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

Sammanfattning: The Subjective Visual Horizontal in the Diagnosis of VestibularDisorders Physiological and Clinical Considerations by Arne Tribukait Departmentof Audiology, Karolinska Hospital, Stockholm In the examination of patients with vestibular disturbances the diagnostic techniquesare almost entirely focused on the semicircular canals. However, a considerable numberof the patients have symptoms which are suggestive of an affection of the otolithorgans, 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 examinationof the otolith system based on two otolith-dependent perceptual phenomena - the subjectivevisual horizontal (SVH) in the upright position and the ability to perceive lateralhead and body tilt - and, in addition, to evaluate the capacity of this test to detectand lateralize defined vestibular lesions. In the upright position and at 10, 20, and 30 degrees of head and body tilt tothe right and to the left, SVH was measured by asking subjects to adjust a luminousline in a completely darkened room so that it looks horizontal. Normal subjects hadSVH 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 tiltangle, i.e the luminous line was set slightly tilted opposite to the tilt of thesubject. In the acute stage of vestibular neuritis, and after unilateral intratympanicgentamicin treatment, in the upright position patients invariably had a pathologictilt of SVH towards the diseased side. In addition, there was a significant reductionin the ability to perceive tilt to the diseased side, while the perception of tiltto the healthy side not showed any significant change. These asymmetries suggesta preference for ipsilateral tilt of the utricle on the intact slde. In patients undergone stapedotomy and in a few patients with untreated Meniere'sdisease SVH was tilted away from the affected side and the perception of head tiltto the affected side was increased Such findings may be explained by an increasedresting activity in otolithic sensory afferents. In patients with unilateral loss of vestibular function there was during follow-upa gradual reduction in the tilt of SVH as well as a recovery of the perception oftilt to the deafferented side, which implies an otolithic component of vestibularcompensation. By measuring SVH during hypergravity (l.5g, 2 0g, 2.5g) in a gondola centrifugeit was possible to demonstrate an influence from the vertical semicircular canalsand the sacculae on SVH. The swing-out of the gondola during centrifuge accelerationis a roll plane canal stimulus, found to cause a tilt of SVH. However, the resultantgravity vector acting in parallel with the subjects' long axis is mainly a saccularstimulus, resulting in smaller and more rapidly decaying tilts of SVH at higher g-levels. In conclusion, measuring SVH in patients with vestibular disorders provides informationon the vestibular organs which complement what is gained by examining the semicircularcanals. The present method was found to be useful for detecting and lateralizingunilateral peripheral dysfunctions and also to give a contribution to the understandingof otolith physlology. Key Words: Vestibular system, vestibular tests, vestibular neuritis, Meniere'sdisease, gentamicin treatment, vestibular compensation, stapedotomy, spatial orientation,gravity otoliths, utricle, saccule, semicircular canals, subjective visual horizontal,roll tilt perception. ISBN 91-628-2789-8