P. P. Vidal

Vestibular projections in the human cortex.

Abstract. There is considerable evidence from studies on cats and monkeys that several cortical areas such as area 2v at the tip of the intraparietal sulcus, area 3av in the sulcus centralis, the parietoinsular vestibular cortex adjacent to the posterior insula (PIVC) and area 7 in the inferior parietal lobule are involved in the processing of vestibular information. Microelectrode recordings from these areas have shown that: (1) most of these cortical neurons are connected trisynaptically to the labyrinthine endorgans and (2) they receive converging vestibular, visual and somatosensory inputs. These data suggest that a multimodal cortical system is involved in postural and gaze control. In humans, recent positron emission tomography (PET) scans and functional magnetic resonance imaging (fMRI) studies have largely confirmed these data. However, because of the limited temporal resolution of these two methods, the minimum time of arrival of labyrinthine inputs from the vestibular hair cells to these cortical areas has not yet been determined. In this study, we used the evoked potential method to attempt to answer this question. Due to its excellent temporal resolution, this method is ideal for the investigation of the tri- or polysynaptic nature of the vestibulocortical pathways. Eleven volunteer patients, who underwent a vestibular neurectomy due to intractable Menieres disease (MD) or acoustic neurinoma resection, were included in this experiment. Patients were anesthetized and the vestibular nerve was electrically stimulated. The evoked potentials were recorded by 30 subcutaneous active electrodes located on the scalp. The brain electrical source imaging (BESA) program (version 2.0, 1995) was used to calculate dipole sources. The latency period for the activation of five distinct cortical zones, including the prefrontal and/or the frontal lobe, the ipsilateral temporoparietal cortex, the anterior portion of the supplementary motor area (SMA) and the contralateral parietal cortex, was 6xa0ms. The short latency period recorded for each of these areas indicates that several trisynaptic pathways, passing through the vestibular nuclei and the thalamic neurons, link the primary vestibular afferents to the cortex. We suggest that all these areas, including the prefrontal area, process egomotion information and may be involved in planning motor synergies to counteract loss of equilibrium.

A radiological analysis of the postural syndromes following hemilabyrinthectomy and selective canal and otolith lesions in the guinea pig.

SummaryX-ray photography was employed to analyse the postural syndromes following unilateral global and selective lesions of the vestibular apparatus in guinea pigs. Following hemilabyrinthectomy, head rotation in the horizontal plane resulted mainly from rotation of the cervical vertebrae whereas lateral head tilt was due to the rotation of thoracic vertebrae about the longitudinal axis. These results support our previous conclusion that because of both resting posture and intrinsic biomechanical constraints, the number of degrees of freedom of the cervical column is reduced and the cervical vertebrae are functionally specialized. Selective lesions of the vestibular apparatus have aided in determining the origin of the different components of the hemilabyrinthectomy syndrome: rotation of thoracic vertebrae was caused by unilateral lesion of the otolithic system whereas rotation of the head in the horizontal plane resulted from unilateral lesion of the horizontal semicircular canal system. We conclude that the functional segmentation of the cervical column corresponds to a differential distribution of vestibular afferents.

NMDA receptors contribute to the resting discharge of vestibular neurons in the normal and hemilabyrinthectomized guinea pig

SummaryExcitatory amino acids (EAA) like L-Glutamate or L-Aspartate have been suggested to be the neurotransmitters at the synapses between primary vestibular afferents and second-order vestibular neurons. In the first part of our work, we have tested the possibility that EAA receptors are implicated in the control of posture by vestibular nuclei. Normal guinea pigs were implanted with minipumps delivering EAA antagonists in the vestibular nuclei. Their resting posture was monitored during the perfusion by using an X-ray photographic method. Chronic infusion of D-L-2-amino-5-phosphonovaleric acid (APV), a specific antagonist of NMDA receptors, in the vestibular nuclei induced a postural and oculomotor syndrome similar to the one observed following acute vestibular deafferentation. Administration of 6-cyano-7-nitro-quinoxaline-2-3-dione (CNQX), a specific antagonist of kainate and quisqualate receptors, failed to induce any postural syndrome or eye deviation. These results suggest that, under physiological conditions, N-methyl-D-aspartate (NMDA) receptors, contrary to kainate and quisqualate receptors, are essential for the maintenance of a symmetric posture and of a normal eye position at rest. Previous electrophysiological studies have demonstrated that following unilateral labyrinthectomy the recovery of a resting discharge in the deafferented vestibular nuclei plays a key role in the compensation of postural disorders. In the second part of this study, we have tested whether NMDA receptors could be implicated in this postural recovery. APV minipumps were implanted in hemilabyrinthectomized guinea pigs after complete compensation. A postural decompensation was induced, which occurred after delivery of the same amount of APV which provoked a vestibular syndrome in intact guinea pigs. This result favors the hypothesis that denervation supersensitivity resulting from an increase in either the number and/or the sensitivity of NMDA receptors could be a factor in the recovery of the static syndrome following hemilabyrinthectomy.

