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 .

Clinical presentation

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
  • acquired
    • intracranial mass lesion
    • lumbar puncture

Radiographic features


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.

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