The plasticity of compensatory eye movements in bilateral vestibular loss. A study with low and high frequency rotatory tests.

Twelve subjects with bilateral vestibular loss, with a mean age of 27 years (18-49) were studied. The loss was based on symptoms of oscillopsia and ice-water caloric tests. Nine subjects were evaluated by low-frequency sinusoidal harmonic acceleration (SHA) rotatory tests (0.01-0.32 Hz), and 3 subjects with high broad-frequency band rotatory tests (0.25-3.25 Hz). During alertness tests (darkness) all subjects had absent or very low gains. When tested with a stationary light in the middle of the swing, the SHA tests showed perfect compensatory eye movements with gains of 1 and phases of approximately 0 degrees. In the broad-frequency test, the gain was near 1 below 1 Hz, decreasing rapidly at higher frequencies. In imaginary stationary target tests (darkness), the gain was enhanced to 'normal' values (0.5) with a phase lead in the low-frequency range (SHA). This could not be replicated in the high-frequency tests. Testing with stationary acoustic guidance (darkness), further enhanced the gain in the SHA tests. A small increase of gain could also be found in the high-frequency tests. Compensatory eye movements, when tested in low-frequency rotatory tests (less than 1 Hz), are to a large extent influenced by non-vestibular mechanisms. By voluntary modifications, normal gains could be produced by patients with 'bilateral vestibular loss'. The phase lead found is proposed to be of central non-vestibular origin. Testing in higher frequency ranges (greater than 1 Hz) could not replicate these findings, thus the broad-frequency band rotatory test should be preferred for adequate vestibular quantification.