Chiari malformation

Scan of Chiari malformation I cerebellar tonsil herniation associated with syringohydromyelia

Chiari Malformations:

Type I

-Caudal decent of cerebellar descent inferior to plane of foramen magnum >3-5mm

- rarely below C2

- not assciated with myelomeningocele

-hydrocephalus in <10%

Type II

- Caudal descent of cerebellar vermis and lower brainstem

-commonly seen below C2

-multiple posterior fossa and cerebellar abnormalities associated with hindbrain hernias including beaking of the dorsal midbrain , enlargement of massa intermedia , medullary kinking , hypoplasia of tentorium

-hydrocephalus >90%

-in conjuction with meningomyelocele very common

Type III

-rare

-protrusion of sac from craniocervical junction that contains portion of cerebellum and brainstem

-associated with hydrocephalus

Type IV

severe hypoplasia or aplasia of cerebellum associated with a diminutive posterior fossa

Many theories have been developed to explain the Chiari malformations and currently the most appealing theory emphasizes in difficulty in rapidly equilibrating the CSF pressure wave between the posterior fossa and cervical subarachnoid space seen during the valsalva maneuver.