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 4. It is considered a progression of Chiari I malformation.
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Epidemiology
Although it is thought to have a lower incidence than Chiari I malformation, the exact range of its incidence is unknown 2.
Clinical presentation
Often asymptomatic. Clinical features, if present, may include intermittent neck pain, more on extension of the cervical spine 2.
Pathology
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.
Etiology
congenital: progression of Chiari I malformation
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acquired
intracranial mass lesion
lumbar puncture
Radiographic features
MRI
MRI is the best method for the diagnosis, with sagittal T1-weighted imaging best to assess tonsillar herniation:
descent >6 mm favors Chiari I malformation and >12 mm suggests Chiari 1.5 malformation
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associated findings may include
posterior angulation of the odontoid process
crowded small posterior fossa
Treatment and prognosis
Posterior fossa decompression surgery for relieving any symptoms. Sometimes repeated surgeries are required.
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