Chiari malformation type I: a new MRI classification.

Thirty patients with Chiari I malformation were examined by MRI over 2-year period. All patients underwent MRI scan before and after surgical decompression of the posterior fossa. Images of the craniocervical junction confirmed tonsillar herniation in all cases and allowed the definition of two anatomically distinct types of Chiari malformation. Twenty-one of the 30 patients (70%) had concomitant syringomyelia and were classified as type A, while the remaining 9 patients (30%) had evidence of frank herniation of the cerebellar tonsils below the foramen magnum without evidence of syringomyelia and were labeled type B. Type A patients had a predominant central cord symptomatology; type B patients exhibited signs and symptoms of brain stem or cerebellar compression. The concomitant cord cavitary lesions (syringomyelia) were noncommunicating (isolated syrinxes), which were separated from the fourth ventricle by a syrinx-free segment of normal spinal cord. Holocord hydromyelic cavities were seen in 8 out of 21 patients with syringomyelia, isolated cervical cavities were seen in 4 patients, while combined cervical and thoracic cavities were seen in 9 patients. Kinking of the medullocervical junction and brain stem was seen in 20 out of 30 patients (67%). MRI has proved to be an excellent, noninvasive means of studying of the craniocervical anatomy; it has allowed a classification of Chiari malformation based on objective anatomic criteria with prognostic and clinical relevance.