DEVIATION OF THE SUBJECTIVE VERTICAL IN LONG-STANDING UNILATERAL VESTIBULAR LOSS

We evaluated changes in the subjectively perceived gravitational verti cal as an index of imbalance in the function of the right and left oto lith organs. In addition to normal subjects (n = 25), we measured pati ents with a longstanding (mean 4.5 year +/- 3.2 SD; range 0.5-11.5 yea rs) unilateral vestibular loss after surgery for acoustic neuroma (n = 32), patients with partial unilateral vestibular loss (n = 7) and pat ients with bilateral vestibular hyporeflexia (n = 8). Normal subjects could accurately align a vertical luminous bar to the gravitational Ve rtical in an otherwise completely dark room (mean setting - 0.14 degre es +/- 1.11 SD). Patients with leftsided (complete; n = 13) or rightsi ded (complete; n = 19 and partial; n = 7) unilateral vestibular loss m ade mean angular settings at 2.55 degrees +/- 1.57 (SD) leftward and 2 .22 degrees (+/- 1.96 SD) rightward, respectively These means differed highly significantly from the normal mean (p < 0.00001). In the time interval investigated (0.5-11.5 years) the magnitude of the tilt angle showed no correlation with the time elapsed since the operation. The mean setting by patients with clinically bilateral vestibular loss (-1 .17 degrees +/- 1.96 SD; n = 8) did not significantly differ from the control group. The systematic tilts of the subjective vertical in pati ents with a unilateral vestibular impairment were correlated with thei r imbalance in canal-ocular reflexes, as reflected by drift during hea d-oscillation at 2 Hz (r(2) = 0.44) and asymmetries in VOR-gain for he ad-steps (r(2) = 0.48-0.67). These correlations were largely determine d by the signs of the asymmetries; correlation between the absolute va lues of the VOR gain asymmetries and subjective vertical angles proved to be virtually absent. We conclude that the setting of the subjectiv e vertical is a very sensitive tool in detecting a left-right imbalanc e in otolith function, and that small but significant deviations towar ds the defective side may persist for many years (probably permanently ) after unilateral lesions of the labyrinth or the vestibular nerve.