Computed Tomography

Among infants with achondroplasia and apnea, CT scanning and MRI have repeatedly demonstrated cord compression resulting from direct impingement of the posterior rim of the foramen magnum and C1 arch. Sleep apnea responds well to decompression of the foramen magnum. In addition, CT scanning shows that in virtually all children with achondroplasia, there is some degree of compromise of the foramen magnum. In about 96% of such children, the foramen magnum is smaller than 3 standard deviations of the mean; both CT scanning and/or MRI can depict this change.

A small spinal canal is present in the cervical region from birth, but symptoms of cervical canal stenosis generally do not occur until middle age or later. If neurologic deficit occurs and does not resolve through conservative measures, laminectomy at multiple levels may be required. Preoperative imaging with CT scanning, CT myelography, and/or MRI is vital for successful surgery. Preoperative and intraoperative myelography, CT evaluation, or MRI defines the level of cord and/or root compression caused by dorsolumbar spinal stenosis. Although MRI has largely replaced conventional myelography, CT myelography and intraoperative myelography may still play a role.

Otitis media is a relatively common complication of achondroplasia that can result in changes affecting the temporal bone that might predispose patients with achondroplastic dwarfism to otitis media. With high-resolution CT examination of the temporal bone, Cobb et al found the most notable change was the rotational effect, which was most pronounced medially and which resulted in abnormal orientations of inner-ear structures relative to middle-ear structures and of middle-ear structures relative to the external auditory canal. ^[6] There were also a number of morphologic changes: (1) poor development of mastoid air cells, (2) shortening of the carotid canals, (3) narrowing of the skull base, (4) towering petrous ridges, and (5) relative rotation of the cochlea and other structures of the temporal bone. ^[6]

The investigators also noted a lack of evidence of otitis media or its sequelae in any of the patients with achondroplasia. In addition, audiogram results showed evidence of mixed hearing loss in the 4 children but only of sensorineural hearing loss in the adults. ^[6] The authors concluded that the persistent hearing loss in patients with achondroplasia is not a sequela of otitis media, as other authors have suggested. Intrinsic vestibulocochlear changes below the limits of resolution of high-resolution CT scanning may be responsible.