Ocular Tilt Reaction Resulting from Vestibuloacoustic Nerve Surgery

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.

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.