Coordination of eye and head movements during smooth pursuit in patients with vestibular failure.
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During pursuit of smoothly moving targets with combined eye and head movements in normal subjects, accurate gaze control depends on successful interaction of the vestibular and head movement signals with the ocular pursuit mechanisms. To investigate compensation for loss of the vestibulo-ocular reflex during head-free pursuit in labyrinthine-deficient patients, pursuit performance was assessed and compared under head-fixed and head-free conditions in five patients with isolated bilateral loss of vestibular function. Target motion consisted of predictable and unpredictable pseudo-random waveforms containing the sum of three or four sinusoids. Comparison of slow-phase gaze velocity gains under head-free and head-fixed conditions revealed no significant differences during pursuit of any of the three pseudo-random waveforms. The finding of significant compensatory eye movement during active head movements in darkness in labyrinthine-deficient patients, which were comparable in character and gain to the vestibular eye movement elicited in normal subjects, probably explains the similarity of the head-fixed and head-free responses. In two additional patients with cerebellar degeneration and vestibular failure, no compensatory eye movement response was observed, implying that the cerebellum is necessary for the generation of such responses in labyrinthine-deficient patients. (+info)
The vestibular system detects motion of the head and maintains stability of images on the fovea of the retina as well as postural control during head motion. Signals representing angular and translational motion of the head as well as the tilt of the head relative to gravity are transduced by the vestibular end organs in the inner ear. This sensory information is then used to control reflexes responsible for maintaining the stability of images on the fovea (the central area of the retina where visual acuity is best) during head movements. Information from the vestibular receptors also is important for posture and gait. When vestibular function is normal, these reflexes operate with exquisite accuracy and, in the case of eye movements, at very short latencies. Knowledge of vestibular anatomy and physiology is important for physical therapists to effectively diagnose and manage people with vestibular dysfunction. The purposes of this article are to review the anatomy and physiology of the vestibular system and to describe the neurophysiological mechanisms responsible for the vestibulo-ocular abnormalities in patients with vestibular hypofunction. (+info)
Usefulness of posturography after epidural block.
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After a nerve block, observation of the course of effects is necessary until discharge of patients. Particularly epidural block in the lumbar region markedly affects ambulation and postural stability. Although there are few methods of objective evaluation of the postural stability, safe and early discharge is desired. We evaluated the influences of epidural block with 5 ml of 1% lidocaine on equilibrium before as well as 30, 60 and 90 minutes after epidural block. Computerized posturography allows the objective evaluation and quantitative assessment of impairment of receptors and the central nervous system involved in the maintenance of postural stability by analyzing of results. Locus length per unit area with the eyes open and that with the eyes closed 30 minutes after epidural block (27.339 +/- 11.761 cm and 25.804 +/- 10.561 cm, respectively) were significantly (P = 0.0067 and 0.0175, respectively) higher than baseline values (19.528 +/- 8.240 cm and 19.496 +/- 7.450 cm, respectively). Sway area with the eyes open 30 minutes after epidural block (3.923 +/- 2.494 cm2) was significantly (P = 0.0190) larger than the baseline value (2.533 +/- 1.309 cm2). These results suggest that marked effect remain after epidural block even when standing appears to be stable, and the observation of the course of effects after epidural block is still necessary for the safety of patients. We considered that locus length per unit area is a useful parameter for the assessment of an early discharge. (+info)
Postural and locomotor control in normal and vestibularly deficient mice.
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We investigated how vestibular information is used to maintain posture and control movement by studying vestibularly deficient mice (IsK-/- mutant). In these mutants, microscopy showed degeneration of the cristae of the semicircular canals and of the maculae of the utriculi and sacculi, while behavioural and vestibulo-ocular reflex testing showed that vestibular function was completely absent. However, the histology of Scarpa's ganglia and the vestibular nerves was normal in mutant mice, indicating the presence of intact central pathways. Using X-ray and high-speed cineradiography, we compared resting postures and locomotion patterns between these vestibularly deficient mice and vestibularly normal mice (wild-type and IsK+/-). The absence of vestibular function did not affect resting posture but had profound effects on locomotion. At rest, the S-shaped, sagittal posture of the vertebral column was the same for wild-type and mutant mice. Both held the head with the atlanto-occipital joint fully flexed, the cervico-thoracic junction fully flexed, and the cervical column upright. Wild-type mice extended the head and vertebral column and could walk in a straight line. In marked contrast, locomotion in vestibularly deficient mice was characterized by circling episodes, during which the vertebral column maintained an S-shaped posture. Thus, vestibular information is not required to control resting posture but is mandatory for normal locomotion. We propose that vestibular inputs are required to signal the completion of a planned trajectory because mutant mice continued rotating after changing heading direction. Our findings support the hypothesis that vertebrates limit the number of degrees of freedom to be controlled by adopting just a few of the possible skeletal configurations. (+info)
The influence of voluntary tonic EMG level on the vestibular-evoked myogenic potential.
