[Type I Chiari malformation as a cause of inferior vertical nystagmus].

Down beat nystagmus is a central nystagmus. In the literature reviewed the two most common causes that can produce it are cerebellar degenerations and Chiari malformation. The site of lesion causing this nystagmus appears to be the brainstem or the cerebellum, although the definitive etiopathogeny remains controversial. We report the case of a 73-year-old woman with sudden unsteadiness of gait and down beating nystagmus, without any other vestibular or neurologic signs. Simple radiology and MRI evidenciated a Chiari malformation type 1 associated to plastybasia.