Torsional and vertical eye movements during head tilt dynamic characteristics.

PURPOSE: As the first response to a Bielschowsky head tilt test (BHTT), a fast transient torsional eye movement in the same direction as the head tilt has been shown with the three-dimensional (3D)-video oculography (3D-VOG) technique, and this movement is paralleled by a transient vertical vergence shift inducing a physiological skew deviation. The purpose of the present study was to investigate these dynamic eye movements further in response to a BHTT paradigm with the magnetic search coil technique.
METHODS: Ten healthy subjects performed a BHTT (15 degrees, 30 degrees, and 45 degrees) toward each shoulder while (search coil) the monocular eye and head positions were recorded. The same head tilt paradigm was repeated in a second test while (3D-VOG) the binocular eye position was recorded.
RESULTS: Subsequent to the initiation of the head tilt (latency, approximately 160 ms) a rapid torsional eye movement (mean peak velocity: 40 deg/s; mean amplitude: 4 degrees) was seen in the same direction as the head movement, followed by a somewhat slower return movement. This torsion was synchronous with a vertical vergence eye movement (mean amplitude 3 degrees). The vertical vergence was always with left eye over right eye in the rightward head tilt and in head straightening from the left shoulder. In the left head tilt and in the head straightening from the right shoulder, this movement was always with the right eye over the left eye.
CONCLUSIONS: A torsional and vertical vergence back-and-forth eye movement induced by a BHTT was confirmed with the search coil technique. Utricular inertia due to an interaural head translation, combined with a stimulation of the vertical semicircular canals, seems to be a plausible explanation for these eye movements.