Chiari 1.5 malformation

Last revised by Dr Dalia Ibrahim on 21 Feb 2021

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 2.

Often asymptomatic. Clinical features, if present, may include intermittent neck pain, more on extension of the cervical spine 2.

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 3.

  • congenital: progression of Chiari 1 malformation
  • acquired
    • 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

Posterior fossa decompression surgery for relieving any symptoms. Sometimes repeated surgeries are required.

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Cases and figures

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  • Case 5
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