Ocular tilt reaction resulting from vestibuloacoustic nerve surgery

Vestibular Test Results in Patients With Horizontal Canal Benign Paroxysmal Positional Vertigo

Introduction

While the mechanism of posterior canal benign paroxysmal positional vertigo (BPPV) is widely accepted as canalolithiasis, the pathophysiology of horizontal canal BPPV remains controversial. We seek to analyze vestibular test results of patients with horizontal canal BPPV with ageotropic nystagmus (AHC) and geotropic nystagmus (GHC) in comparison to patients with posterior canal BPPV (PC) to better understand its pathophysiology.

Methods

In a retrospective chart review of adults with BPPV at a tertiary referral balance center, we reviewed the clinical characteristics and compared videonystagmography, caloric, rotary chair, subjective visual vertical (SVV)/ subjective visual horizontal (SVH), and vestibular evoked myogenic potential (VEMP) results between groups.

Results

We included 11 AHC and seven GHC patients and randomly selected 20 PC patients as the comparison group. All groups had a high rate of migraine and low rates of diabetes and head trauma, but no difference between groups. Ipsilateral caloric weakness was more prevalent in the GHC group compared to the PC group (p=0.02). One of two AHC patients and both GHC patients who had SVV/SVH testing had abnormal findings. The only AHC patient who had ocular VEMP testing had abnormal results. Additionally, we observed a significant downbeating component to nystagmus (4 deg/sec or greater) exclusively in the AHC group (5/10 patients, p=0.001).

Conclusions

Patients with AHC and GHC have unique vestibular testing results. In particular, only AHC patients showed a downbeating component to their nystagmus, which may suggest utricular dysfunction in the pathophysiology of AHC.

The Anatomical and Physiological Basis of Clinical Tests of Otolith Function. A Tribute to Yoshio Uchino

Otolithic receptors are stimulated by gravitoinertial force (GIF) acting on the otoconia resulting in deflections of the hair bundles of otolithic receptor hair cells. The GIF is the sum of gravitational force and the inertial force due to linear acceleration. The usual clinical and experimental tests of otolith function have used GIFs (roll tilts re gravity or linear accelerations) as test stimuli. However, the opposite polarization of receptors across each otolithic macula is puzzling since a GIF directed across the otolith macula will excite receptors on one side of the line of polarity reversal (LPR at the striola) and simultaneously act to silence receptors on the opposite side of the LPR. It would seem the two neural signals from the one otolith macula should cancel. In fact, Uchino showed that instead of canceling, the simultaneous stimulation of the oppositely polarized hair cells enhances the otolithic response to GIF—both in the saccular macula and the utricular macula. For the utricular system there is also commissural inhibitory interaction between the utricular maculae in each ear. The results are that the one GIF stimulus will cause direct excitation of utricular receptors in the activated sector in one ear as well as indirect excitation resulting from the disfacilitation of utricular receptors in the corresponding sector on the opposite labyrinth. There are effectively two complementary parallel otolithic afferent systems—the sustained system concerned with signaling low frequency GIF stimuli such as roll head tilts and the transient system which is activated by sound and vibration. Clinical tests of the sustained otolith system—such as ocular counterrolling to roll-tilt or tests using linear translation—do not show unilateral otolithic loss reliably, whereas tests of transient otolith function [vestibular evoked myogenic potentials (VEMPs) to brief sound and vibration stimuli] do show unilateral otolithic loss. The opposing sectors of the maculae also explain the results of galvanic vestibular stimulation (GVS) where bilateral mastoid galvanic stimulation causes ocular torsion position similar to the otolithic response to GIF. However, GVS stimulates canal afferents as well as otolithic afferents so the eye movement response is complex.

Skew Deviation: Case Report and Review of the Literature

OBJECTIVE

To present a patient with a sudden onset ocular tilt reaction (OTR) and review recent knowledge and evolving insights of the underlying pathophysiological mechanisms of skew deviation and OTR.

