Ultralow vestibuloocular reflex time constants

Vestibular Perception and Navigation in the Congenitally Blind

Vestibular input is required for accurate locomotion in the dark, yet blind subjects’ vestibular function is unexplored. Such investigation may also identify visually dependent aspects of vestibular function. We assessed vestibular function perceptually in six congenitally blind (and 12 sighted) subjects. Cupula deflection by a transient angular, horizontal acceleration generates a related vestibular nerve signal that declines exponentially with time constant ≈4–7 s, which is prolonged to 15 s in the evoked vestibular-ocular reflex by the brain stem “velocity storage.” We measured perceptual velocity storage in blind subjects following velocity steps (overall perceptual vestibular time constant, experiment 1) and found it to be significantly shorter (5.34 s; range: 2.39–8.58 s) than in control, sighted subjects (15.8 s; P < 0.001). Vestibular navigation was assessed by subjects steering a motorized Bárány-chair in response to imposed angular displacements in a path-reversal task, “go-back-to-start” (GBS: experiment 2); and a path-completion task, “complete-the-circle” (CTC: experiment 3). GBS performances (comparing response vs. stimulus displacement regression slopes and r2) were equal between groups ( P > 0.05), but the blind showed worse CTC performance ( P < 0.05). Two blind individuals showed ultrashort perceptual time constants, high lifetime physical activity scores and superior CTC performances; we speculate that these factors may be inter-related. In summary, the vestibular velocity storage as measured perceptually is visually dependent. Early blindness does not affect path reversal performance but is associated with worse path completion, a task requiring an absolute spatial strategy. Although congenitally blind subjects are overall less able to utilize spatial mechanisms during vestibular navigation, prior extensive physical spatial activity may enhance vestibular navigation.

Gentamicin ototoxicity: clinical features and the effect on the human vestibulo-ocular reflex

CONCLUSIONS

Gentamicin ototoxicity presents with gait imbalance and oscillopsia, but only rarely with hearing loss and vertigo. Sinusoidal rotational stimuli with high accelerations such as the bedside head-thrust test or rotational step changes in velocity are useful to diagnose bilateral vestibulopathy.

OBJECTIVE

To describe the salient clinical features and vestibular testing results in gentamicin ototoxicity.

PATIENTS AND METHODS

A retrospective review of the quantitative vestibular function testing results for patients presenting to the UCLA Neurotology Clinic with gentamicin ototoxicity over the past 10 years (n=35).

RESULTS

All patients presented with imbalance and 33 out of 35 had oscillopsia. Three patients reported a noticeable change in hearing and five reported vertigo. Of the 35 patients, 15 were in renal failure at the time of gentamicin administration. Patients with pre-existing peripheral neuropathy compensated poorly. Sinusoidal rotational testing demonstrated profoundly decreased gain and increased phase lead over the entire frequency range, with a subset of patients having relatively preserved gain at the intermediate frequencies (0.8-1.6 Hz) and low acceleration (<30 degrees/s). There was little or no response to high acceleration step changes in velocity. The time constant measured both by sinusoidal and step responses was ultra-low. All patients tested had a positive head-thrust test bilaterally.

Vestibular function in severe bilateral vestibulopathy

OBJECTIVES

To assess residual vestibular function in patients with severe bilateral vestibulopathy comparing low frequency sinusoidal rotation with the novel technique of random, high acceleration rotation of the whole body.

METHODS

Eye movements were recorded by electro-oculography in darkness during passive, whole body sinusoidal yaw rotations at frequencies between 0.05 and 1.6 Hz in four patients who had absent caloric vestibular responses. These were compared with recordings using magnetic search coils during the first 100 ms after onset of whole body yaw rotation at peak accelerations of 2800 degrees /s(2). Off centre rotations added novel information about otolithic function.

RESULTS

Sinusoidal yaw rotations at 0.05 Hz, peak velocity 240 degrees/s yielded minimal responses, with gain (eye velocity/head velocity)<0.02, but gain increased and phase decreased at frequencies between 0.2 and 1.6 Hz in a manner resembling the vestibulo-ocular reflex. By contrast, the patients had profoundly attenuated responses to both centred and eccentric high acceleration transients, representing virtually absent responses to this powerful vestibular stimulus.

CONCLUSION

The analysis of the early ocular response to random, high acceleration rotation of the whole body disclosed a profound deficit of semicircular canal and otolith function in patients for whom higher frequency sinusoidal testing was only modestly abnormal. This suggests that the high frequency responses during sinusoidal rotation were of extravestibular origin. Contributions from the somatosensory or central predictor mechanisms, might account for the generation of these responses. Random, transient rotation is better suited than steady state rotation for quantifying vestibular function in vestibulopathic patients.

