Pitch, loudness and frequency selectivity in low-frequency hearing loss

Sammanfattning: Patients with Ménière's disease and cochlear hydrops show fluctuating low-frequency hearing loss (FLFHL). At present these changes are followed as patients' subjective reports and occasional measurements. Consecutive long-term measurements should provide more comprehensive information on the hearing fluctuations than the occasional audiogram used today and constitute an approach to quantify the fluctuations. Quantifications could potentially be used to define disease subgroups and to evaluate treatments. This thesis aims to introduce 'home audiometry' to monitor hearing function in monaural FLFHL. The approaches contain assessment of other manifestations of the diseases such as frequency selectivity and frequency coding of the auditory system. Long-term monitoring of binaural loudness and pitch matches showed that patients had daily fluctuations not present in normal-hearing subjects. The average day-to-day difference was considered as a measure of disease activity. This measure showed that patients with Ménière's disease had more fluctuations than patients with cochlear hydrops, and that both these groups had had more fluctuations than normal-hearing references. There was no simple relation between the measurements and simultaneous symptom ratings, corroborating the importance of the measurements. It seems possible to separate disease subgroups using long-term measurements of loudness and pitch matches. This could prove to be an essential feature in understanding the diseases and in clinical treatment trials. The deviant pitch matches observed during the long-term measurements suggest changes in inner ear physiology not only related to pure tone hearing. The probable cause is excessive fluid volume in the affected inner ear. Indeed, after pressure exposure in the hypobaric pressure chamber, no average hearing threshold improvements were seen in patients with FLFHL. However, there were improvements in individual subjects regarding speech recognition, outer hair cell function, and frequency selectivity. But deteriorations were also seen, mainly in frequency selectivity. Moreover, after experimentally reducing conflicting frequency information in the affected ear, the pitch matching precision improved to some extent. It is concluded that the long-term measurements provide more and comprehensive information on the hearing fluctuations than the occasional audiograms used today. More advanced measurements can be done as well, which provide more information than the blunt pure tone audiometry. Separate disease subgroups can be identified by quantifications of the fluctuations. The methods can set a new standard for the hearing evaluation of treatment in FLFHL.