METHODS

A middle-aged hypertensive man who had previously suffered stroke with good recovery presented with sudden-onset double vision, slurred speech, ataxia, and a head tilt. Romberg test was positive. The patient denied having disturbances of visual acuity, eye pain, or recent trauma. The right eyeball was pushed upward. The patient complained of double vision in any gaze direction. Movements of the extraocular muscles (EOMs) in the horizontal plane were normal, whereas vertical version and convergence were not possible. We administered a Hess-Lancaster test, cover test, fundoscopic examination, Parks-Bielschowsky three-step test, upright-supine test, brain magnetic resonance imaging (MRI), transcranial doppler (TCD) ultrasonography, electrocardiogram (ECG), Holter monitor (24 h), and echocardiography.

RESULTS

The Hess-Lancaster test showed superior rectus muscle and inferior obliquus muscle palsy to the left and rectus inferior muscle and superior obliquus muscle palsy to the right. The right eyeball fell behind when looking downward and the left eyeball when looking upward. Cover alternating test was positive from vertical, R/L. Examination of the ocular fundus showed incyclotorsion of elevated right eye and excyclotorsion of depressed eye. The Parks-Bielschowsky three-step test was negative. A brain MRI with gadolinium revealed a small zone of diffusion restriction in the medial portion of the right cerebral peduncle and right thalamus. There was a gradual improvement in the patient's neurological status following treatment.

CONCLUSION

Skew deviation, a not uncommon clinical condition, should be promptly recognized when binocular vertical diplopia cannot be interpreted by trochlearis and oculomotor nerve lesion, myasthenia gravis, or orbital pathology. Maddox rod, cover test, Parks-Bielschowsky three-step, and other tests should help to establish the diagnosis. The prognosis depends on etiology, but it is commonly favorable; the majority of patients recover spontaneously after less than a year. More invasive management options should be discussed thereafter.

Skew Deviation Revisited

Skew deviation is a vertical misalignment of the eyes caused by damage to prenuclear vestibular input to ocular motor nuclei. The resultant vertical ocular deviation is relatively comitant in nature, and is usually seen in the context of brainstem or cerebellar injury from stroke, multiple sclerosis, or trauma. Skew deviation is usually accompanied by binocular torsion, torticollis, and a tilt in the subjective visual vertical. This constellation of findings has been termed the ocular tilt reaction. In the past two decades, a clinical localizing value for skew deviation has been assigned, and a cogent vestibular mechanism for comitant and incomitant variants of skew deviation has been proposed. Our understanding of skew deviation as a manifestation of central otolithic dysfunction in different planes of three-dimensional space is evolving. The similar spectrum of vertical ocular deviations arising in patients with congenital strabismus may further expand the nosology of skew deviation to include vergence abnormalities caused by the effects of early binocular visual imbalance on the developing visual system.

[Ocular tilt reaction. A rare complication after acoustic neuroma surgery]

Visual disturbances after acoustic neuroma surgery are very rare complications. We present a patient with diplopia after a middle fossa approach for resection of an acoustic neuroma. The patient presented ocular torsion and tilts of subjective visual vertical which is typical for the ocular tilt reaction. It is generally agreed that this reaction is secondary to a dysfunction of the tonic bilateral vestibular inputs that stabilize the eyes and head in a normal upright position in the roll plane. It has also been described after denervation of the utricle and injury of the brain stem.

Surgical management of skew deviation

There are no published data on the outcomes of realignment surgery for skew deviation. A retrospective chart review disclosed 10 patients who had undergone surgical correction of skew deviation by three surgeons at a single institution between 1991 and 2002. Nine of 10 patients had satisfactory relief of diplopia with an acceptable field of single binocular vision. Vertical rectus recession or resection was the most common procedure. Four patients required more than one procedure. For nonalternating hypertropias, resection of the inferior rectus muscle or recession of the superior rectus muscle of the hypertropic eye was successful. For alternating hypertropia, resection of both inferior rectus muscles was successful. Oblique muscle surgery was not associated with good outcomes.

