Optokinetic tracking is the part of the VNG battery where the patient is simply asked to “follow the lights” as they sail by, first in one direction, then the other. In real life, the optokinetic system kicks in when the vestibular system fails to ‘‘keep up’’ during sustained head rotation. Optokinetic tracking is similar to smooth pursuit tracking in its mechanism of origin, and abnormalities in this test suggest the possibility of a cerebellar disorder. Optokinetic tracking is analyzed for symmetry and gain.
As noted a few weeks back, the oculomotor tests are neurologic disorder screening tests, and making a diagnosis of “cerebellar dysfunction” based on these tests alone is a stretch.
The Vestibular Ocular Reflex (VOR) will keep your eye stable on a target as you move your head around, but, after several seconds of sustained rotation, the fluid movement in the inner ear becomes “exhausted” and stops moving. The fluid dynamics of the vestibular apparatus are such that it cannot maintain stimulation during sustained rotation. That is why you get dizzy on a merry go round. As the vestibular response gradually decays, the optokinetic and pursuit systems provide input to stabilize the eyes and maintain stable vision.
Optokinetic nystagmus can be simulated by exposing the patient to repetitive moving visual stimuli. Various techniques can accomplish this. Neurologists sometimes use a long, narrow striped cloth that they move side to side and ask the patient to count or watch the stripes. Older ENG systems frequently supplied an ‘‘optokinetic drum,’’ a striped drum that could be spun around in front of the patient. Some laboratories built customized striped drums that would be lowered over the patient’s head and spun around to create the repetitive stimuli.
Current ENG systems provide optokinetic stimuli by having lights move along a light bar, asking the patient to ‘‘count each light’’ as it passes by, or a laser projector that projects long stripes on the wall, moving side to side. Enclosed rotary-chair systems use a projector to provide full-field (floor to ceiling) stimulation of moving lighted stripes. These are very different tests because patients can easily look away from the light bar, but in full-field stimulation they cannot avoid the moving visual stimulus unless they close their eyes. Kveton, Limb, and Bell (1999) report that standard light-bar stimulation results in a high number of false-positives and question the clinical value of this test as part of the standard ENG battery. More about that next week.
