Evaluation of diagnostic tests of the otolith organs and their application in various vestibular pathologies

Abstract

Current vestibular testing is limited. The general function of the vestibular system on both sides of the head can be tested, and one part of the peripheral vestibular organ, the horizontal semicircular canal, can be tested unilaterally. However, recently a test for the function of the otolith organs has been developed, the ocular Vestibular Evoked Myogenic Potential (oVEMP) test. This oVEMP test makes use of air-conducted sound (ACS) or bone-conducted vibration (BCV) to stimulate the otolith organs. Subsequently, the response can be recorded in the inferior eye muscles. Another relatively new test that can be employed for testing otolith function, is the unilateral centrifugation test. This method uses eccentric head positions during high speed rotation to stimulate the otolith organs unilaterally. In this thesis three main research questions regarding the tests described above were evaluated. 1. To explore the options of extending vestibular testing in relation to the otolith organs using recording of the oVEMPs. The ACS and BCV oVEMP were extensively studied and successfully implemented in vestibular testing. 2. To find new applications of oVEMP recording in relation to the diagnosis of specific vestibular diseases. The ACS oVEMP is a relevant addition in diagnosing Ménière’s disease. In Ménière’s patients the oVEMP response rates are lower, the amplitudes smaller and thresholds higher than in normal subjects. This effect is seen in both ears of Ménière’s patients. The affected ear is altered more than the clinically unaffected ear. Furthermore, it was found that in normal subjects the oVEMP tunes to a stimulus frequency of 500 Hz, with the highest amplitude and lowest threshold at this particular frequency, while for Ménière’s ears the best stimulus frequency was 1000 Hz. This ‘tuning effect’ was used to find a criterion to distinguish normal ears from Ménière’s ears. In otosclerotic disease the BCV oVEMP demonstrated that no significant differences in all oVEMP characteristics between normal and otosclerotic ears, and between otosclerotic ears before and after stapedotomy. Therefore it could be concluded that no or undetectably little damage to the utricle is caused by both otosclerotic disease and stapes surgery. Therefore screening with the BCV oVEMP in otosclerotic disease is not useful. On the contrary, in superior canal dehiscence the BCV oVEMP shows a significantly lower threshold and a higher amplitude in the dehiscent ear. 3. To develop a relatively simple unilateral centrifugation test for evaluation of the otolith organs. A new method of unilateral centrifugation was tested through registering of the tilt of the haptic SV induced by head tilt during on-axis body rotation. This provided a relatively uncomplicated alternative to test unilateral otolith function as compared to body and head translation during rotation. This thesis is an addition to the present knowledge with respect to diagnostic testing of the otolith organs, both on the basics of otolith testing, as well as more practically in the diagnosing of specific vestibulo-otologic diseases. This work will help the process of integrating otolith testing in daily practice.