Effects of age, viewing distance and target complexity on static ocular counterroll

Clinical application of 3D-VOG analysis for quantitative evaluation of Otolith-Ocular reflex in the roll and pitch planes

BACKGROUND

Management of vestibular falls requires precise evaluation of otolithic function and gravity recognition. Tilt of the head in the roll plane causes ocular counter rolling (OCR) with a gain of 5-10%, which can be a quantitative parameter related to the otolith-ocular reflex (OOR) in that plane. However, the OOR in the pitch plane is indiscernible because of our ability to voluntarily gaze and pursue. 3 D eye movements and its Listing's plane description can provide novel parameters with which to evaluate OOR, even in the pitch plane.

OBJECTIVES

To evaluate the utility of 3 D eye movement analysis in video oculography (VOG) as a quantitative parameter reflecting otolithic function during head movements associated with daily life.

MATERIALS AND METHODS

In healthy subjects, we analyzed the changes in 3 D eye movements and the Listing's plane that occur with static gravitational modification.

RESULTS

OOR during head tilt in the roll plane revealed that OCR amplitude had a saturating/peaked conformation and was not fitted by a sine curve. OOR in the pitch plane revealed anteroposterior leaning of the Listing's plane.

CONCLUSIONS

These novel tools for minimally invasive evaluation of OOR in a clinical setting could potentially enable us to quantitatively evaluate peripheral static disequilibrium.

Analysis of torsional eye movements using the corneal birefringence pattern

The literature mentions several invasive methods to measure the degree of the compensatory torsional eye movement during a head-tilt. Nevertheless, none of them have yielded universally clinical tests. This study focuses on an optical system (with a circular polarizer) for noninvasive acquisition of corneal birefringence patterns (isochromes). The acquired isochromes are quadrangular in shape and unique for each eye, as well as independent of the head-rotation angle. The results obtained suggest that isochrome orientation analysis could be an effective method to accurately measure the degree of compensatory torsional eye movement.

Enhancing vestibular function in the elderly with imperceptible electrical stimulation

Age-related loss of vestibular function can result in decrements in gaze stabilization and increased fall risk in the elderly. This study was designed to see if low levels of electrical stochastic noise applied transcutaneously to the vestibular system can improve a gaze stabilization reflex in young and elderly subject groups. Ocular counter-rolling (OCR) using a video-based technique was obtained in 16 subjects during low frequency passive roll tilts. Consistent with previous studies, there was a significant reduction in OCR gains in the elderly compared to the young group. Imperceptible stochastic noise significantly increased OCR in the elderly (Mean 23%, CI: 17–35%). Increases in OCR gain were greatest for those with lowest baseline gain and were negligible in those with normal gain. Since stimulation was effective at low levels undetectable to subjects, stochastic noise may provide a new treatment alternative to enhance vestibular function, specifically otolith-ocular reflexes, in the elderly or patient populations with reduced otolith-ocular function.

Effects of head tilt on visual field testing with a head-mounted perimeter imo

PURPOSE

A newly developed head-mounted perimeter termed "imo" enables visual field (VF) testing without a fixed head position. Because the positional relationship between the subject's head and the imo is fixed, the effects of head position changes on the test results are small compared with those obtained using a stationary perimeter. However, only ocular counter-roll (OCR) induced by head tilt might affect VF testing. To quantitatively reveal the effects of head tilt and OCR on the VF test results, we investigated the associations among the head-tilt angle, OCR amplitude and VF testing results.

SUBJECTS AND METHODS

For 20 healthy subjects, we binocularly recorded static OCR (s-OCR) while tilting the subject's head at an arbitrary angle ranging from 0° to 60° rightward or leftward in 10° increments. By monitoring iris patterns, we evaluated the s-OCR amplitude. We also performed blind spot detection while tilting the subject's head by an arbitrary angle ranging from 0° to 50° rightward or leftward in 10° increments to calculate the angle by which the blind spot rotates because of head tilt.

RESULTS

The association between s-OCR amplitude and head-tilt angle showed a sinusoidal relationship. In blind spot detection, the blind spot rotated to the opposite direction of the head tilt, and the association between the rotation angle of the blind spot and the head-tilt angle also showed a sinusoidal relationship. The rotation angle of the blind spot was strongly correlated with the s-OCR amplitude (R2≥0.94, p<0.0001). A head tilt greater than 20° with imo causes interference between adjacent test areas.

CONCLUSIONS

Both the s-OCR amplitude and the rotation angle of the blind spot were correlated with the head-tilt angle by sinusoidal regression. The rotated VF was correlated with the s-OCR amplitude. During perimetry using imo, the change in the subject's head tilt should be limited to 20°.

Clinical measurement of compensatory torsional eye movement during head tilt

PURPOSE

To measure the degree of compensatory torsional eye movement during head tilt using a fundus photography method.

METHODS

We enrolled 55 healthy subjects who were 20-66 years of age. Fundus photographs were obtained in the presumed baseline position and in stepwise head tilt positions to evaluate ocular torsion using a non-mydriatic fundus camera. Horizontal marks on the nose were photographed simultaneously to evaluate head tilt. Images were analysed using Photoshop to measure the degree of ocular torsion and head tilt.

RESULTS

A consistent compensatory torsional eye movement was observed in all subjects during head tilt. The degree of compensatory torsional eye movement showed a positive correlation with the angle of head tilt. Ocular torsional disconjugacy was observed during head tilt, with larger excycloductional eye movement than incycloductional eye movement (4.88 ± 2.91° versus 4.50 ± 2.76°, p < 0.001). In multiple linear regression analysis, the degree of compensatory torsional eye movement was significantly associated with the degree of head tilt (β = 0.191, p < 0.001), and the direction of cycloduction (β = -0.548, p < 0.001).

CONCLUSIONS

The fundus photography method is a non-invasive, accurate and objective tool for measuring compensatory torsional eye movement. Considering the availability of fundus photography in clinical ophthalmology practice, the proposed method can be used as a clinical tool to measure compensatory torsional eye movement.

New understanding on the contribution of the central otolithic system to eye movement and skew deviation

The otolith organs consist of the utricle and saccule. The utricle mediates the utriculo-ocular reflex by detecting horizontal head translation and static head tilt. Skew deviation is a vertical strabismus caused by imbalance of the utriculo-ocular reflex pathway and is commonly caused by lesions in the brainstem or cerebellum. It is associated with abnormal utriculo-ocular reflexes including asymmetric reduction of the translational vestibulo-ocular and ocular counterroll responses. Skew deviation is also associated with head position-dependent changes in ocular torsion and vertical strabismus. The reduction in ocular torsion and vertical strabismus when changing from an upright to supine position in skew deviation allows us to devise a new bedside 'upright-supine test' to differentiate skew deviation from fourth nerve palsy and other causes of vertical strabismus.

Understanding skew deviation and a new clinical test to differentiate it from trochlear nerve palsy

Skew deviation is a vertical strabismus caused by a supranuclear lesion in the posterior fossa. Because skew deviation may clinically mimic trochlear nerve palsy, it is sometimes difficult to differentiate the 2 conditions. In this review we compare the clinical presentations of skew deviation and trochlear nerve palsy and examine the pathophysiology that underlies skew deviation. We then describe a novel clinical test-the upright-supine test-to differentiate skew deviation from trochlear nerve palsy: a vertical deviation that decreases by > or =50% from the upright to supine position suggests skew deviation and warrants investigation for a lesion in the posterior fossa as the cause of vertical diplopia.