Asymptomatic Chiari Type I malformations identified on magnetic resonance imaging

Object. Chiari Type I malformation (CMI) is a congenital disorder recognize d by caudal displacement of the cerebellar tonsils through the foramen magn um and into the cervical canal. Frequently, associated findings include abn ormalities of nearby bony and neural elements as well as syringomyelia. Cer ebellar tonsillar ectopia is generally considered pathological when greater than 5 mm below the foramen magnum. However, asymptomatic tonsillar ectopi a is an increasingly recognized phenomenon, the significance of which is po orly understood. Methods. The authors retrospectively reviewed the records of all brain magn etic resonance (MR) images obtained at our hospital over a 43-month period in an attempt to ascertain the relative prevalence and MR imaging character istics of asymptomatic CMIs. Of 22,591 patients who underwent MR imaging of the head and cervical spine, 175 were found to have CMIs with tonsillar he rniation extending more than 5 mm below the foramen magnum. Of these, 25 (1 4%) were found to be clinically asymptomatic. The average extent of ectopia in this population was 11.4 +/- 4.86 mm, and was significantly associated with a smaller cisterna magna. Syringomyelia and osseous anomalies were fou nd in only one asymptomatic patient. Conclusions. The authors suggest that the isolated finding of tonsillar her niation is of limited prognostic utility and must be considered in the cont ext of all available clinical and radiographic data. Strategies for treatin g patients with asymptomatic CMIs are discussed.