In 1891 and 1896, Chiari classified malformations of the hind brain that caused congenital hydrocephalus. In this classification were three of the four types of malformations that are now included in the group of Anold-Chiari malformations. In type 1, the cerebellar tonsils and medulla are displaced caudally into the foramen magnum. The medulla is often elongated, and a meningomyelocele is rarely present. Symptoms are often first noted in adult life.
We report here a 46-year-old female with Arnold-Chiari malformation type 1 and review the literature on this subject. Neuro-otological examination of this patient revealed horizontal rebound nystagmus and down-beat nystagmus. The preoperative and post-operative findings indicated that:
1. Horizontal rebound nystagmus was due to bilateral lesions of the nucleus fastigii and dentate.
2. Down-beat nystagmus was due to lesions of the inferior brain stem and inferior cerebellum.
