Prolonged conductive hearing loss in rat pups causes shorter brainstem transmission time.

A carefully controlled study was conducted in rats to determine whether a reversible conductive hearing loss during the neonatal period could induce changes in central conduction and thereby perhaps contribute to an understanding of learning problems seen in children following conductive hearing loss in infancy. Ear plugs were inserted from post-natal day 9 in rat pups and auditory nerve brainstem evoked responses were recorded in the presence of ear plugs and following their removal on post-natal day 23. The I-IV interpeak latency (brainstem transmission time) was significantly shorter in the experimental rats several days after plug removal (day 28) compared to untreated control rats. This difference was not related to residual conductive loss, nor to different body weights and was present even in response to equal sensation level stimuli, and was not present in adult rats with similarly induced conductive hearing losses. It seems therefore that conductive hearing loss in the young animal (critical period?) can induce changes in central auditory conduction and may be related to the findings of smaller brainstem auditory neurons in sound deprived animals. These results have implications for neonatal hearing loss in humans.