Intratympanic gentamicin injections for Meniere disease: vestibular hair cell impairment and regeneration.

The authors treated 22 patients with intratympanic gentamicin. Vestibular function was measured using caloric and head impulse tests and vestibular evoked myogenic potentials induced by high amplitude sounds and short duration galvanic currents. Roughly one-third of the patients, after initially losing their caloric responses and displaying refixation saccades to head impulse tests, recovered within 2 years of the lesion. Vertigo did not recur in patients in whom the galvanic response was abolished.

Sensory reweighting in controls and stroke patients

OBJECTIVEnTo test sensitivity to proprioceptive, vestibular and visual stimulations of stroke patients with regard to balance.nnnMETHODnThe postural control of 20 hemiparetic patients after a single hemispheric stroke that had occurred at least 6 months before the study along with 20 controls was probed with vibration, optokinetic, and vestibular galvanic stimulations. Balance was assessed using a force platform (PF) with two miniature inertial sensors placed on the head (C1) and the trunk (C2) under each sensory condition and measured by three composite scores as the mean displacement of the body (PF, C1, C2) during the stimulation. A subject with a composite score greater than the 75th percentile of the composite scores found in the control subjects was arbitrarily considered to be sensitive to that stimulation.nnnRESULTSnBoth control and stroke patients showed large inter-individual variations in response to the three types of sensory stimulation. Among the hemiparetic patients, nearly 65% were sensitive to the optokinetic stimulation, 60% to the galvanic stimulation and 65% to the vibration stimulation. In contrast to the control group, all the hemiparetic subjects were sensitive to at least one type of stimulation.nnnCONCLUSIONnStroke patients are highly dependent on visual, proprioceptive and vestibular information in order to control their standing posture and individually differ in their relative sensitivity to each type of sensory stimulation.nnnSIGNIFICANCEnContrarily to what one might suppose, the increased visual dependence manifested by stroke patients does not necessarily entail any neglect of proprioceptive and vestibular information.

Chapter 22 Vestibular control of skeletal geometry in the guinea pig: a problem of good trim?

Motor control of different segments of the body with multiple degrees of freedom appears to be coordinated by utilizing preferred axes of motor activity. This hypothesis may also be applied to vestibular control of posture. To explore this question we studied the anatomical relationship between the head and the cervical vertebral column by taking radiographs of the head-neck region in unrestrained alert guinea pigs. We determined that biomechanical constraints contribute to the stereotypical skeletal geometry observed in the resting animal and to a functional segmentation of the head-neck movement apparatus. Subsequent lesion studies of vestibular end organs with quantification of the resulting postural syndromes suggest that the functional segmentation of the cervical vertebral column corresponds to a functional partitioning of vestibular afferents. Our findings also indicate that the sensorimotor transformation mechanisms necessary to convert a given head velocity signal into the appropriate neck motor frame are already embedded in the networks provided by second-order vestibular neurons. Good trim of postural control will be the end result of an appropriate internal representation of the objective vertical.

Early post-stroke period: A privileged time for sensory re-weighting?

BACKGROUNDnShortly after stroke, patients exhibit excessive sensitivity to visual, proprioceptive and vestibular perturbations regarding balance control.nnnOBJECTIVEnTo evaluate the stability of this perceptual behaviour after stroke and test the relationships between sensory sensitivity and balance.nnnMETHODSnThirty subjects following a hemispheric stroke (mean age 54.7 (standard deviation (SD) 10.6 years), 21 men, right hemisphere lesionu2009=u200913) and 30 control subjects (mean age 52.0 (SD 12.0), 14 men). Sensitivity to sensory perturbations was evaluated using the displacement of the centre of pressure during tendon vibration (proprioception score), optokinetic (visual score) and galvanic perturbations (vestibular score) while standing on a force-platform a mean of 2 months after stroke, and 1 month later. Balance and independence were evaluated using the Berg Balance Scale (BBS), Timed Up and Go test (TUG) and Barthel Index (BI).nnnRESULTSnGlobal sensitivity to perturbations decreased (pu2009=u20090.001). Patients remained more sensitive to visual perturbation than did controls (pu2009=u20090.033). The Vestibular Score was correlated with BBS (Rsu2009=u2009-0.576, pu2009=u20090.006), TUG (Rsu2009=u20090.408, pu2009=u20090.045), BI (Rsu2009=u2009-0.481, pu2009=u20090.016); the Visual Score was correlated with BBS (Rsu2009=u2009-0.500, pu2009=u20090.019), TUG (Rsu2009=u20090.401, pu2009=u20090.049).nnnCONCLUSIONnThe initial months following stroke appear to be a period of individual perceptual motor adaptation. Sensory re-weighting is likely to be a major component of that process.

