Asymptomatic Chiari Type I malformations identified on magnetic resonance imaging.

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Chiari Type I malformation (CMI) is a congenital disorder recognized by caudal displacement of the cerebellar tonsils through the foramen magnum and into the cervical canal. Frequently, associated findings include abnormalities of nearby bony and neural elements as well as syringomyelia. Cerebellar tonsillar ectopia is generally considered pathological when greater than 5 mm below the foramen magnum. However, asymptomatic tonsillar ectopia is an increasingly recognized phenomenon, the significance of which is poorly understood. The authors retrospectively reviewed the records of all brain magnetic resonance (MR) images obtained at our hospital over a 43-month period in an attempt to ascertain the relative prevalence and MR imaging characteristics 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 herniation extending more than 5 mm below the foramen magnum. Of these, 25 (14%) 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 found in only one asymptomatic patient. The authors suggest that the isolated finding of tonsillar herniation is of limited prognostic utility and must be considered in the context of all available clinical and radiographic data. Strategies for treating patients with asymptomatic CMIs are discussed.