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T HE ARNOLD-CHIARI malformation is a deformity of the hindbrain in which a tongue-like projection of the cerebellar tonsils protrudes through the foramen magnum down onto the cervical cord. The caudad portion of the 4th ventricle likewise is elongated downward. The upper cervical nerve roots pursue a cephalad direction, giving the impression that the brain stem has been drawn down through the foramen magnum.Tile malformation was named after Arnold 3 and Chiari 6 who independently described the anomaly in 1894 and 1895 respectively. Schwalbe and Gredig 23 published an embryologic and anatomic treatise on the subject in 1907.We have operated upon 17 patients, adults and adolescents, with the Arnold-Chiari malformation and our experience suggests that the fundamental mechanism is obstructive hydrocephalus with resulting foraminal herniation of the hindbrain.Interest in the anomaly was revived in 1935 when Russell and Donald 22 discussed its importance as a possible cause of internal hydrocephalus in spina bifida. They presented 10 examples of myelomeningoeele associated with hydrocephalus and Arnold-Chiari malformation in infants. McConnell and Parker 16 reported surgical treatment of 5 patients ranging in age from 10 to 3e years with e successful results. All were found to have some degree of hydrocephalus. D'Errico s in 1989 described 10 cases of myelomeningocele with hydrocephalus in infants. The myelomeningocele was repaired in 7, and when hydrocephalus followed, operation for the Arnold-Chiari malformation was done. Three remained in good health for a period of ~ years thereafter. He stressed the advisability of doing a suboccipital decompression in all cases of spina bifida associated with myelomeningocele and hydrocephalus. In 1989, MeKissoek 17 stated that he routinely treated hydrocephalus with associated myelomeningocele by first decompressing the Arnold-Chiari malformation, and that this frequently benefited both the hydrocephalus and the sac protrusion.Chamberlain 5 in 1989 described concomitant findings of Arnold-Chiari malformation with platybasia and described the latter condition as follows: "The morphologic changes shown by roentgenograms give the impression of softening of the base of the skull and moulding through the force of gravity. It is as though the weight of the head has caused the ears to approach the 199

Bony anomalies encountered in the ‘no neck’ form of Klippel-Feil syndrome (KFS) are a wide, short, fused, bifid, retroflexed spinal canal; craniolacunia, cranium bifidum, and acrania. The only symptom may be mirror movement (MM). The CNS anomalies are hindbrain hernia, hydrocephalus, hydromyelia, syringomyelia, meningocele, myelocele, encephalocele, and anencephalus. In severe KFS, i.e. iniencephalus (IN) and in anencephalus (AN), the inion is in contact with the back. In both there is hindbrain hernia and the left thorax may contain the stomach tethered to an anterior spina bifida. KFS results from distortion of somites by an overdistended neural tube. A neural tube that fails to close cannot overdistend.