Biomedical subjects

H Galiana

Publications and source records attributed to H Galiana.

3 recordsLinked to original sources

PubMed2000

Head-shaking nystagmus (HSN): the theoretical explanation and the experimental proof.

Head-shaking nystagmus (HSN) is induced by oscillating the head at high frequency in the horizontal plane. This test is used in the clinic to detect the presence of a unilateral loss of vestibular function. HSN has been described as monophasic with fast-phase direction towards either side, or biphasic with the direction of fast phases reversing after a few seconds. Loss of vestibular function amplifies existing non-linearities in the vestibular system, so that imposed sinusoids can induce biases which are the source of HSN. Fifty-one patients suffering from loss of peripheral vestibular function (43 partial, 11 total unilateral tests) were exposed to whole-body sinusoidal stimulation, with increasing head velocities (90-220 degrees/s) at 1/6Hz, to explore the consistency of per-rotatory induced biases. A bias was induced in all cases, but it wandered on either side, healthy or pathologic, unless test head velocities were larger than approximately 180 degrees/s. Given this condition, the slow-phase bias was located towards the pathologic side for all patients with significant bias ( > 5 degrees/s). These observations demonstrate that the sign and amplitude of the bias is variable and is not correlated with the lesioned side, unless high head velocities are imposed. This explains why the direction of the initial phase of HSN in the clinic seems so labile. Subsequent monophasic or biphasic characteristics of HSN are simply the reflection of interactions between two main time constants associated with "velocity storage" and "gaze holding" in the vestibular central processes.

Adult↗

PubMed1991

Does the vestibulo-ocular reflex (VOR) merit redefinition?

The VOR has historically been described by parameters of the slow phase of nystagmus while fast phases have been systematically ignored. However, ample neurophysiologic and theoretical evidence suggests that fast phases are an integral part of the VOR and are not to be discarded in evaluation of vestibular function. In a series of experiments, normal subjects and compensated patients with total or partial loss of unilateral peripheral vestibular function were tested. They were exposed to sinusoidal oscillations (20 degrees, 60 degrees, and 120 degrees/second at 1/6 Hz and 25 degrees/second at 1 Hz) with no actual or imaginary visual targets (doing mental arithmetic) and the usual slow-phase parameters (gain, phase, etc.) were computed. The gaze (total eye position, slow and fast phase of nystagmus vs. head + head position in space) was also plotted vs. head position in space. It was found that in patients, fast phases of nystagmus are unequally (right vs. left) produced, extending the linear range of VOR function. It was also found that in normal subjects and patients, gaze essentially tracks the head in the absence of visual targets. It is concluded that fast phases of nystagmus contribute to gaze control during head rotations in darkness and that the definition and the testing of the VOR have to be modified to accommodate these observations.

Eye Movements↗

PubMedDate not supplied

Postural instability on one foot in patients with loss of unilateral peripheral vestibular function.

The vestibular system is a major contributor to postural control and the vestibular nuclei are closely connected to the limb spinal motor neurons. Acute loss of unilateral peripheral vestibular function results in, among others, postural instability which improves with time due to central compensatory processes. It has been shown, however, that a number of processes, such as sensorimotor restriction, drugs, and so forth, may affect the speed and the quality of compensation. Due to the difference of ipsilateral vs contralateral connectivity between the vestibular nuclei and the limb spinal motor neurons, and the peculiarities of compensatory processes, the ipsilateral may not compensate as well as the contralateral foot to lesion in patients who lost unilateral peripheral vestibular function. To explore this possibility, the sway, while standing on each foot, was recorded in normal subjects (N = 30) and compensated (except three) patients (N = 37) with loss of unilateral peripheral vestibular function and the percentage difference between the two sways was calculated. No difference was found between patients swaying more on the ipsilateral vs patients swaying more on the contralateral foot to lesion (p > .80). There was, however, a statistical difference between patients as a group vs normal subjects as a group (p = .005). These data suggest, although not evident in all patients of the group, that sway on the contralateral foot was less than the sway on the ipsilateral foot to lesion, as initially speculated. They may, however, signify that one foot compensates better than the other. This is a preliminary report and more testing is on-going to confirm these results.

Adolescent↗