比较不同类型神经元膜特性的差异.向胞内注入刺激电流以模拟头部作直线加速和匀速旋转运动时外周前庭的传入信号,观察神经元的放电反应.结果 在大鼠前庭内侧核神经元可记录到放电活动,在低钙高镁人工脑脊液中放电活动仍然存在.神经元可被分为有单相后超极化及A样整流的A型(33%),有双相后超极化的B型(63%),以及同时具有或不具有以上特征的其他型(4%);A、B型神经元的部分主动膜特性存在明显差异,对同样的模拟传入信号可作出不同的反应.结论 大鼠前庭内侧核神经元的放电基于其内在膜特性的自发活动;大部分神经元的放电表现为典型的A型或B型,但仍有少量非典型放电存在;A、B型神经元膜特性和放电反应特性的差异是它们执行不同生理功能的基础.     

Optokinetic response in patients with vestibular areflexia

Optokinetic nystagmus (OKN) responses (stimuli 40°/s and 60°/s) were evaluated in 121 patients with vestibular areflexia (VA) and were compared with a control group of 99 control subjects matched by age. The mean response levels were significantly higher in the VA group than in the control group: 1.7°/s at 40°/s stimulation, and 4.4°/s at 60°/s. The VA group showed a significantly wider scattering and greater variances and, as a group, they exhibited higher OKN gains than the control subjects. We suggest that the higher gain of OKN responses in VA patients can be attributed to an increased efficiency in signal processing by the cortical optokinetic system. This enhancement may be similar to the enhancement which, in healthy subjects, is produced by "optokinetic training".     

Loss of vestibular neurons in clinical otosclerosis

Summary Although the principal functional deficit caused by otosclerosis is hearing loss, some patients also experience dysequilibrium and/or vestibular test abnormalities. A study was performed to determine the populations of vestibular neurons in subjects with clinical otosclerosis. Reduced neuronal counts were found when the otosclerotic lesions were large enough to involve the cribrose areas. Decreased cell counts were found principally in the superior division of the vestibular ganglion. In some cases, however, normal populations of neurons were found in spite of involvement of the cribrose areas. In some ears there were bundles of vestibular nerve fibers deviating from their normal course to pass within the focus of otosclerotic bone. It seems probable that a loss in vestibular neuronal population caused by involvement of dendritic fibers in the cribrose areas is at least partially responsible for the dysequilibrium or vestibular test abnormalities occurring in some patients with otosclerosis.Supported by a grant from the Research Fund of the American Otological Society     

The auditory brain stem response recorded with 60 Hz notch filters

Line-frequency (60 Hz in the USA) interference is a common source of contamination during recording of the auditory brain stem response (ABR). Notch filters are undeniably effective in eliminating this artifact, but concerns have been expressed as to their effect on the ABR itself. Theoretical considerations suggest that the degree of distortion should be minimal if the filter is sufficiently sharp and the stimulus rate has no multiples very close to 60 Hz. Responses simultaneously recorded through three notch filters of varying sharpness confirmed this: Latency was virtually unaffected with any filter or stimulus rate, and distortion was minimal when a sharply tuned filter was used in conjunction with a stimulus rate of 23.3 Hz, whose nearest harmonic to 60 is 69.9 Hz.     

Vestibular ototoxicity of gentamicin assessed by the recording of a short-latency vestibular-evoked response in cats

The short-latency vestibular-evoked response (VsER) and the auditory brain stem response (ABR) were recorded with scalp electrodes in four cats before, during, and after systemic administration of gentamicin. The VsER was altered and later disappeared in three cats, and in one cat it became asymmetric, typical of a unilateral vestibular lesion. In all cats the ABR was unaffected and remained normal through the end of the experiment 4 months later. Histopathological examination of the temporal bones of three cats showed severe damage to the vestibular end-organ, particularly in the summit of the cristae, but in lesser amounts in the hair cells on the slopes of the cristae and in the maculae. In one cat, the pathological condition was greater in one ear, corresponding to the asymmetry in the VsER records. This new method of inducing and recording the VsER has been demonstrated by the present study to be an effective tool for experimentally assessing vestibular end-organ and nerve function in animal models. Furthermore, these results indicate that the VsER is generated in the vestibular labyrinth, most probably by the cristae of the semicircular canals.     

Neurofilament immunoreactivity in vestibular ganglion neurons of the adult rat

Immunocytochemical methods were used to study the distribution of neurofilament (NF) proteins in vestibular ganglion neurons of the adult rat. Monoclonal antibodies against the three triplet proteins were used. By indirect immunofluorescence and the peroxidase-antiperoxidase method, two populations of neurons were distinguished. One population with large perikarya showed strong NF immunoreactivity. A second population of neurons presented only slight or no immunoreactivity. The strong NF immunoreactivity in the perikarya of certain neurons seems to be a general feature of many sensory ganglia.     

The effect of loop diuretics on the vestibular system. Assessment by recording the vestibular evoked response

Whether or to what extent loop diuretics, e.g., ethacrynic acid and furosemide, affect the vestibular system is controversial. We studied this problem by recording in cats the short-latency vestibular evoked response (VsER) to acceleration stimuli by skin electrodes before and after local or systemic administration of loop diuretics. The effect on the VsER was minimal in contrast to the major changes that appeared in the auditory evoked response, in which, among the waves known to originate from the brain stem, the most affected was N1. These findings suggest that the vestibular end-organ function is minimally affected by loop diuretics and that the changes in the vestibulo-ocular reflex reported by several authors might be due to the effect of the drugs on the central nervous system. Thus, recording of the VsER in experimental animal models might serve as a useful tool for direct evaluation of the effect of certain drugs and conditions on the vestibular system.     

Firing mechanisms in the single vestibular neurons in the cat.

Spontaneous unitary discharges in nucleus vestibularis lateralis (NVL) neurons were studied in locally anesthetized cats. The mean +/-S.E. firing rate of spontaneous unitary discharges of NVL neurons was 19.8 +/- 0.9 Hz. About 12% of NVL neurons showed a random firing. The spontaneous unitary discharge rate of the gamma distribution at lambda=2 was relativley low. However, the others were high. Patterns of peak interval time of the distribution with one peak and the interpeak interval time of those with two and three peaks were almost the same.     

Selective effect of cinnarizine on the vestibular nucleus neurons

Summary Effects of cinnarizine were studied on the lateral vestibular nucleus (LVN) and spinal trigeminal nucleus (STN) of cats anesthetized with -chloralose. Cinnarizine did not produce any obvious alterations of the field potential and spike generation of type B interneurons in STN elicited by trigeminal nerve stimulation as well as the field potential in LVN by vestibular nerve stimulation. Spike generation of monosynaptic LVN neurons elicited by the suprathreshold stimulus to the vestibular nerve was unaffected by cinnarizine up to 4 mg/kg. When the subthreshold stimulus was applied to the vestibular nerve, however, the spike number of LVN monosynaptic neurons was significantly increased after cinnarizine treatment. The enhancement of spike firing by cinnarizine upon both supra- and subthreshold stimuli to the vestibular nerve was found to be more pronounced in LVN polysynaptic neurons than monosynaptic ones. Since the effect of cinnarizine on LVN neurons was not dose-dependent, it is suggested that the enhanced responsiveness of the neurons by the drug might be due to an increase of blood flow, but not to a direct excitation of the neurons themselves.