Vertical divergence and counterroll eye movements evoked by whole-body position steps about the roll axis of the head in humans

In healthy human subjects, a head tilt about its roll axis evokes a dynamic counterroll that is mediated by both semicircular canal and otolith stimulation, and a static counterroll that is mediated by otolith stimulation only. The vertical ocular divergence associated with the static counterroll too is otolith-mediated. A previous study has shown that, in humans, there is also a vertical divergence during dynamic head roll, but this report was not conclusive on whether this response was mediated by the semicircular canals only or whether the otoliths made a significant contribution. To clarify this issue, we applied torsional whole-body position steps (amplitude 10 degrees, peak acceleration of 90 degrees /s(2), duration 650 ms) about the earth-vertical (supine body position) and earth-horizontal (upright body position) axis to healthy human subjects who were monocularly fixating a straight-ahead target. Eye movements were recorded binocularly with dual search coils in three dimensions. The dynamic parameters were determined 120 ms after the beginning of the turntable movement, i.e., before the first fast phase of nystagmus. The static parameters were measured 4 s after the beginning of the turntable movement. The dynamic gain of the counterroll was larger in upright (average gain: 0.48 +/- 0.10 SD) than in supine (0.36 +/- 0.10) position. The static gain of the counterroll in the upright position (0.21 +/- 0.06) was smaller than the dynamic gain. Divergent eye movements (intorting eye hypertropic) evoked during the dynamic phase were not significantly different between supine (average vergence velocity: 0.87 +/- 0.51 degrees /s) and upright (0.84 +/- 0.64 degrees /s) positions. The static vertical divergence in upright position was 0.32 +/- 0.14 degrees. The results indicate that the dynamic vertical divergence in contrast to the dynamic ocular counterroll is not enhanced by otolith input. These results can be explained through the different patterns of connectivity between semicircular canals and utricles to the eye muscles. Alternatively, we hypothesize that the small dynamic vertical divergence represents the remaining vertical error necessary to drive an adaptive control mechanism that normally maintains a vertical eye alignment.

Assessing otolith function by the subjective visual vertical

The effects of peripheral vestibular diseases on the subjective visual vertical (SVV) are resumed and provide the basis for some insights into the otolith pathophysiology. With a normal range of 0 +/- 2 deg (when measured in an upright body position), the SVV was shifted by 11 +/- 6 deg toward the ipsilateral ear in 40 patients following an acute unilateral vestibular deafferentiation (UVD), but in the opposite direction in 9 of 52 patients after stapes surgery. These opposite effects suggest a push-pull mechanism of the pairs of otolith organs with respect to the SVV. The dissociation between the SVV and the perception of body position indicates influences by unconscious reflexive mechanisms such as ocular cyclotorsion on the SVV. In chronic UVD patients, lateral shifts of the subjects during constant angular velocity rotation into various eccentric positions (+/- 16 cm) revealed a shift of the "center of graviception" close to the remaining intact contralateral inner ear. To date, this seems to be the most consistent test for clinical identification of a chronic compensated unilateral loss of otolith function. The findings regarding asymmetries in otolithic sensitivity to medially and laterally directed roll-tilts remain controversial, probably mainly because of influences of extravestibular cues.

Fast eye movement initiation of ocular torsion in mesodiencephalic lesions

Three patients with episodic ocular torsion and skew deviation due to mesodiencephalic lesions were studied by using binocular three-dimensional scleral search coils. The conjugate ocular torsion (upper pole of each eye rotating toward the side of the brainstem lesion) was initiated by a torsional fast eye movement. During prolonged episodes, torsional nystagmus was also present. Cessation of the ocular torsion and skew deviation occurred by slow eye movements with exponentially decreasing velocities in 2 patients, and by multiple fast torsional movements in 1 patient. In 1 patient, the abnormal eye movements were temporally linked to dystonic movements in the limbs on the side opposite the brainstem lesion. The occurrence of skew deviation with conjugate ocular torsion in brainstem lesions has been attributed to functional asymmetry in vestibular pathways responsible for the slow-phase compensatory eye movement response to roll. In comparison, the findings in our patients show that in mesodiencephalic lesions conjugate ocular torsion with skew deviation may be generated by torsional fast eye movements, indicating activation of the burst cells of the rostral interstitial nucleus of the medial longitudinal fasciculus.

Vertigo

Insight into the pathomechanisms of the benign positional vertigo syndromes has led to quick and effective treatment with positional manoeuvres. Our understanding of other causes of vertigo is progressing at a slower pace, but several recent findings regarding vestibular neuritis, vascular compression of the eighth nerve and psychogenic vertigo are of immediate clinical relevance.

