Comparison of head thrust test with head autorotation test reveals that the vestibulo-ocular reflex is enhanced during voluntary head movements

Objectives: To compare 2 clinical tests of vestibular function, the head autorotation test (HART) and the head thrust test (HTT), and to determine why they give disparate results in patients with known unilateral vestibular deficiency (UVD) due to labyrinthectomy. Methods: We used scleral coils to measure the horizontal (yaw) vestibulo-ocular reflex (VOR) in 5 healthy human subjects and in I I patients who underwent labyrinthectomy. We used 2 paradigms. Using HART, subjects visually fixated a target during self-generated, swept-frequency, sinusoidal, horizontal head rotations. Using HTT, patients fixated the target during horizontal head thrusts delivered randomly in direction and time. Results: In subjects without UVD, eye movements were almost perfectly compensatory for both paradigms. in subjects with UVD, VOR gain for ipsilesional head thrusts was low for both paradigms, but significantly (P<.001) higher (less abnormal) for HART (0.60 +/- 0.13) than for HTT (0.14 +/- 0.13). Contralesional gain was reduced for both, to 0.64 +/- 0.20 for HART and to 0.57 +/- 0.17 for HTT. Because ipsilesional and contralesional gains were not statistically different for HART (P = .69), comparison of VOR gains for half-cycle responses to the HART stimulus could not reliably identify the side of the known lesion. In contrast, HTT consistently identified the side of the lesion for all subjects with UVD. To investigate whether preprogramming contributes to the boost in VOR as measured by HART, we compared the gain and response delay of eye movements during actively self-generated and passively received head thrusts. For subjects without UVD, response delays were shorter for active (6 +/- 1 milliseconds) than for passive (12 +/- 1 milliseconds) HTT. For ipsilesional rotations of subjects with UVD, active HTT yielded a significantly higher gain (0.44 +/- 0.20) (P<.001) and a shorter delay (15 +/- 6 milliseconds) (P<.001) than did passive HTT (0.14 +/- 0.13 and 37 +/- 15 milliseconds, respectively). Contralesional test results revealed a similar performance boost for active head movements. Data are given as mean +/- SD. Conclusion: When comparison of half-cycle gains is used to identify the lesion side, self-generated predictable head movement paradigms, such as HART and active HTT, are less accurate than passive HTT in the characterization of UVD, in part because preprogramming can augment the VOR during voluntary head movements.