G. Michael Halmagyi

Responses of guinea pig primary vestibular neurons to clicks

Responses of single neurons in the vestibular nerve to high-intensity clicks were studied by extracellular recording in anaesthetised guinea pigs. One hundred and two neurons in the posterior division of the superior branch or in the inferior branch of the vestibular nerve were activated at short latency by intense clicks. The latency of activation was short (median 0.9 ms) and the threshold was high: the click intensity for evoking the response of these cells was around 60 dB above the auditory brainstem response threshold. Animals were tilted and rotated to identify physiologically the sensory region of the labyrinth from which the activated neurons originated. Seventeen neurons responded to static tilt as well as clicks. These results show that vestibular receptors, probably the otoliths, respond to clicks at intensities corresponding to those used in a new clinical test of the vestibulo-collic pathway.

Impulsive Testing of Semicircular-Canal Function Using Video-oculography

The head impulse test (HIT) is a safe, quick way of assessing horizontal semicircular‐canal function in patients with peripheral vestibular loss. At the bedside, the clinician identifies “overt” catch‐up saccades back to the target after brisk passive head rotation as an indirect sign of canal paresis. However, saccades during head rotation (“covert” saccades) may not be detectable by the naked eye, and so lead to incorrect diagnosis. Up to now, the scleral search coil technique has been the standard for HIT measurement, but that technique is not practical for routine diagnostic use. A new lightweight, nonslip, high‐speed video‐oculography system (vHIT) that measures eye velocity during horizontal head impulses has been developed. This system is easy to use in a clinical setting, provides an objective measure of the vestibulo‐ocular reflex (VOR), and detects both overt and covert catch‐up saccades in patients with vestibular loss.

The video head impulse test (vHIT) of semicircular canal function – age dependent normative values of VOR gain in healthy subjects

Background/hypothesis The video Head Impulse Test (vHIT) is now widely used to test the function of each of the six semicircular canals individually by measuring the eye rotation response to an abrupt head rotation in the plane of the canal. The main measure of canal adequacy is the ratio of the eye movement response to the head movement stimulus, i.e., the gain of the vestibulo-ocular reflex (VOR). However, there is a need for normative data about how VOR gain is affected by age and also by head velocity, to allow the response of any particular patient to be compared to the responses of healthy subjects in their age range. In this study, we determined for all six semicircular canals, normative values of VOR gain, for each canal across a range of head velocities, for healthy subjects in each decade of life. Study design The VOR gain was measured for all canals across a range of head velocities for at least 10 healthy subjects in decade age bands: 10–19, 20–29, 30–39, 40–49, 50–59, 60–69, 70–79, 80–89. Methods The compensatory eye movement response to a small, unpredictable, abrupt head rotation (head impulse) was measured by the ICS impulse prototype system. The same operator delivered every impulse to every subject. Results Vestibulo-ocular reflex gain decreased at high head velocities, but was largely unaffected by age into the 80- to 89-year age group. There were some small but systematic differences between the two directions of head rotation, which appear to be largely due to the fact that in this study only the right eye was measured. The results are considered in relation to recent evidence about the effect of age on VOR performance. Conclusion These normative values allow the results of any particular patient to be compared to the values of healthy people in their age range and so allow, for example, detection of whether a patient has a bilateral vestibular loss. VOR gain, as measured directly by the eye movement response to head rotation, seems largely unaffected by aging.

Superior semicircular canal dehiscence simulating otosclerosis

This is a report of a patient with an air-bone gap, thought 10 years ago to be a conductive hearing loss due to otosclerosis and treated with a stapedectomy. It now transpires that the patient actually had a conductive hearing gain due to superior semicircular canal dehiscence. In retrospect for as long as he could remember the patient had experienced cochlear hypersensitivity to bone-conducted sounds so that he could hear his own heart beat and joints move, as well as a tuning fork placed at his ankle. He also had vestibular hypersensitivity to air-conducted sounds with sound-induced eye movements (Tullio phenomenon), pressure-induced nystagmus and low-threshold, high-amplitude vestibular-evoked myogenic potentials. Furthermore some of his acoustic reflexes were preserved even after stapedectomy and two revisions. This case shows that if acoustic reflexes are preserved in a patient with an air-bone gap then the patient needs to be checked for sound- and pressure-induced nystagmus and needs to have vestibular-evoked myogenic potential testing. If there is sound- or pressure-induced nystagmus and if the vestibular-evoked myogenic potentials are also preserved, the problem is most likely in the floor of the middle fossa and not in the middle ear, and the patient needs a high-resolution spiral computed tomography (CT) of the temporal bones to show this.