Abnormal supranuclear eye movements in the child: a practical guide to examination and interpretation

Abnormal eye movements in the infant or voting child can be congenital or acquired. They may be a result of abnormal early visual development or a sign of underlying neurologic or neuromuscular disease. It is important to be able to detect these abnormalities and to distinguish them from normal but immature eye movements. The spectrum of disease in children differs from that in adults. Serious, potentially fatal but treatable disorders can be acquired in infancy, and abnormal eye movements in a sometimes apparently well child should never be labeled as congenital or benign without careful investigation. Eye movement analysis can indicate the presence of an underlying condition and help the clinician to classify different neurologic diseases. It is important to carefully examine the ocular motor system in any children at risk of neurologic disease. This review provides a practical guide to the examination and interpretation of eye movements in the child and includes recent literature on eye movement disorders of childhood. We describe supranuclear abnormalities of the ocular motor system in the order in which we would normally examine it: saccades, pursuit, convergence, vestibulo-ocular reflex, and optokinetic nystagmus. Nystagmus, internuclear ophthalmoplegia, cranial nerve abnormalities, and "miswiring" phenomena (such as Duane's syndrome and synergistic divergence) are not discussed.

Oculomotor abnormalities and MRI findings in idiopathic cerebellar ataxia

Extensive oculomotor testing and quantitative MRI evaluation was performed in seven patients with idiopathic cerebellar ataxia without extracerebellar symptoms (IDCA-C) and in ten patients with additional extracerebellar symptoms (IDCA-P). The most severe oculomotor deficits were disturbed smooth pursuit, optokinetic nystagmus and suppression of the vestibulo-ocular reflex (VOR). The symptoms correlated well and consistently with the amount of atrophy of the flocculus and the dorsal vermis. These correlations, however, were not specific, and deficits also correlated with the amount of atrophy of other cerebellar structures. No correlation was found between saccade velocity and brainstem atrophy or between saccade metrics and atrophy of the dorsal vermis. Although patients with IDCA-P had more severe oculomotor deficits than patients with IDCA-C, the pattern of the oculomotor disturbances was the same for both groups. Thus, eye movement analysis alone is not sufficient to distinguish between patients with pure cerebellar ataxia and those with additional extracerebellar symptoms.

The dynamics of the vestibulo-ocular reflex in patients with vestibular neuritis

The dynamics of the vestibulo-ocular reflex (VOR) were studied in 14 patients at the onset of vestibular neuritis, and at follow-up 1 year later. A velocity step stimulus of 150 degrees/s was used to investigate the VOR time constant and gain, and the results were related to the caloric response. In the acute, vertiginous phase of the disease, the VOR time constant was reduced but was almost normalized 1 year later, both among patients who regained normal caloric side-difference and among those who did not. However, the increase in VOR time constant was greater among those who regained normal caloric excitability, and regression analysis showed a correlation between the prolongation of the VOR time constant and the recovery of caloric excitability. These findings suggest that VOR dynamics are modulated during the acute phase of vestibular neuritis, and that there is recovery with vestibular compensation. Furthermore, the recovery of the VOR time constant is not solely dependent on the recovery of normal caloric excitability. This implies that central storage of velocity information may be involved in the VOR, even in cases of asymmetric vestibular input after vestibular compensation.

Short vestibulo-ocular reflex time constants associated with oculomotor pathology in multiple sclerosis

Significantly short vestibulo-ocular reflex (VOR) time constants were found in significant high numbers in patients with multiple sclerosis who had oculomotor dysfunction, especially when this dysfunction comprised both the smooth pursuit/optokinetic and the saccadic systems. Some time constants were in the range of 2.5-6 s, i.e. shorter than the presumed time constant of the primary afferent canal neuron. Increase in oculomotor impairment was accompanied by a decrease in time constants in some patients, whereas others showed a combined renormalization of previously disturbed oculomotor function and short time constants. Significantly high numbers of combinations of asymmetry in oculomotor dysfunction and a directional preponderance in time constant were observed, with the shorter time constant on the side of the (more severe) defect.

Vestibular responses in Wernicke's encephalopathy

Two patients with Wernicke's encephalopathy were evaluated with quantitative vestibulo-ocular reflex and ocular motor testing. Vestibulo-ocular reflex testing included caloric irrigation, earth vertical axis rotational sinusoids, and rotational impulses. Both patients demonstrated hypoactive vestibular responses to both caloric and rotational stimuli at the time of presentation. One patient had unbeating nystagmus that diminished with upgaze, downgaze, or convergence. Following treatment with thiamine, both patients' vestibular responses improved but remained abnormal, with a short vestibulo-ocular reflex time constant and increased low-frequency rotational phase lead. Impairment of the velocity storage element attributable to damage to the vestibular nucleus and nucleus prepositus hypoglossi may account for this permanent effect on the vestibulo-ocular reflex.