The visual consequences of deviations in the orientation of the axis of rotation of the human vestibulo-ocular reflex

The 3D orientation and amplitude of the movement of each eye evoked by predictable, sinusoidal and non-predictable, sum-of-sines rotation about roll, yaw, pitch and intermediate axes were measured in seven subjects. The rotation axis of the eyes was not always perfectly aligned with the stimulus axis but showed systematic deviations that depended on the orientation of the rotation axis of the head. Misalignment of the oculomotor response with the stimulus axis is equivalent to adding an orthogonal, non-compensatory vector that potentially could introduce retinal slip, rather than compensate for it. The variations in orientation could not be readily explained as an artifact arising from the differential processing of the roll component. Instead, differences in the movements of the left and right eyes had trends appropriate for compensating for the geometrical demands of the translation of the eyes that must necessarily accompany natural head rotation.

The protective effect of brain-derived neurotrophic factor after gentamicin ototoxicity

HYPOTHESIS

Brain-derived neurotrophic factor (BDNF) influences the process of hair cell recovery in the vestibular sensory epithelium of the chinchilla after local application of gentamicin (GM).

BACKGROUND

Hair cell regeneration in the inner ear after GM ototoxicity has been demonstrated. However, the mechanisms responsible for this recovery have yet to be completely elucidated. This report examines the protective and proliferative effects that BDNF exerts on vestibular hair cells in experiments designed to further elucidate the mechanisms of hair cell regeneration.

METHODS

The inner ears of three separate groups of chinchillas were treated with GM only, GM and BDNF simultaneously, and GM followed by BDNF 1 week later. The numbers of hair and supporting cells in the horizontal cristae of each group were then estimated at 1, 2, 4, and 8 weeks, and the data were compared.

RESULTS

Type I hair cells after GM treatment completely disappeared. After simultaneous BDNF and GM treatment, their numbers decreased to 23% at 1 week and progressively disappeared by week 8. When BDNF was applied 1 week after GM administration, type I hair cells recovered to 12% at week 4 and 28% at week 8. Type II hair cells after GM treatment decreased to 15%, but recovered to 83% 4 weeks later. Simultaneous administration of BDNF and GM prevented the ototoxic effects of GM alone. When BDNF was administered 1 week after GM, type II hair cell recovery was accelerated and was greater than after GM alone (81% versus 18%). Supporting cells after GM treatment decreased to 74% at 1 week after treatment, recovered to 91% at 2 weeks, and remained at 86% at 4 weeks and 85% at 8 weeks. With the simultaneous administration of BDNF and GM, supporting cells significantly decreased at 2 weeks after treatment (63%), but recovered to normal by week 8.

CONCLUSIONS

These results suggest that BDNF provided simultaneously with GM minimizes the ototoxic effect of GM on type II hair cells. The increase in the number of new hair cells when BDNF is provided after ototoxic damage is evidence of the proliferative capacity of this neurotrophic factor.

Hair cell recovery in the chinchilla crista ampullaris after gentamicin treatment: a quantitative approach

The mechanisms for hair cell recovery were investigated after intraortic application of 50 microg gentamicin into the perilymphatic space of the superior semicircular canal of the chinchilla. Histologic evaluation of one normal group and four posttreatment groups (7, 14, 28, and 56 days) was made with light and transmission electron microscopic techniques. The numeric changes of hair cells and supporting cells was quantified with the dissector technique. At 7 and 14 days after treatment, no type I hair cells were present, and 85% and 88% of type II hair cells were lost. Supporting cells decreased to 76% at 7 days, but they recovered to 91% at 14 days. Recovery of the epithelia was evident 28 days after treatment; 83% were type II hair cells, and 3% were type I hair cells. The supporting cell number remained close to normal (86%). Between 14 and 28 days after treatment, there was an increase of 1758 of type II hair cells, representing approximately 125 new hair cells per day. At the same time interval the number of supporting cells remained near normal. These results suggest that new hair cells might be the result of supporting cell mitotic division and differentiation.

