Vestibular functional testing using motor responses

Basic data for this project

Description

The otolith organs, utricle and saccule, are highly responsive sensors for linear acceleration. But the effect used to test their function in patients with vertigo has little to do with this natural task. What is commonly exploited is that the vestibular system may be stimulated also by high-intensity tone pulses or bone-conducted vibrations. The reflexes that are triggered by such stimuli appear, in the averaged electromyogram (EMG) of the muscles involved, as a vestibular evoked myogenic potential (VEMP). The cVEMP, recorded from cervical muscles, is an expression of the vestibulo-collic reflex, whereas the oVEMP, arising from the extraocular muscles, is an expression of the vestibulo-ocular reflex. Under physiological conditions, these two reflexes stabilize the head position in space and the view. Although VEMP-measurements are increasingly used in clinical diagnostics, many fundamental questions are still insufficiently investigated, and some results are controversial. A central aim of this project is to establish relationships between VEMPs recorded under the non-physiological conditions that are common at present, and EMG changes caused by linear accelerations. The latter kind of stimulation, corresponding to the actual function of the otolith organs, will be realized by imparting mechanical impulses to the resting head. Our previous work has established two important preconditions for this research project. First, theoretical studies paved the way for a model-based interpretation of experimental data and for making predictions as to the outcome of conceivable new experiments. Second, our experimental studies led to the introduction of a new measure, the VEMP-associated variance modulation. Especially for slower EMG changes as they are to be expected in the experiments envisaged, the variance modulation promises significant advantages over the VEMP. The investigations are not only expected to contribute to gaining a more fundamental understanding of the VEMP phenomenon and its variants, but they are also supposed to bring about stimulation procedures that patients conceive as significantly less stressful than the currently established ones, where the VEMP threshold may be of the order of the maximum tolerable stimulus level even in healthy subjects. In order to test how well different methodological approaches lead to consistent diagnostic conclusions, not only healthy individuals, but also patients with specific vestibular disorders (e. g., vestibular neuritis, Meniere's disease) will be considered.