To assess the contributions of the vestibular system to whole-body motion discrimination in the dark, we measured direction-recognition thresholds as a function of frequency for yaw rotation, superior-inferior translation (“z-translation”), inter-aural translation (“y-translation”), and roll-tilt for 14 normal subjects and for three patients following total bilateral vestibular ablation. The patients had significantly higher average threshold measurements than normal (p<0.01) for yaw-rotation (depending upon frequency, 5.4× to 15.7× greater), z-translation (8.3× to 56.8× greater), y-translation (1.7× to 4.5× greater), and roll tilt (1.3× to 3.0× greater) – establishing the predominant contributions of the vestibular system for these motions in the dark.

Fatigue in MG is highly prevalent, mainly physical, and influenced by depressive symptoms, disease severity, female sex and sleep debt. Cognitive fatigue in MG may not be a direct disease manifestation, but secondary to depression. The FSS and FIS represent reliable and validated tools, appropriate to discern meaningful clinical aspects of fatigue in MG. Clinical recognition of the complexity of fatigue may foster individualized treatment approaches for affected MG patients.

We measured vestibular perceptual thresholds in patients with idiopathic bilateral vestibulopathy to assess the distribution of peripheral vestibular damage in this disorder. Thresholds were measured with standard psychometric techniques in 4 patients and compared with thresholds in normal subjects and patients with completely absent peripheral vestibular function. Motion paradigms included yaw rotation (testing the lateral canals), interaural translation (testing the utricles), superior-inferior translation (testing the saccules), and roll tilt (testing the vertical semicircular canals and the otolith organs). We found that perceptual thresholds were abnormally elevated in the patients with idiopathic bilateral vestibulopathy for yaw rotation at all frequencies and for interaural translation at only the lower frequencies. Thresholds were normal for the other 2 motion paradigms. The results demonstrate that the distribution of vestibular dysfunction in this disorder is not uniform but, rather, can affect lateral canal and utricular thresholds while relatively sparing vertical canal and saccular function.

Controlling posture requires continuous sensory feedback about body motion and orientation, including from the vestibular organs. Little is known about the role of tilt vs. translation vs. rotation vestibular cues. We examined whether inter-subject differences in vestibular function were correlated with inter-subject differences in postural control. Vestibular function was assayed using vestibular direction-recognition perceptual thresholds, which determine the smallest motion that can be reliably perceived by a subject seated on a motorized platform in the dark. In study A, we measured thresholds for lateral translation, vertical translation, yaw rotation and head-centered roll tilts. In study B, we measured thresholds for roll, pitch, left anterior-right posterior and right anterior-left posterior tilts. Center-of-pressure (CoP) sway was measured in Sensory Organization Tests (study A) and Romberg tests (study B). We found a strong positive relationship between CoP sway and lateral translation thresholds but not CoP sway and other thresholds. This finding suggests that the vestibular encoding of lateral translation may contribute substantially to balance control. Since thresholds assay sensory noise, our results support the hypothesis that vestibular noise contributes to spontaneous postural sway. Specifically, we found that lateral translation thresholds explained more of the variation in postural sway in postural test conditions with altered proprioceptive cues (vs. a solid surface), consistent with postural sway being more dependent on vestibular noise when the vestibular contribution to balance is higher. These results have potential implications for vestibular implants, balance prostheses and physical therapy exercises.

Straumann, D; Bockisch, C J (2010with the visual vertical during whole-body roll tilt and eccentric rotation in healthy subjects and patients with unilateral vestibular loss, to determine which test was most sensitive in discriminating impaired utricle function. Methods:OVEMPs and the visual vertical were measured in 11 patients and 11 healthy subjects. Visual vertical was measured during roll tilts between -9.6 and 9.6°, and during rotation at 400°/s with the head upright and the vertical rotation axis located between ±3.5 cm from the head center.Results: OVEMPs in patients were strikingly asymmetric, whereas they were approximately symmetric in healthy subjects. Patients showed impaired visual vertical gain during eccentric rotation and increased errors for both roll tilt and eccentric rotation tests. OVEMPs were superior at discriminating between patients and healthy subjects, although eccentric rotation performed nearly as well. Conclusions:OVEMPs provide a powerful test for discriminating between healthy subjects and patients with chronic unilateral vestibular loss, and testing the visual vertical testing during eccentric rotation was superior to testing during whole-body roll tilt.Significance: OVEMPs are easier to administer, less demanding on patients, and in general are more effective at identifying chronic unilateral vestibular loss than visual vertical measurements.

