Chiari 1.5 malformation
Chiari 1.5 malformation, or bulbar variant of Chiari I malformation, is a term used in the literature to describe the combination of cerebellar tonsillar herniation (as seen in Chiari I malformation) along with caudal herniation of some portion of the brainstem (often obex of the medulla oblongata) through the foramen magnum. It is considered a progression of Chiari I malformation.
Although it is thought to have a lower incidence than Chiari I malformation, the exact range of its incidence is unknown .
Often asymptomatic. Clinical features, if present, may include intermittent neck pain, more on extension of the cervical spine .
Chiari 1.5 malformation likely results from a Chiari I malformation combined with a smaller posterior fossa that in turn leads to overcrowding and caudal displacement of the medulla .
- congenital: progression of Chiari 1 malformation
- intracranial mass lesion
- lumbar puncture
MRI is the best method for the diagnosis with sagittal T1 WI to assess tonsillar herniation:
- descent >6 mm favors Chiari I malformation and >12 mm suggests Chiari 1.5 malformation
- associated findings may include
Treatment and prognosis
Posterior fossa decompression surgery for relieving any symptoms. Sometimes repeated surgeries are required.