Acquired Chiari I malformation by a transtentorial pseudocyst

Case contributed by Dr Mostafa El-Feky


Chronic headache.

Patient Data

Age: 45 years
Gender: Male

The hugely dilated right lateral ventricle is protruding into the posterior fossa through a deficiency in its medial wall resulting in a posterior fossa pseudocyst:

  • indenting the superior vermis, tectum and cerebellum
  • measuring 5 x 3.5 x 2.7 cm in maximum dimensions
  • communicating with the atrium of the right lateral ventricle through a wide-pore opening (9 mm in anteroposterior dimension)
  • no patent communication to the subarachnoid spaces

Downwards cerebellar tonsillar descend about 18 mm below the opisthion-basion line indenting the cervicomedullay junction, consistent with Chiari I malformation.

Moderate to marked supratentorial hydrocephalic changes with reduced mamillopontine distance (1 mm) and effaced cortical sulci. No subependymal periventricular permeation.

Obstructed inferior aspect of the cerebral aqueduct of Sylvius, ectatic third ventricle with markedly stretched depressed third ventricular floor and cisterns indenting the posterosuperior clivus and sella, associated with deficient posterior clinoid processes reflecting long standing third ventricular outflow tract obstruction.


CSF flow dynamic study with ultra-thin 3D sagittal T2 weighted images and phase contrast sequence revealed:

  • patency of the aqueduct with absent flow through its lumen  
  • CSF flow noted between the medial aspect right lateral ventricle and the posterior fossa pseudocyst
  • aliasing noted at foramen magnum, sequel to Chiari I malformation

Case Discussion

This case demonstrates transtentorial posterior fossa pseudocyst communicating with the dilated right lateral ventricle complicated by supratentorial hydrocephalus and compression of the cerebellum resulting in its downward descent and tonsillar herniation in keeping with acquired Chiari I malformation

The mass effect and combined impedance of CSF flow at the foramen magnum and distal aqueductal obstruction results in marked supratentorial hydrocephalus.

Tonsillar herniation is assessed measuring its descent from the line that connects the inner margins of the foramen magnum (basion to opisthion). It measures 18 mm in this case (above 5 or 6 mm is considered a Chiari I malformation).

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