Auditory Neuropathy Spectrum Disorder (ANSD) is diagnosed by the presence of outer hair cell function, and absence or severe abnormality of the auditory brainstem response (ABR). Within the spectrum of ANSD, level of severity varies greatly in two domains: hearing thresholds can range from normal levels to a profound hearing loss, and degree of speech perception impairment also varies. The latter gives a meaningful indication of severity in ANSD. As the ABR does not relate to functional performance in ANSD, there is a lack of clinically useful information after diagnosis in infants until they are developmentally able to perform behavioural assessment. Some neurodegenerative conditions, such a Friedreich's Ataxia (FA) and Charcot-Marie-Tooth disease (CMT) can also present with an ANSD-type perceptual pattern of auditory impairment. There is currently little research into the auditory profile of these conditions. In these neurodegenerative conditions, where physical mobility and test fatigue can be an issue, behavioural testing can be difficult. Thus finding an alternative objective test to the ABR would be useful in both the infant ANSD population, and in adults with neurodegenerative ANSD-type hearing loss. Cortical auditory evoked potentials (CAEPs) are thought to be a promising objective tool in these populations. The aim of this research was to investigate the relationship between CAEPs and speech perception in these participants for whom speech perception does not relate to hearing thresholds or ABR. The first study of this thesis, a pilot, tested CAEP presence and speech perception ability in FA and CMT. This pilot showed significantly poorer performance amongst those with FA. CAEPs were not present in those with the poorest speech scores, but the sample was too small to identify a relationship. A larger follow-up was carried out in adults with FA, CMT and sensorineural hearing loss (SNHL). This study aimed to assess if CAEPs related to speech perception ability, and to compare the auditory profile of those with FA and CMT to those with the more typical SNHL. This second study did not find a significant correlation between CAEP measures and speech perception, and showed no significant differences in performance between groups. This was likely due to the significantly worse hearing levels of those with SNHL, which was a major limitation of the study. The second study found no significant differences between FA and CMT groups on CAEP measures, hearing levels, speech perception or gap-detection thresholds. The significant differences between groups in the pilot study are likely attributable to differences in global progression, rather than an inherent difference between these neurodegenerative conditions. Following the adult studies, the relationship between CAEP presence and speech discrimination in three infants with ANSD was assessed. These case studies did not demonstrate a relationship between CAEP presence and degree of ANSD severity, and further research in this area is required with a larger sample. Young infants spend most of the day in sleep, and it can be difficult to maintain a resting state suitable for CAEP testing when awake. The final study revealed that CAEP responses were recordable in infants during sleep, but that it was state dependent. REM sleep was the optimal recording state.
|Date of Award||31 Dec 2015|
- The University of Manchester
|Supervisor||Kai Uus (Supervisor) & Kevin Munro (Supervisor)|