Development of vestibular evoked myogenic potentials and audiometry for the clinical diagnosis of superior canal dehiscence syndrome

Sammanfattning: Often in Audiology and Neurotology the focus is on disorders characterized by loss of hearing and vestibular function. A complementary approach is to look at the manifestations of abnormally augmented auditory or vestibular functions, commonly named auditory and vestibular hypersensitivity. The present thesis deals with several aspects of a prototype of hypersensitivity syndrome, the superior canal dehiscence syndrome (SCDS). This relatively rare syndrome is characterized by cochlear and vestibular hypersensitivity to sound and vibrations, but also to pressure stimulation which normally has no influence on hearing or balance. By use of a recently introduced vestibular test method, vestibular evoked myogenic potentials (VEMP), it is possible to obtain an objective measure of vestibular sensitivity to sound and vibrations. The thesis describes the background to the research field presenting: the normal function of the ear, the function in presence of conditions giving sound/vibration and pressure hypersensitivity (third mobile window syndromes), a detailed description regarding the superior canal dehiscence syndrome and the vestibular evoked myogenic potentials. The thesis then presents and discuss in details the four scientific papers which are the core of this research project. In the first paper a new vestibular evoked myogenic method is presented. This method, based on low frequency bone conducted stimulation, showed a high diagnostic ability for SCDS, especially valuable in those sporadic conditions when other methods, based on sound stimulation, cannot be applied. The second paper presents a new approach for testing the cochlear hypersensitivity to body sounds. This phenomenon, expressed clinically by the symptom autophony, is particularly evident in SCDS. The study showed that it is possible to retrieve a measure of the internal body sound hypersensitivity by means of a specific audiometric application based on the delivery of bone conducted stimuli at distance sites and not at the mastoid as normally performed. The third paper is a comparative study on the actual ocular VEMP (oVEMP) methods investigating their diagnostic ability for SCDS in a large cohort of dizzy patients. The study confirmed the diagnostic superiority of oVEMP evoked by sound stimuli over the two other methods based on oVEMP evoked by bone conducted stimuli. In the fourth paper SCDS the diagnostic ability of sound induced VEMP protocols is studied. The testing is performed at reduced stimulus intensity levels according to the newly released recommendation on acoustic stimulation restrictions during VEMP testing. Specifically, VEMP was tested at sound intensity levels compatible with safe acoustic exposure levels in audiological testing. The study showed that these reduced sound levels did not affect the diagnostic accuracy of VEMP for SCDS. This is clinically relevant, as it will make it possible to carry out a SCDS diagnosis even among patients affected by abnormal acoustic susceptibility.