MRI of Chiari malformations and other lesions at the craniocervical junction

Magnetic resonance imaging (MRI) can be used in diagnosing Chiari malformations and other lesions at the craniocervical junction. The authors evaluated the MR findings in 16 cases of Chiari Type I malformation, 43 cases of Type II, and two cases of a basilar impression (BI) with an atlantoaxial dislocation (AAD). T1 and proton density-weighted (spin echo with short repetition and echo times) sagittal and coronal images were useful in evaluating the grade of the downward herniation of posterior fossa structures and the accompanying anomalies. A downward herniation of cerebellar tonsil was the main characteristic finding in the Chiari Type I malformations. On the contrary, there was a downward herniation of the cerebellar vermis and tonsil and of the brainstem, in addition to the presence of many other brain anomalies and hydrocephalus, in the Chiari Type II malformations. A quantitative analysis was made by introducing some index points - the C1-line, the foramen magnus, the midbrain-pontine junction, the ponto-medullary junction, and the fastigium point - in Chiari malformations. The study disclosed an enlargement of the foramen magnum and a low position of each index point. The herniation of posterior fossa structures was especially more marked in symptomatic than in asymptomatic cases of the Chiari Type II anomaly. In basilar impression MRI provides satisfactory information for planning surgical treatment. (author)