Static and dynamic posturography in patients with vestibular and cerebellar lesions

OBJECTIVE

To assess the diagnostic usefulness of posturography in 2 well-defined patient groups with impaired balance.

PATIENTS

Ten control subjects, 10 patients with bilateral vestibular loss, and 10 patients with cerebellar atrophy.

OUTCOME MEASURES

Amplitude, velocity, and frequency of sway in the anteroposterior and medial-lateral directions on a static platform, on foam, and on a moving platform.

RESULTS

Both patient groups consistently had increased sway compared with controls, particularly when standing on foam or on a moving platform with eyes closed. Sway amplitude and velocity were increased about the same amount. The Romberg ratio (sway with eyes closed/sway with eyes open) did not reliably differentiate patients from controls or the 2 patient groups from each other. Some patients with cerebellar atrophy exhibited a characteristic body tremor at about 3 Hz in the anteroposterior direction.

CONCLUSIONS

Although sway amplitude and velocity were consistently increased in patients with bilateral vestibular loss and patients with cerebellar atrophy, none of the posturography measurements reliably distinguished the 2 patient groups. The finding of increased frequency of sway in the anteroposterior direction in patients with cerebellar atrophy was of limited value since the tremor was visible at the bedside.

Clinical-pathologic correlation in a patient with selective loss of hair cells in the vestibular endorgans

We found a selective loss of vestibular hair cells in a patient followed for more than 10 years with imbalance and oscillopsia due to idiopathic progressive loss of vestibular function. Hearing function and cochlear hair cells were normal. The vestibulo-ocular reflex (VOR) gain at high frequencies was relatively maintained despite marked shortening of the dominant VOR time constant (to less than 500 ms). Ultrastructural examination of remaining hair cells showed mitochondrial abnormalities. The ultrashort VOR time constant probably resulted from changes in firing patterns of the primary afferent nerves due to loss of hair cells and impaired energy metabolism in remaining hair cells.

Quantification of the process of hair cell loss and recovery in the chinchilla crista ampullaris after gentamicin treatment

The degree of ototoxic drug sensitivity and hair cell repair was determined in the chinchilla horizontal crista ampullaris after intraotic administration of gentamicin. Histological evaluation was made of 22 cristae ampullaris from one normal and six post-treatment (PT) animal groups killed at 1, 4, 7, 14, 28, and 56 days. New hair cell production was quantified, using the dissector technique. Transmission electron microscopy was used to investigate the ultrastructural characteristics of the hair cells in the regenerated epithelium. At 1 day PT, type I and II hair cells presented cytoplasmic vacuolization, swollen nerve calyces and 20% of type I and 18% of type II hair cells were lost. At 4 days PT, 95% of type I hair cells and 14% of type II hair cells had disappeared. In addition, most of the type II hair cells showed clumping of nuclear material. Nerve fibers were not found in the sensory epithelium, but were still observed below the basal lamina. Supporting cells appeared unaffected, maintaining their location in the crista. At 1 and 4 days PT, the damage to hair cells was more pronounced in the central region of the crista ampullaris. The degree of ototoxic damage at 7 days was similar to that of 14 days: no type I hair cells were present and most of the type II hair cells had disappeared; supporting cell nuclei began to occupy the apical part of the sensory epithelium and most of the nerve fibers had retracted. Quantitatively, 87 and 93% of type II hair cells were lost at 7 and 14 days PT, respectively. Initial signs of hair cell recovery began at 28 days PT; immature type II-like hair cells appeared, supporting cell nuclei began to align at the base of the sensory epithelium and nerve fibers penetrating the basal lamina were observed. No type I hair cells were found, but 40% of the normal number of type II hair cells were present. Hair cells appeared to regenerate in the peripheral areas of the cristae ampullaris first. At 56 days PT, an increase in the number of mature type II hair cells was present, supporting cells were aligned at the base of the epithelium, and more nerve fibers appeared to penetrate the basal lamina to the sensory epithelium. Although type I hair cells were absent from the epithelium 55% of the normal number of type II hair cells were present. At this time, more regenerated hair cells were located in the center of the cristae ampullaris as compared to the periphery. At the transmission electron microscopic level, type II hair cells at different stages of maturation were observed. Some exhibited mature stereocilia, a cuticular plate, and terminal endings with synaptic specialization opposing these hair cells. In conclusion, type I hair cells were more sensitive than type II hair cells to gentamicin intoxication (as they disappeared as early as 4 days PT). After 56 days PT, the number of type II hair cells reached 55% of normal. No type I hair cells had regenerated at this time. These results demonstrate quantitatively the differential ototoxic sensitivity and regenerative capacity of hair cells.

