Tilted perception of the subjective 'upright' in unilateral loss of vestibular function

Abstract

Objective: To demonstrate that unilateral vestibular loss (UVL) may cause an erroneous perception of the subjective (bodily) vertical.

Study design: Comparison of patients with UVL with age-matched controls.

Setting: The study was performed in a clinical neurophysiology laboratory.

Patients: Seven patients with unilateral, mixed acute, and chronic losses of vestibular function (six surgical), two patients with bilaterally absent vestibular function, one patient with a plugged posterior canal, and twenty-two healthy individuals.

Interventions: Subjects were seated, whole body restrained, in a flight simulator. The simulator executed intermittent stepwise tilts, in roll, up to 28 degrees that subjects had to correct to maintain an "earth upright" attitude using the joystick. Subjects performed in both "calm" conditions and also when the simulator was oscillating in roll at 1 Hz, 4 degrees peak displacement to simulate "turbulence." The purpose of the turbulence was to destabilize (or mask) somatosensory cues to uprightness.

Main outcome measures: Accuracy of corrections of subjective attitude to earth upright after tilts.

Results: All subjects accepted the initial attitude of the simulator as "upright." In response to imposed tilts, normal subjects (n = 22) corrected their attitude to a mean 0.4 degrees SD 1.5 degrees (tilted slightly rightward) in calm and 0.7 degrees SD 1.3 degrees in turbulence. On average, corrections were normometric. All patients with UVL responded to imposed tilts by corrections that left them 'flying' tilted slightly to the side of their lesion, mean 3.2 degrees SD 2.5 degrees when calm, 6.4 degrees SD 2.7 degrees in turbulence (p<0.01). Their corrections were hypometric in response to imposed tilts to the lesioned side (i.e., undershooting true upright) and hypermetric in response to tilts to the intact side.

Conclusions: Unilateral vestibular loss causes a "vestibular perception" of an erroneous tilt of the body that is probably caused by an imbalance of otolith signals and apparently never fully compensates. The tilt is enhanced when rapid perturbations of posture make somatosensory cues difficult to interpret. An erroneous perception of upright may contribute to vestibular ataxia, which is provoked when motion context involves rapid change.