The subjective visual horizontal in the diagnosis of vestibular disorders : physiological and clinical considerations

In the examination of patients with vestibular disturbances the diagnostic techniques are almost entirely focused on the semicircular canals. However, a considerable number of the patients have symptoms which are suggestive of an affection of the otolith organs, e.g. false sensations of tilt or linear motion and even sudden falls.

The aim of the present work was to standardize a clinical test procedure for examination of the otolith system based on two otolith-dependent perceptual phenomena - the subjective visual horizontal (SVH) in the upright position and the ability to perceive lateral head and body tilt - and, in addition, to evaluate the capacity of this test to detect and lateralize defined vestibular lesions.

In the upright position and at 10, 20, and 30 degrees of head and body tilt to the right and to the left, SVH was measured by asking subjects to adjust a luminous line in a completely darkened room so that it looks horizontal. Normal subjects had SVH in the upright position not deviating more than 2.5 degrees from the true horizontal. At tilt to either side most subjects made an overestimation of the physical tilt angle, i.e. the luminous line was set slightly tilted opposite to the tilt of the subject.

In the acute stage of vestibular neuritis, and after unilateral intratympanic gentamicin treatment, in the upright position patients invariably had a pathologic tilt of SVH towards the diseased side. In addition, there was a significant reduction in the ability to perceive tilt to the diseased side, while the perception of tilt to the healthy side not showed any significant change. These asymmetries suggest a preference for ipsilateral tilt of the utricle on the intact side.

In patients undergone stapedotomy and in a few patients with untreated Meniere's disease SVH was tilted away from the affected side and the perception of head tilt to the affected side was increased. Such findings may be explained by an increased resting activity in otolithic sensory afferents.

In patients with unilateral loss of vestibular function there was during follow-up a gradual reduction in the tilt of SVH as well as a recovery of the perception of tilt to the deafferented side, which implies an otolithic component of vestibular compensation.

By measuring SVH during hypergravity (l.5g, 2 0g, 2.5g) in a gondola centrifuge it was possible to demonstrate an influence from the vertical semicircular canals and the sacculae on SVH. The swing-out of the gondola during centrifuge acceleration is a roll plane canal stimulus, found to cause a tilt of SVH. However, the resultant gravity vector acting in parallel with the subjects' long axis is mainly a saccular stimulus, resulting in smaller and more rapidly decaying tilts of SVH at higher g-levels.

In conclusion, measuring SVH in patients with vestibular disorders provides information on the vestibular organs which complement what is gained by examining the semicircular canals. The present method was found to be useful for detecting and lateralizing unilateral peripheral dysfunctions and also to give a contribution to the understanding of otolith physiology.