Arrangement of vestibular nerve fibers in the semicircular canal crista of the chinchilla

The topographic arrangement of vestibular nerve fibers innervating semicircular canal cristae of the chinchilla was studied using computer-aided video-microscopy and three-dimensional reconstruction. At the level 20 microns proximal to the base of the crista, bundles consisting of 30-50 nerve fibers each were identified. Nerve fibers in bundles were classified into seven categories depending on the diameter. We confirmed that large nerve fibers were more frequently found in the central bundles and small nerve fibers were more frequently found in the peripheral bundles. The central bundle might function as a physiological unit coding various types of head movements, whereas the peripheral bundle might contribute more to the detection of slow and long-lasting movements giving rise to tonus and posture changes. The canalicular nerve may code rotational acceleration of the head via function- and locus-specific nerve fiber bundles.

Projections of the individual vestibular end-organs in the brain stem of the squirrel monkey

The central nervous system (CNS) projections of primary afferent neurons from individual vestibular receptors were studied using horseradish peroxidase (HRP) or biocytin labeling in 14 ears from 7 adult squirrel monkeys using the technique developed in the chinchilla (Lee et al., 1989, 1992). The specificity of labeling was verified by examining the location of the labeled fibers and cell bodies in the vestibular nerve and Scarpa's ganglion. Labeled fibers and cells were restricted to nerves and areas belonging to groups of cells in either the superior or the inferior ganglion of the vestibular nerve. In the vestibular nerve root, labeled primary afferent fibers also exhibited a receptor-dependent segregation at the entrance to the medulla. Fibers from the HSC and the SSC were found rostrally and those from the PSC and the SAC were found in the caudal area. The UTR fibers were situated intermediate between these two groups of fibers. (A bundle of fibers, probably vestibular efferents, was identified immediately rostrally and ventromedially to the UTR fibers.) The primary afferent fibers bifurcated into secondary ascending and descending fibers at the lateral border of the vestibular nuclei, forming a longitudinal rostrocaudal vestibular tract. The secondary fibers from individual end-organs occupied specific locations in the tract: the UTR fibers were dorsal to the SSC and the HSC fibers, PSC fibers were found most medially, and the SAC fibers occupied the lateralmost area. The secondary UTR fibers overlapped considerably with those of the SSC and the HSC. The orderly receptor-dependent segregation of fibers was more prominent in the descending tracts than in the ascending tracts. In the vestibular nuclei complex the location of the tertiary branches of various end-organs exhibited considerable overlap within the major vestibular nuclei (SN, superior nucleus; LN, lateral nucleus; MN, medial nucleus; DN, descending nucleus). There were still differences, however, in the projection pattern. Fibers from the SAC ran primarily in the lateral area, fibers from the SSC and the UTR were found ventromedially to the SAC fibers, and the HSC projected slightly medially to the fibers from the SSC. The PSC fibers projected most medially. The UTR and SAC sent numerous fibers to the cerebellum. Fibers from the semicircular canals projected through the rostrodorsal region of the SN and presumably also projected to the cerebellum. The precise termination of fibers was evaluated by studying the location of labeled boutons, which were identified in all major vestibular nuclei. Labeled boutons from all the receptors were in the rostral and central areas of the SN, and in the MN mainly in the rostral two-thirds. In the LN, boutons from all the receptors were in the rostroventral part, most of which were from the UTR and SAC. No labeled boutons were in the caudodorsal part of this nucleus. Labeled boutons in the DN primarily surrounded the descending tract fibers and were particularly prominent medially. In specimens in which superior vestibular nerve receptor organs were scratched vestibular efferent fibers were also labeled. These fibers traveled in the most ventral part of the vestibular nerve root and projected in the ventral aspect of the LN to labeled soma in the ipsilateral and contralateral brain stem. Specificity the in projection patterns of efferent fibers from different end-organs could not be ascertained.