OBJECTIVE To explore whether ocular vestibular evoked myogenic potentials (oVEMP) can be used to detect a decrement in the extraocular muscle activity of patients with myasthenia gravis (MG). METHODS Twenty-seven patients with MG, including 13 with isolated ocular and 14 with generalized MG, and 28 healthy controls participated. We applied repetitive vibration stimuli to the forehead and recorded the activity of the inferior oblique muscle with 2 surface electrodes placed beneath the eyes. To identify the oVEMP parameters with the highest sensitivity and specificity, we evaluated the decrement over 10 stimulus repetitions at 3 different repetition rates (3 Hz, 10 Hz, and 20 Hz). RESULTS Repetitive stimulation at 20 Hz yielded the best differentiation between patients with MG and controls with a sensitivity of 89% and a specificity of 64% when using a unilateral decrement of 15.2% as cutoff. When using a bilateral decrement of 20.4% instead, oVEMP allowed differentiation of MG from healthy controls with 100% specificity, but slightly reduced sensitivity of 63%. For both cutoffs, sensitivity was similar in isolated ocular and generalized MG. CONCLUSION Our study demonstrates that the presence of an oVEMP decrement is a sensitive and specific marker for MG. This test allows direct and noninvasive examination of extraocular muscle activity, with similarly good diagnostic accuracy in ocular and generalized MG. Thus, oVEMP represents a promising diagnostic tool for MG. CLASSIFICATION OF EVIDENCE This study provides Class III evidence that oVEMP testing accurately identifies patients with MG with ocular symptoms (sensitivity 89%, specificity 64%). Author contributions:Yulia Valko collected and analyzed the data and wrote the manuscript.Sally Rosengren contributed to the study design, data analysis and writing of the manuscript.Hans Jung contributed to the patient examination and review of the manuscript.Dominik Straumann contributed to the study design, data analysis and review of the manuscript.Klara Landau contributed to the study design, patient examination and review of the manuscript.Konrad Weber designed the study, contributed to data collection, data analysis and writing of the manuscript, and conducted the statistical analysis.Study funding: the study was supported by a research grant of the University of Zurich (Forschungskredit der Universität Zürich).Disclosure Agreement: All authors report no disclosures.Valko et al. 3 AbstractObjective: To explore whether ocular vestibular evoked myogenic potentials (oVEMP)can be used to detect a decrement in the extraocular muscle activity of myasthenia gravis (MG) patients.Methods: Twenty-seven MG patients, including 13 with isolated ocular and 14 with generalized MG and 28 healthy subjects participated. We applied repetitive vibration stimuli to the forehead and recorded the activity of the inferior oblique muscle with two surface electrodes placed beneath the eyes. To identify the oVEMP parameters with the highest sensitivity and specificity, we evaluated th...

Visuo-vestibular interactions in monkeys can be accurately modelled using the classical Raphan and Cohen's model. This model is composed of direct vestibular and visual contributions to the vestibulo-ocular reflex (VOR) and of a velocity storage. We applied this model to humans and estimated its parameters in a series of experiments: yaw rotations at moderate (60°/s) and high velocities (240°/s), suppression of the VOR by a head-fixed wide-field visual stimulus, and optokinetic stimulation with measurements of optokinetic nystagmus (OKN) and optokinetic afternystagmus (OKAN). We found the velocity storage time constant to be 13 s, which decreased to 8 s during visual suppression. OKAN initial velocity was 12% of the OKN stimulus velocity. The gain of the direct visual pathway was 0.75 during both visual suppression and OKN; however, the visual input to the velocity storage was higher during visual suppression than during OKN. We could not estimate the time constant of the semicircular canals accurately. Finally, we inferred from high-velocity rotations that the velocity storage saturates around 20–30°/s. Our results indicate that the dynamics of visuo-vestibular interactions in humans is similar as in monkeys. The central integration of visual cues, however, is weaker in humans.

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