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Vestibular-evoked myogenic potentials (VEMPs) are proposed as a reliable test to supplement the current vestibular test battery by providing diagnostic information about saccular and/or inferior vestibular nerve function. VEMPs are short-latency electromyograms (EMGs) evoked by high-level acoustic stimuli and recorded from surface electrodes over the tonically contracted sternocleidomastoid muscle. VEMP amplitude is influenced by the EMG level, which must be controlled. This study examined the ability of subjects to achieve the EMG target levels over a range of target levels typically used during VEMP recordings. In addition, the influence of target EMG level on the latency and amplitude of the click- and tone-evoked VEMP was examined. The VEMP amplitude increased as a function of EMG target level, and the latency remained constant. EMG target levels ranging from 30 microV to 50 microV are suggested for clinical application of the VEMP. (+info)
Subjective visual vertical in pitch and roll in right hemispheric stroke.
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BACKGROUND AND PURPOSE: Patients with right hemispheric stroke usually present an anticlockwise deviation of the subjective visual vertical (SVV) in the frontal (roll) plane. However, the occurrence of a similar disorder in the sagittal (pitch) plane has never been assessed. We investigated the subjective visual vertical in both planes in those patients. METHODS: Eight patients, 4 with spatial neglect (N+) and 4 without neglect (N-), were compared with 4 healthy participants (C). They sat facing a luminous bar adjustable in rotation, either in the roll or in the pitch plane, and had to orient it in a vertical position, in the dark. RESULTS: Compared with N- (-0.1 degrees) and C (+1.1 degrees) groups, N+ patients presented with a significant backward deviation (-4.5 degrees) of the SVV in pitch. In accordance with other studies, they also showed a significant anticlockwise deviation (-8.8 degrees) of the SVV in roll, as compared with N- (-1.9 degrees) and C (+0.4 degrees) subjects. This was associated with an opposite trunk deviation in both planes. CONCLUSIONS: While confirming the anticlockwise deviation already reported in the frontal plane, we showed for the first time to our knowledge a backward deviation of the SVV in neglect patients, which has to be put in relation with their balance disorders. (+info)
Vestibular perception and action employ qualitatively different mechanisms. I. Frequency response of VOR and perceptual responses during Translation and Tilt.
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To investigate the neural mechanisms that humans use to process the ambiguous force measured by the otolith organs, we measured vestibuloocular reflexes (VORs) and perceptions of tilt and translation. One primary goal was to determine if the same, or different, mechanisms contribute to vestibular perception and action. We used motion paradigms that provided identical sinusoidal inter-aural otolith cues across a broad frequency range. We accomplished this by sinusoidally tilting (20 degrees, 0.005-0.7 Hz) subjects in roll about an earth-horizontal, head-centered, rotation axis ("Tilt") or sinusoidally accelerating (3.3 m/s2, 0.005-0.7 Hz) subjects along their inter-aural axis ("Translation"). While identical inter-aural otolith cues were provided by these motion paradigms, the canal cues were substantially different because roll rotations were present during Tilt but not during Translation. We found that perception was dependent on canal cues because the reported perceptions of both roll tilt and inter-aural translation were substantially different during Translation and Tilt. These findings match internal model predictions that rotational cues from the canals influence the neural processing of otolith cues. We also found horizontal translational VORs at frequencies >0.2 Hz during both Translation and Tilt. These responses were dependent on otolith cues and match simple filtering predictions that translational VORs include contributions via simple high-pass filtering of otolith cues. More generally, these findings demonstrate that internal models govern human vestibular "perception" across a broad range of frequencies and that simple high-pass filters contribute to human horizontal translational VORs ("action") at frequencies above approximately 0.2 Hz. (+info)
Acrophobia and pathological height vertigo: indications for vestibular physical therapy?
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BACKGROUND AND PURPOSE: Acrophobia (fear of heights) may be related to a high degree of height vertigo caused by visual dependence in the maintenance of standing balance. The purpose of this case report is to describe the use of vestibular physical therapy intervention following behavioral therapy to reduce a patient's visual dependence and height vertigo. CASE DESCRIPTION: Mr N was a 37-year-old man with agoraphobia (fear of open spaces) that included symptoms of height phobia. Exposure to heights triggered symptoms of dizziness. Intervention. Mr N underwent 8 sessions of behavioral therapy that involved exposure to heights using a head-mounted virtual reality device. Subsequently, he underwent 8 weeks of physical therapy for an individualized vestibular physical therapy exercise program. OUTCOMES: After behavioral therapy, the patient demonstrated improvements on the behavioral avoidance test and the Illness Intrusiveness Rating Scale, but dizziness and body sway responses to moving visual scenes did not decrease. After physical therapy, his dizziness and sway responses decreased and his balance confidence increased. DISCUSSION: Symptoms of acrophobia and sway responses to full-field visual motion appeared to respond to vestibular physical therapy administered after completion of a course of behavioral therapy. Vestibular physical therapy may have a role in the management of height phobia related to excessive height vertigo. (+info)