Cochlear neuronal populations in developmental defects of the inner ear. Implications for cochlear implantation.

A histological study was made to determine the cochlear neuronal populations of 20 human ears having hearing loss caused by developmental defects. The neuronal populations ranged from 7677 in an ear with Mondini dysplasia to 30 753 in an ear with DiGeorge's syndrome, the norm for young human subjects being 35 000 neurons. The length of the cochlear (spiral) ganglion varied from 7.3 mm to 14.8 mm, the norm for human subjects being 12 mm. The sensorineural hearing losses in all cases were attributable to malformation or degeneration of the sensorineural structures. The hearing loss was moderate in one case of Alport's syndrome, severe in one case of Usher's syndrome and in one case of severe Mondini dysplasia; it was profound in one case of maternal rubella, one of congenital deafness of unknown cause, another case of severe Mondini dysplasia and one of Down's syndrome (Trisomy 21). One case of mild Mondini dysplasia and one of DiGeorge's syndrome were known to have normal hearing. In 4 other cases hearing status was absent.