Skew deviation following vestibular nerve surgery

Eighteen consecutive patients undergoing vestibular nerve surgery underwent pre- and postoperative examination of ocular motility. Five patients developed a skew deviation following surgery, with the lower eye on the operated side and an incomitant pattern of deviation in all cases. Three patients experienced diplopia lasting from 1 day to 6 months. The magnitude of skew deviation was increased by head tilt away from the operated side in only 1 case. The development of skew deviation was not determined by preoperative ocular alignment or binocular function, or any particular type or pattern of vestibular disease. There was an association with large changes in ocular torsion and subjective visual vertical, which correlated with lesser degrees of canal paresis to preoperative caloric testing on the operated side. No patient developed a head tilt postoperatively. There is a gradation of responses to surgical vestibular deafferentation in humans, skew deviation only occurring in patients suffering marked changes in subjective visual vertical and ocular torsion.

Intact binocular function and absent ocular torsion in children with alternating skew on lateral gaze

BACKGROUND

A form of skew deviation, called alternating skew on lateral gaze, resembles bilateral superior oblique overaction. Oblique muscle overaction has been recently speculated to result from loss of fusion with subsequent "free-wheeling" of the torsional control mechanisms of the eyes, causing sensory intorsion or extorsion with attendant superior or inferior oblique muscle overaction, respectively. We wanted to investigate whether loss of fusion plays a role in the pathogenesis of alternating skew on lateral gaze.

SUBJECTS AND METHODS

We examined seven consecutive patients with posterior fossa tumors, enrolled in a multi-disciplinary pediatric neuro-oncology program, who displayed alternating skew on lateral gaze. All patients underwent a thorough ophthalmologic evaluation.

RESULTS

Visual acuities in the study patients ranged from 20/20 to 20/40. Five of the seven patients were orthotropic, and showed 40 sec of arc stereopsis. Three patients showed associated downbeat nystagmus. No ocular torsion was found in any of the five patients who showed normal stereopsis upon inspection of fundus landmarks on indirect ophthalmoscopy.

CONCLUSION

Patients with alternating skew on lateral gaze often have normal binocular vision and stereopsis, and lack ocular intorsion so typical of superior oblique overaction. Alternating skew on lateral gaze is neurologically mediated, with no role for defective fusion in its pathogenesis.

Skew deviation of the eyes in normal human subjects induced by semicircular canal stimulation

Computerised video-oculography and scleral search coils were used to record the horizontal, vertical and torsional binocular eye movements of human subjects exposed to roll oscillation at 0.4 Hz about earth-horizontal and earth-vertical naso-occipital axes in darkness. The stimuli provoked a dominant torsional ('ocular counter-rolling') response with a ratio of peak slow phase eye velocity to stimulus velocity which was not significantly different for earth-horizontal (0.39, SD 0.08) or earth-vertical axis orientations (0.40, SD 0.08). For all conditions the responses also had a head-vertical component which was disconjugate ('skew deviation'). The cumulative, vertical, slow phase divergence was 5.8 degrees, SD 1.3 degrees, about upright and 4.3 degrees, SD 0.6 degrees, when supine. This is the first demonstration that dynamic roll stimuli provoke a skew deviation in normal human subjects. At the frequency tested, the skew was driven by vertical semicircular canal stimulation.

The effect of roll-tilt on ocular skew deviation

Static roll-tilt of normal healthy subjects causes the ocular tilt reaction (OTR) one component of which is disconjugate vertical eye position (skew deviation). In this study the magnitude of skew was measured subjectively by the use of a computerized Hess test at three static roll-tilt angles (head erect, left ear down and right ear down) and two viewing distances (20 cm and 60 cm). The results showed that during static roll-tilt there was a small skew deviation, the magnitude of which was increased at close viewing distances.

Cyclodeviation in skew deviation

We examined four patients with dorsolateral pontomedullary lesions and skew deviation, with ocular torsion of varying symmetry. In three patients, the hypotropic eye was excyclodeviated relative to the fellow eye. Observations of these patients, combined with recent evidence, suggest that cyclodeviation is a frequent component of skew deviation and may result from variable involvement of utricular, semicircular canal, and cerebelloocular pathways. Although cyclodeviation is a feature of trochlear-nerve palsies, its presence does not exclude skew deviation and underlying brainstem lesions.