-We re p o rton a 49 year old man with impacted cisterna magna without the presence of syringoh y d romyelie (SM). The clinical picture was characterized by spastic paraparesis. Magnetic resonance imaging depicted a cisterna magna filled by the cerebellar tonsils. Six months after osteodural-neural decomp ression of the posterior fossa there was resolution of neurological symptoms and signs with the exception of hyperactive patellar and Achilles reflexes.KEY WORDS: tight cisterna magna, impacted cisterna magna, Chiari malformation, posterior fossa decompression, spastic paraparesis. Relatamos o caso de paciente de 49 anos com cisterna magna impactada sem a pre s e n ç a de siringo-hidromielia (SM). O quadro clínico caracterizava-se por paraparesia espástica. A re s s o n â n c i a magnética evidenciou a cisterna magna preenchida pelas tonsilas cerebelares. Seis meses após a descomp ressão osteodural-neural da fossa posterior, houve resolução dos sintomas e sinais neurológicos, com exceção da hiperatividade dos reflexos patelares e aquileus.PA L AV R A S -C H AVE: cisterna magna impactada, malformação de Chiari, descompressão da fossa posterior, paraparesia espástica.Hans Chiari 1 , 2 described four types of cere b e l l a r anomalies (CM). Type I characterized by downward displacement of the cerebellar tonsils and the medial portions of the inferior cerebellar lobes which accompanied the medulla into the cervical spinal canal. Type II showed downward displacement of port i o n s of the cerebellum (1891), and portions of the inferior vermis (1895), pons, medulla oblonga and, at least, p a rt of lengthened fourth ventricle, which re a c h e d the disc C 4-C 5, into the enlarged cervical spinal canal. In type III, the hydrocephalic cerebellum, pons and medulla oblonga were inside a cervical meningocele (hydroencephaloceles cerebellaris cerv i c a l i s ) , t h rough a spina bifida of the first three cervical vertebrae. In type IV, there was hipoplasia of the cerebellum without herniation of cerebellar stru c t u res into the spinal canal. Iskandar et al. 3 (1998) related five cases of syringohidromyelia (SM), in which the cist e rna magna was filled by the cerebellar tonsils. After d e c o m p ression of the posterior fossa, there was clinical improvement, as well as marked reduction in the size of the syrinx in all cases. The authors admitted t h a t this dramatic response to decompression indicates that this entity has a Chiari-like pathophysiology.