Comparison of static and dynamic posturography in young and older normal people

OBJECTIVE

To measure sway velocity during static and dynamic posturography in "normal" older people and to determine which tests best distinguish young from older subjects.

SUBJECTS

A sample of 30 young (18-39 years) and 82 community-dwelling older (> 75 years) subjects who reported normal balance underwent a battery of balance tests.

MEASUREMENTS

Velocity and frequency of sway, Tinetti gait and balance score, self-reported fear of falling and number of falls.

RESULTS

Mean sway velocity and the ratio of high to low frequency sway velocity were significantly increased in older subjects compared with younger subjects for static and dynamic tests with eyes open and with eyes closed. Measures of anterior-posterior sway velocity during angular tilt of the platform with eyes closed best distinguished young from older subjects (almost three-fourths of older subjects had values greater than 2 standard deviations from the young normal mean). Older subjects who reported falls in the past year did not have a significantly higher sway velocity on static or dynamic tests compared with those who did not report falls. However, subjects who reported a fear of falling had significantly higher sway velocity on dynamic tests with eyes closed compared to those who did not report a fear of falling.

CONCLUSIONS

On average, velocity of sway is higher in older subjects compared with younger subjects, and the difference between young and old is greater with dynamic posturography than with static posturography.

The electrically evoked vestibulo-ocular reflex: I. Normal subjects

Recent animal studies indicate that electric currents applied through perilymphatic-space electrodes stimulate vestibular primary afferent neurons directly. These findings suggest that electrical stimulation may provide a testing method by which the vestibular nerve and central pathways could be evaluated separately from the vestibular end-organ. The goal of this study was to obtain normative data on human beings for an electrically evoked vestibulo-ocular reflex (EVOR). Sinusoidal electrical stimuli (0.0125 to 0.8 Hz, 4 mA peak intensity) were applied along the interaural axis through mastoid electrodes in 10 subjects. Horizontal eye movements were recorded by an infrared limbus-tracking device. The subjects also underwent rotational stimulation at the same frequencies so that their horizontal vestibulo-ocular reflex (VOR) could be evaluated. Nystagmus was observed in the EVOR at lower stimulus frequencies, whereas purely sinusoidal eye deviations occurred at higher frequencies. The phase of the EVOR slow-component eye velocity consistently lagged the stimulus. This contrasts with the phase measurements of the VOR in the same subjects, which exhibited a lead relative to head velocity. These findings suggest that currents applied to human beings may activate vestibular primary afferents independent of peripheral receptor mechanisms and thereby provide a "site-of-lesion" testing method by which the vestibular nerve and central pathways can be evaluated separately from the vestibular end-organ.

Horizontal vestibulo-ocular reflex after acute peripheral lesions

We studied the dynamics of the horizontal vestibulo-ocular reflex using a broad range of sinusoidal rotational stimuli in 14 patients with acute unilateral peripheral vestibular lesions. From plots of slow phase velocity (SPV) versus stimulus velocity (phase shifts removed) we calculated the DC offset and gain to ampullopetal (AP) and ampullofugal (AF) stimulation of the remaining intact labyrinth. DC offset values were highly correlated with spontaneous nystagmus SPV values for all frequencies (0.0125-0.8 Hz) and amplitudes (30-120-deg/sec) of stimulation. Mean AP gain values were consistently higher than mean AF values with the greatest difference occurring at low frequencies of stimulation. Saturation nonlinearities were present with AF stimulation at high amplitudes. These findings can be explained by dynamic asymmetries at the level of the primary and secondary vestibular neurons.

