Chiari 1

chiari type I

Chiari 1 malformation is characterised by inferior herniation of the cerebellar tonsils through the foramen magnum, due essentially to a mismatch between size and content of the posterior fossa. Chiari 1 needs to be distinguished from tonsilar ectopia, which is an asymptomatic and incidental finding in normal individuals, whereby the the tonsils protrude through foramen magnum by no more than 5mm. The distance is measured by drawing a line from the inner margins of the foramen magnum, and measuring the inferior most part of the tonsils. (click here for diagram).

On sagittal imaging the tonsils are pointed and “peg like”, and the sulci are vertically oriented, forming “sergeant stripes”. Axial images through the the foramen show crowding of the medulla by the tonsils. (click here for image)

A cervical cord syrinx is present in 15 – 75% of cases and is thought to result from abnormal CSF flow dynamics through the central canal of the cord.

For more on Chiari 1 malformations visit Radiopaedia.org here.

Credit: Dr Frank Gaillard