Auditory outcomes after implantation and electrical stimulation of the lateral ampullar nerve in guinea pig.

Objective: To determine in a guinea pig model the factors of invasiveness of a bipolar electrode implanted in the horizontal semicircular canal (HSC) and to evaluate the consequences on hearing of electrical stimulation of the ampullary nerve. Design: Sixteen guinea pigs divided into four groups underwent surgical opening of the HSC of one ear as follows: control (group 1), cyanoacrylate glue application on the HSC opening (group 2), electrode implantation with cyanoacrylate glue on the HSC opening (group 3), and electrode implantation with electrical stimulation (1 hr/day) for 9 days (group 4). Auditory brainstem responses were recorded before and after surgery and after electrical stimulation. The effectiveness of electrical stimulation in producing a horizontal vestibulo-ocular reflex was evaluated by recording eye movement with video-oculography. Results: Group 1 animals showed hearing loss, and in group 2, sealing the HSC opening with cyanoacrylate glue preserved the hearing thresholds. After electrode implantation, seven of the eight animals showed hearing loss compared with preoperative values. Electrical stimulation did not induce additional hearing loss. Conclusion: Electrode implantation at the canal level entailed a risk of hearing loss in an animal model, but electrical stimulation of the horizontal ampullary nerve did not further alter hearing function.

Probing residual vestibular function with galvanic stimulation in vestibular loss patients.

Introduction: Galvanic vestibular stimulation (GVS) was used to probe the residual vestibular function in unilateral vestibular loss. Methods: 39 control subjects, 60 patients areflexic to caloric tests, 21 patients with pre-operative acoustic neurinoma, and 26 patients with surgical deafferentation were tested with 4mA trapezoidal GVS. Horizontal slow phase velocity and vertical slow phase velocity components of the slow phase of induced nystagmus were recorded with videonystagmography. Results: All patients had lower horizontal slow phase velocity compared with intact subject. In caloric areflexic patients, the intact ear responded similarly to controls. The lesioned ear was less excitable but could be inhibited as in controls. In patients with acoustic neurinoma, the intact ear was normally excitable but less sensitive to inhibition. The lesioned ear, although not excitable, could be inhibited. In patients with surgical deafferentation, the intact ear could be barely excited and not inhibited. The lesioned ear could not be excited but slightly inhibited. These results fit well with previous data in animal models after unilateral vestibular loss. Conclusion: Unilateral vestibular patients exhibit a progressive deterioration of the horizontal canal function, corresponding to 4 different patterns of GVS response. This study confirms that GVS is useful to probe evolution of the residual vestibular function in patients.

An Automated Recording Method in Clinical Consultation to Rate the Limp in Lower Limb Osteoarthritis.

For diagnosis and follow up, it is important to be able to quantify limp in an objective, and precise way adapted to daily clinical consultation. The purpose of this exploratory study was to determine if an inertial sensor-based method could provide simple features that correlate with the severity of lower limb osteoarthritis evaluated by the WOMAC index without the use of step detection in the signal processing. Forty-eight patients with lower limb osteoarthritis formed two severity groups separated by the median of the WOMAC index (G1, G2). Twelve asymptomatic age-matched control subjects formed the control group (G0). Subjects were asked to walk straight 10 meters forward and 10 meters back at self-selected walking speeds with inertial measurement units (IMU) (3-D accelerometers, 3-D gyroscopes and 3-D magnetometers) attached on the head, the lower back (L3-L4) and both feet. Sixty parameters corresponding to the mean and the root mean square (RMS) of the recorded signals on the various sensors (head, lower back and feet), in the various axes, in the various frames were computed. Parameters were defined as discriminating when they showed statistical differences between the three groups. In total, four parameters were found discriminating: mean and RMS of the norm of the acceleration in the horizontal plane for contralateral and ipsilateral foot in the doctor’s office frame. No discriminating parameter was found on the head or the lower back. No discriminating parameter was found in the sensor linked frames. This study showed that two IMUs placed on both feet and a step detection free signal processing method could be an objective and quantitative complement to the clinical examination of the physician in everyday practice. Our method provides new automatically computed parameters that could be used for the comprehension of lower limb osteoarthritis. It may not only be used in medical consultation to score patients but also to monitor the evolution of their clinical syndrome during and after rehabilitation. Finally, it paves the way for the quantification of gait in other fields such as neurology and for monitoring the gait at a patient’s home.