The Ocular Tilt Reaction and Skew Deviation

The ocular tilt reaction composes a triad of ipsiiateral head-tilt, ipsilaterai conjugate ocular torsion, and skew deviation with ipsilaterai hypotropia. It results from a disruption of central vestibular pathways conveying information from the semicircular canals and otoliths. Skew deviation is one component of the syndrome which may or may not be associated with the concomitant torsion and head-tilt of the ocular tilt reaction. Because of the frequent clinical presentation of skew deviation it merits special attention. Westheimer and Blair (1975) produced the ocular tilt reaction in monkeys by unilateral electrical stimulation of a region of the brainstem tegmentum dorsolateral to the oculomotor and trochlear nuclei from the caudal wall of the third ventricle to the rostral floor of the fourth ventricle. This area encompasses, but does not consist entirely of, the Interstitial nucleus of Cajal. They observed 1) a skew deviation of the eyes resulting from an equal and opposite movement of the globes, 2) a conjugate torsional movement ipsiiateral to the stimulated side, and 3) an ipsiiateral head tilt (Westheimer and Blair, 1975). Generally the eye ipsiiateral to stimulation becomes hypotropic. Rarely the opposite is seen with contralateral hypotropia and contralateral torsion. Further experimental studies in animals (Hyde and Eason, 1959; Hyde and Toczek, 1962; Mabuchi, 1970) and in humans (Sano et al, 1972) have confirmed that the rostral midbrain in and around the interstitial nucleus of Cajal is the crucial area for eliciting the ocular tilt reaction. The presence of ocular counter-roll in humans, although disputed in the past, has unquestionably been confirmed. In an elegant study utilizing the search coil technique, Collewijn et al (1985) documented the degree of counter-roll present under static conditions, during a roll movement of the head, and under conditions of voluntary pseudo-sinusoidal head rotation, Confirming previous observations dating back to the 19th century (Nagel 1896) they found static counter-roll to be primarily an otolith function though likely with a small additional input from neck proprioception. Static counter-roll compensates only slightly for the degree of head rotation In roll with a gain on the order of ten percent. Gain is increased during an active step in head roll in which the torsional eye movements consist of conjugate smooth cyclorotary movements opposite to the direction of head roll Interspersed with conjugate cyclorotary saccades (approximately two to four per second) in the direction of head roll, As the saccadic eye movements compensate largely, though not fully, for the preceeding smooth eye movements, this results in an accumulation of torsional deviation which, at the end of the head roll, represents the static torsional component of counter-roll. Continuous voluntary pseudo-sinusoidal oscillation of the head in roll again shows an increase in gain of the counter-roll with increased frequency of the oscillation and with the amount of visual information. This study documents the progressive increase in gain of counter-roll eye movements secondary to canal input (rotary head acceleration) and visual input, The role of the utricle In the generation of the ocular tilt reaction has been confirmed by animal studies. (Suzuki et al, 1969; Curthoys, 1987), The utricle projects to both the lateral vestibular nucleus and the superior vestibular nucleus. From the lateral vestibular nucleus there are descending vestibulo-spinal projections presumably contributing to head tilt. An ipsiiateral projection from the superior vestibular nucleus follows the medial longitudinal fasciculus and terminates in the ipsilaterai trochlear nucleus, the inferior rectus subdivision of the oculo-motor nucleus bilaterally, the Ipsiiateral interstitial nucleus of Cajal and the rostral interstitial nucleus of the MLF. Contralateral projections of the superior vestibular nucleus terminate In the trochlear nucleus and the superior rectus and inferior oblique subdivisions of the oculomotor nucleus. There are scattered cross projections to the interstitial nucleus of Cajal. The lateral vestibular nucleus projects ipsilaterally to the oculomotor nucleus via the ascending tract of Dieters. Brant and Dleterich (1987) propose that the typical eye movements of the ocular tilt reaction may be due to activation of the Ipsiiateral Inferior rectus and Inferior oblique (? via ipsiiateral MLF oripsilateral ascending tract of Dieters) as well as excitation of the contralateral superior rectus and superior oblique (via the contralateral ascending tract of the MLF), It goes without saying that these proposed pathways are purely speculative, The ascending tract of Dieters may in fact not be involved in vertical eye movements,