Unilateral testing of utricular function

A modified rotatory chair test is reported in which radial acceleration, generated by eccentric displacement of the subject during constant angular velocity, is exploited as a unilateral stimulation to the otolith organs. During constant angular rate rotation, the test subject is displaced laterally on the rotating turntable by 3.5 cm, so that one labyrinth becomes aligned with the rotatory axis while the second - eccentric - labyrinth is solely exposed to the altered gravito-inertial acceleration (GIA). Previously reported results showed that the direction of the response is independent of the direction of turntable rotation, ruling out any canal influence, and indicated that in a normal population the response, measured in one eye, was symmetrical for displacement of the left and right labyrinths. This mode of stimulus thus appears to elicit a unilateral otolith-ocular response (OOR). Examination of this unilateral OOR was extended in the present study; comparative testing with head-tilt to gravity, i.e. involving bilateral stimulation to the otolith organs, was carried out. Movements of both eyes were recorded (by three-dimensional video-oculography), in order to examine response conjugacy. To verify the specificity of the unilateral stimulus, tests were performed with patients who had previously undergone unilateral section of the vestibular nerve as treatment for acoustic neuroma. The eccentric displacement profile (EDP) and head-tilt stimulus each included ten cycles of left-right oscillation in order to permit signal averaging. In the normal subjects (n=12) the torsional component of the OOR proved to be both labyrinth-symmetrical and conjugate, during both bilateral and unilateral otolith stimulation. OOR gain (ocular torsion/GIA tilt) was higher for bilateral than unilateral stimulation. Bilateral OORs, obtained from three of the five unilaterally deafferented patients, proved less symmetrical and conjugate than in the normals. Unilateral OORs in all five patients were characteristically asymmetrical, with little or no response during stimulation of the diseased labyrinth.