Abstract:
Auditory Neuropathy Spectrum Disorder (ANSD) describes a variety of processing deficits which impact on the ability to acquire and comprehend language. Extreme prematurity – defined by birth around 28 weeks gestation – and the hypoxic-ischemic insult that often follows is a risk factor for ANSD. This insult results in lesions at the inner hair cell synapse, auditory nerve, and the subsequent auditory pathway causing dys-synchronous activity, atrophy of auditory nuclei and degraded processing of signals. The present study investigated the effects of hypoxia-ischemia on the auditory brainstem in a novel model of extreme prematurity developed by Oorschot et al. (2013). Male rat pups aged P1-3 were exposed to repeated bouts of hypoxia (1.5% O2, 5% CO2 and 93.5% N2 at 37oC every 2 hours for 12.25 hours per day) (Oorchot et al., 2013) and sacrificed at P14, P21 and P98 for analysis of structural and functional deficits. Total reference volume and absolute neuron number were measured for the ventral and dorsal cochlear nucleus, superior olivary complex and the lateral lemniscus. Generally, volume loss with variable decrease in neuron population was observed across structures, but particularly in the ventral cochlear nucleus and lateral lemniscus in the repeated hypoxic group for each age group. Features of the auditory brainstem response waves II-V, which are associated with the aforementioned structures were analysed. Smaller amplitudes with longer latencies were observed in repeated hypoxic rats aged P21, whereas larger amplitudes with shorter latencies were recorded at P98. In addition, the interpeak intervals I-III, I-V and III-V were analysed and were found to be longer in the repeated hypoxia group at P21 and similar to control at P98. The threshold of response was similar for repeated hypoxic and normoxic rats across ages. Although variable, the present results uncovered multiple trends which indicate that this model of prematurity produces lesions throughout the auditory brainstem, affecting the structural integrity and the functional activity of the studied auditory nuclei.