Clinical testing of the statolith-ocular reflex.

Quantification of eye torsion induced by stimulation of the statolith organ has recently become available (in the clinic) thanks to the development of video nystagmography. A major step forward has been the development of real-time measurement of eye torsion which enables interactive testing of the statolith-ocular reflex. The function of the statolith organs can be evaluated by the study of the impact of statolith stimulation upon the canal-mediated dynamic vestibulo-ocular reflex (VOR) or by measurement of the static VOR. The latter is more appropriate because instruction or alertness seems not to affect the reflex substantially. Four ways of inducing the static reflex are discussed and related experiments are described. In normal subjects, the gain of eye torsion upon lateroflexion (up to 45 degrees) and whole-body roll (up to 90 degrees) ranges from 0.13 to 0.22; eye torsion is 10.3 degrees maximal. Eccentric rotation inducing a maximal centrifugal force of 1 g results in a gain of eye torsion ranging from 0.15 to 0.23 with a maximal eye torsion of 7.8 degrees in normals. Sinusoidal sideward translation at accelerations of about 0.5 g at 0.3 Hz, which can be considered as specific dynamic statolith stimulation, induces a small but substantial eye torsion of maximally 4.1 degrees and with an eye torsion gain ranging from 0.10 to 0.15. Lateroflexion and body flexion do not require sophisticated stimulation equipment and therefore are attractive for implementation in the out-patient department. In patients, impairment of the statolith-ocular reflex is frequently found both in patients with normal as well as in those with defective dynamic VORs (caloric asymmetries). A qualitative concept is presented to describe the deflection pattern of the cilia upon lateroflexion, eccentric rotation and sidewards translational acceleration. According to this theoretical concept, a reduced static eye torsion might point to both a saccular and/or an utricular lesion. An asymmetry of ocular torsion is only expected upon eccentric rotation or sidewards translation in case of a unilateral saccular lesion.