The Clivus Deformity of the Arnold-Chiari Malformation

Of 24 patients with Arnold-Chiari malformation, 16 had a satisfactory demonstration of the clivus on a midline sagittal tomogram of the posterior fossa. In all but 1 there was obvious deformity of the basiocciput: marked concavity with extreme thinning of the bone near the basion. The deformity is thought due to the pulsatile pressure from the crowded posterior fossa contents and the low-lying hindbrain onto the bone directly, enhanced by the paucity of the cerebrospinal fluid between brain and bone.