Chiari 1 malformation

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 foramen magnum, and measuring the inferior most part of the tonsils. (see figure 3).

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 (see figure 2).

A cervical cord syrinx is present in 15 - 75% of cases (see figure 4) and is thought to result from abnormal CSF flow dynamics through the central canal of the cord.

Associations

• Klippel-Feil syndrome
• Sprengel deformity
• Platybasia
• NF1

Staging

• I = asymptomatic - treatment if any is controversial.
• II = brain stem compression
• III = syrinx

Treatment

Usually only for symptomatic patients or those with a syrinx, consists of decompressing the posterior fossa, by removing part of the occipital bone, and posterior arch of C1 as well as performing a duroplasty.

Differential diagnosis

• incidental tonsillar ectopia - <5mm
• Chiari 2
• aquired tonsillar ectopia
  • lumbar puncture
  • lumboperitoneal shunt
  • basilar invagination
  • push from above - tonsillar herniation