Why is this woman having left-sided weakness post Chiari decompression?

This 30-year-old female presents with left-sided weakness post Chiari decompression.

Q1 – What is the most likely diagnosis?

(a) Inadequate decompression
(b) Hydrocephalus with slit ventricles
(c) Pseudomeningocele
(d) Cerebellar ptosis
(e) Dandy-Walker continuum

Try to answer the question by looking at the first five images. Then, have a look at the latter five (images 6 through 10) duplicate images with arrows added for key findings.

Sagittal T1 FLAIR

Sagittal T1 FLAIR

Sagittal T1 FLAIR

Axial T1

Axial T1

Sagittal T1 FLAIR

Sagittal T1 FLAIR

Sagittal T1 FLAIR

Axial T1

Axial T1

 

A1 – (d) Cerebellar ptosis

When evaluating causes of failure of posterior fossa decompression in Chiari, it is important to systematically check that each of the following are NOT potential causes:

  • Insufficient posterior fossa decompression
  • Overvigorous decompression
  • Unrecognized hydrocephalus
  • Pseudomeningocele formation

Chiari decompression may be indicated if a syrinx is present and in situations where neurologic findings are present or in a sterotyped clinical presentation of tussive occipital or suboccipital headaches with syrinx. The goal of surgery is decompression of the cerebellar tonsils and opening of the CSF pathways at the skull base.

Decompression requires suboccipital craniectomy and often C1 laminectomy and may also include duraplasty sufficient to unblock the CSF pathways at the skull base. Overvigorous decompression above the equator of the cerebellum may result in downward migration of the cerebellum (ptosis) which may reproduce the obstruction at the skull base. This may result in recurrent symptoms and persistence, or re-expansion of a collapsed syrinx. This is a common but poorly appreciated cause of failure of Chiari surgery.

The failure to recognize that tonsillar displacement is secondary to increased intracranial pressure or an intracranial mass, and not a true Chiari, may also result in failed surgery. This is a serious error.

Pseudomeningocele occurs most commonly when duraplasty is performed but the repair breaks down or is not water tight.

Remember that any suspected Chiari malformation seen on a cervical spine MR should always prompt an examination of the brain for other causes of tonsillar displacement such as increased intracranial pressure or decreased intrathecal pressure (intracranial hypotension), the latter of which may cause tonsillar sagging.

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Reference:

1. Holly LT, Batzdorf U. JNS 2001 Jan: 94, 21-26.

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