Maintained ocular torsion produced by bilateral and unilateral galvanic (DC) vestibular stimulation in humans

Abstract This study was designed to measure ocular movements evoked by galvanic (DC) stimulation using computerised video-oculography. Long duration (>30 s) galvanic vestibular stimulation at currents of up to 5 mA through large-area surface electrodes over the mastoid processes causes maintained changes in the ocular torsional position of both eyes in healthy human subjects. With the subject seated and the head held firmly, torsion was measured by a computer-based image-processing system (VTM). Torsion was recorded in darkness, with or without a single fixation point. With bilateral stimulation, the upper poles of both eyes always torted away from the side of cathode placement and toward the anode. For unilateral stimulation, torsion was directed away from the cathode or toward the anode. The magnitude of ocular torsion was dependent on current strength: with bilateral stimulation the peak torsion was on average 2.88° for 5-mA current intensity compared with 1.58° for 3 mA. A smaller amplitude of torsion was obtained for unilateral stimulation. The average peak torsion was the same for both eyes for all forms of stimulation. Our findings indicate that low-intensity galvanic stimulation evokes ocular torsion in normal subjects, an effect which is consistent with an action on otolith afferents.

The vestibular evoked-potential profile of Ménière’s disease

OBJECTIVE To define the ocular and cervical vestibular evoked myogenic potential (oVEMP and cVEMP) profile in Ménières Disease (MD), we studied air-conducted (AC) sound and bone-conducted vibration (BCV)-evoked responses in 77 patients and 35 controls. METHODS oVEMPs were recorded from unrectified infra-orbital surface electromyography (EMG) during upward gaze. cVEMPs were recorded from rectified and unrectified sternocleidomastoid EMG during head elevation against gravity. Responses to AC clicks delivered via headphones and BC forehead taps delivered with a mini-shaker (bone-conduction vibrator) and a triggered tendon-hammer were recorded. RESULTS In clinically definite unilateral MD (n=60), the prevalence of unilateral VEMP abnormalities was 50.0%, 10.2% and 11.9% for click, minitap and tendon-hammer evoked oVEMPs, 40.0%, 22.8% and 10.7% for click, minitap and tendon-hammer evoked cVEMPs. The most commonly observed profile was abnormality to AC stimulation alone (33.3%), followed by abnormalities to both AC and BCV stimuli (26.7%). Isolated abnormalities to BCV stimuli were rare (5%) and limited to the minitap cVEMP. The prevalence of abnormalities for each of the AC VEMPs was significantly higher than for any one BCV VEMP. For click cVEMP, click oVEMP and minitap cVEMP, average Reflex Asymmetry Ratios (AR) were significantly higher in MD compared with controls. Test results for AC cVEMP, AC oVEMP, minitap cVEMP and caloric asymmetry were significantly correlated with hearing loss. CONCLUSIONS Predominance of abnormalities in oVEMP and cVEMP responses to AC sound is characteristic of MD and indicative of saccular involvement. SIGNIFICANCE This pattern of VEMP abnormalities may enable separation of Ménières disease from other peripheral vestibulopathies.

A Mathematical Model of Human Semicircular Canal Geometry: A New Basis for Interpreting Vestibular Physiology

We report a precise, simple, and accessible method of mathematically measuring and modeling the three-dimensional (3D) geometry of semicircular canals (SCCs) in living humans. Knowledge of this geometry helps understand the development and physiology of SCC stimulation. We developed a framework of robust techniques that automatically and accurately reconstruct SCC geometry from computed tomography (CT) images and are directly validated using micro-CT as ground truth. This framework measures the 3D centroid paths of the bony SCCs allowing direct comparison and analysis between ears within and between subjects. An average set of SCC morphology is calculated from 34 human ears, within which other geometrical attributes such as nonplanarity, radius of curvature, and inter-SCC angle are examined, with a focus on physiological implications. These measurements have also been used to critically evaluate plane fitting techniques that reconcile many of the discrepancies in current SCC plane studies. Finally, we mathematically model SCC geometry using Fourier series equations. This work has the potential to reinterpret physiology and pathophysiology in terms of real individual 3D morphology.

Cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS): a review of the clinical features and video-oculographic diagnosis

The association of bilateral vestibulopathy with cerebellar ataxia was first reported in 1991 and delineated as a distinct syndrome with a characteristic and measurable clinical sign—an absent visually enhanced vestibulo‐ocular reflex—in 2004. We reviewed 27 patients with this syndrome and show that a non‐length‐dependent sensory deficit with absent sensory nerve action potentials is an integral component of this syndrome, which we now call “cerebellar ataxia with neuropathy and bilateral vestibular areflexia syndrome” (CANVAS). All patients had brain MRI and 22/27 had evidence of cerebellar atrophy involving anterior and dorsal vermis, as well as the hemispheric crus I. Brain and temporal bone pathology in one patient showed marked loss of Purkinje cells and of vestibular, trigeminal, and facial ganglion cells, but not of spiral ganglion cells. There are two sets of sibling pairs, suggesting CANVAS is a late‐onset recessive disorder. The characteristic clinical sign—the visual vestibulo‐ocular reflex deficit—can be demonstrated and measured clinically using video‐oculography.

Vestibular Abnormalities in Charge Association

We report the vestibular abnormalities in 5 patients with the CHARGE association (Coloboma, Heart disease, Atresia of choanae, Retarded growth and development and/or central nervous system anomalies, Genital hypoplasia, and Ear anomalies). All patients had absent vestibular function as indicated by absent vestibulo-ocular reflexes and severe imbalance on simultaneous deprivation of proprioception and vision, as well as delayed motor development. All 6 semicircular canals were aplastic in each of the patients. While cochlear function was severely reduced in 6 of the 10 ears, it was absent only in 3 ears and was actually intact below 3 kHz in 1 ear. All 10 bony cochleas were present on computed tomography, and although 7 appeared abnormal, 3 appeared normal. This study confirms that absence of the bony semicircular canals in the presence of a bony cochlea is a characteristic finding in CHARGE association. It also demonstrates that these disproportionate structural abnormalities are reflected in the functional abnormalities: absent vestibular function with preservation of some cochlear function.

Stenting of the transverse sinuses in idiopathic intracranial hypertension

Although the mechanism of idiopathic intracranial hypertension (IIH) is uncertain, the cerebral venous system has been implicated since the 1930s. Dandy (1) hypothesized that the volume of cerebrospinal fluid (CSF) or cerebral blood might be increased. Others postulated that the cerebral microvasculature was the source of cerebral edema in this condition (2–4). Intracranial venous hypertension was subsequently proposed as a unifying mechanism or final common pathway of IIH. Johnston and Paterson (5) first suggested that increased sagittal sinus pressure causing decreased CSF absorption was the underlying cause of IIH. A syndrome clinically identical to IIH is produced by cerebral venous sinus thrombosis (6–8). Interest in the dural venous sinuses as the source of increased intracranial pressure (ICP) in IIH was heightened in the 1990s, with studies of direct venous manometry in IIH patients and studies performed in IIH patients undergoing bariatric surgery. Intracranial and central systemic venography and manometry were performed in 10 patients with increased ICP associated with various disorders (1 congenital stenosis, 2 idiopathic, 5 morbid obesity, 1 tumor compressing the dural sinus, 1 craniodiaphyseal dysplasia, and bony overgrowth of the skull) (9). All patients had CSF pressures above 200 mm H2O and had no ventriculomegaly. No venous outflow obstruction was found in the obese patients, but some degree of stenosis or occlusion was seen in the other 5 patients (including those with congenital stenosis and tumor). Superior sagittal sinus pressure was elevated in all 7 patients in whom it was measured, including the 5 obese patients. The mean increase was small (1.8 mm Hg above normal) in patients without obstruction, and it is uncertain whether the elevation was statistically significant. Central venous pressure was measured in 6 of the 7 patients who had venous sinus manometry and was abnormal in 5 patients. Patients with venous sinus occlusion were treated with angioplasty/thrombolysis or shunting. The patient with the tumor underwent a CSF diversion procedure (shunt), and the patients with IIH were treated with a combination of shunt, optic nerve sheath fenestration, or gastric stapling. None of the manometric measurements was repeated following treatment.