Eye movements induced by linear acceleration on a parallel swing

1. Horizontal and vertical eye movements were induced in normal human subjects by sinusoidal linear acceleration on a parallel swing. The swing frequency was 0.3 Hz and the peak horizontal and vertical acceleration ranged from 0.17 to 0.48 and 0.03 to 0.34 g, respectively. Eye movements were recorded with the scleral search coil technique. 2. With the subjects seated in the dark to stimulate the otolith-ocular reflex, swing displacement along the interaural axis induced horizontal eye movements with a mean sensitivity to translation (ST) (peak eye velocity/peak swing velocity) of 3.8 to 4.7 degrees/m and a mean phase shift (eye velocity re swing velocity) of -152 to -160 degrees. Vertical eye movements had ST and phase values comparable to those of the horizontal eye movements. When the subjects sat facing forward so that the horizontal linear accelerations occurred in the occipitonasal axis, almost identical vertical but no consistent horizontal eye movements were induced. In each case the horizontal and vertical eye movements were proportional to the horizontal and vertical displacement of the swing. 3. With the subject seated in the light looking to an earth-fixed target (synergistic visual-vestibular interaction), the gain (peak eye velocity/peak target velocity) of induced eye movements was near 1, and the phase was compensatory (i.e., approximately -180 degrees) for all stimuli (even at target velocities at which the pursuit gain was less than 1). Subjects were able to suppress the otolith-ocular responses by fixating on a target attached to the swing. The ST decreased by an order of magnitude compared with measurements in the dark without a fixation target. 4. Subjects were able to augment the ST (horizontal and vertical) by imagining an earth-fixed target. Halving the distance of the imagined target approximately doubled the ST. 5. In two of three subjects tested, the ST measured with mental alerting in the dark adaptively increased (approximately doubled) after 20 min of continuous synergistic visual-vestibular interaction. The subject who did not show an adaptive increase in ST began with the highest value of the 10 normal subjects. 6. We conclude that during linear accelerations of the head the otolith signal is correctly interpreted as head movement and not rotation of the gravity vector. The otolith-ocular reflex interacts with the visual pursuit system to improve ocular stability during translational head movements.

Ultralow vestibuloocular reflex time constants

We report detailed oculomotor studies in 3 patients with central nervous system lesions and markedly decreased time constants (less than 2 seconds) of the vestibuloocular reflex (VOR). In 1 patient with Chiari type I malformation, serial measurements over 3 years documented a progressive decrease in the duration of postrotatory nystagmus (100 deg/sec steps, acceleration 140 deg/sec2) until finally there was no sustained nystagmus. At this time, the patient had no response to caloric stimulation or to sinusoidal rotation below 0.2 Hz but normal gain (peak slow-phase eye velocity/peak chair velocity) above 0.4 Hz (phase lead increased). Gaze holding, saccades, smooth pursuit, and optokinetic nystagmus were normal, but optokinetic-after-nystagmus disappeared. The other 2 patients (combined brainstem-cerebellar atrophy) had impaired gaze holding, abnormal smooth pursuit and optokinetic nystagmus, and absent optokinetic-after-nystagmus. VOR gain to step and high-frequency sinusoidal stimuli was increased. The neural mechanism that normally prolongs the VOR time constant may have reduced it in our patients.

[Thrombocyte function in vasomotor and migraine headaches]

The number of circulating platelet aggregates was assessed using a modification of the method of Wu and Hoak. The index for the number of circulating aggregates was 1.04 +/- 0.12 for 35 healthy probands aged 35 +/- 11 years and was different from the index of 25 patients with vasomotor headache aged 37 +/- 14 years of 1.1 +/- 0.14 (P less than 0.1). The number of circulating platelet aggregates with an index of 1.38 +/- 0.24 was significantly higher (P less than 0.001) in 48 migraine patients aged 36 +/- 12 years.