CT of the temporal bone in achondroplasia.

In an attempt to better define the changes affecting the temporal bone that might predispose achondroplastic dwarfs to otitis media, nine achondroplastic subjects who were evaluated for hearing loss underwent high-resolution CT scanning of the temporal bone. Comparisons were made with 10 nonachondroplastic subjects. A number of morphologic changes were seen, including (1) poor development of mastoid air cells, (2) foreshortening of the carotid canals, (3) narrowing of the skull base, (4) "towering" petrous ridges, and (5) relative "rotation" of the cochlea and other temporal bone structures. The most significant change was the rotational effect, which was more pronounced medially, resulting in an abnormal orientation of inner ear structures relative to middle ear structures and of middle ear structures relative to the external auditory canal. There was a notable lack of evidence for otitis media or its sequelae in any of the achondroplastic subjects. Audiograms were obtained in six of the nine achondroplastic subjects (two adults and four children). There was evidence of mixed hearing loss in the four children, but only of sensorineural hearing loss in the adults. We believe that the persistent hearing loss in achondroplasia is not due to sequelae of otitis media as some authors have suggested. Intrinsic vestibulocochlear changes below the limits of resolution of high-resolution CT scanning may be responsible.