Temporal processing in auditory and visual systems: evidence of wider central nervous system involvement in glaucoma

© 2011 Fleur O'HareBackground: Open Angle Glaucoma (OAG) is a condition that is characterised by a loss of retinal ganglion cells; which in turn inhibits visual signals, through a reduction in the levels of information being sent to the brain for processing. The degree and rate at which this inhibition occurs can vary greatly between individuals and therefore may be a consequence of innate differences in sensory nerve susceptibility to injury. In the event of an underlying neuronal susceptibility, signs of auditory processing dysfunction may be observed in individuals with OAG. Aims: The purpose of this study was to examine whether individuals with OAG display signs of central auditory processing dysfunction. Concurrent investigation of specific visual processing functions was conducted to assess if any similarities in neural processing dysfunction were evident across both visual and auditory systems within the same individuals. Methods: Experiment 1: A comprehensive range of peripheral and central auditory processing tests were conducted in 42 OAG individuals and matched controls. Central auditory function tests included evoked potential measures, tests for auditory temporal processing and speech perception. Experiment 2: Based on the findings of Experiment 1, specific auditory temporal processing tasks were performed in conjunction with hierarchical visual temporal processing tasks. Results: Experiment 1: Compared to age equivalent controls, OAG participants displayed significantly poorer amplitude modulation detection and speech perception under high levels of background noise. Amplitude modulation detection moderately correlated with speech perception and ABR performance within both groups. Collectively, signs indicated evidence of impaired auditory neural temporal processing in a significant proportion of the OAG cohort. Experiment 2: OAG Individuals showed a tendency toward poorer performance on both auditory and visual discriminatory tasks that require a high degree of temporal resolution. Specifically, poorer speech perception under competing noise as well as impaired visual speed discrimination for slow velocities were identified in a significant proportion of OAG individuals (39.13% and 47.82% respectively, were below the 90th percentile of control performance). Overall, performance on auditory processing tasks did not correlate with performance on visual processing tasks for either group. Conclusions: A subgroup of OAG individuals display evidence of central auditory neural processing dysfunction which implies that glaucoma may be associated with other non-visual sensory neural deficits. As such, the results indicate increased neuronal susceptibility in these individuals. Clinical Significance: This is the first study to explore auditory function in OAG individuals beyond assessing sound detection or the integrity of the middle and inner ear. A proportion of OAG individuals have shown signs of central auditory processing dysfunction with subsequent difficulty in perceiving speech in noisy environments. It is therefore suggested that individuals with OAG, who report hearing difficultly, be subsequently assessed to explore the impact and extent of this difficulty in relation to challenging auditory environments. This process will ensure a more comprehensive assessment is undertaken which will lead to more direct and appropriate treatment options offered. In regard to the visual findings, OAG individuals with impaired speed and motion discrimination may require interventional strategies to improve driving/pedestrian safety and navigation, an area for future research. Overall, further research to explore other specialised auditory functions in glaucoma patients would be warranted along with an investigation into how having both auditory and visual impairment impacts upon communication and quality of life