Acquired cerebellar tonsillar ectopia

Case contributed by Dr Mahmoud Yacout Alabd


Gradual onset weakness.

Patient Data

Age: 8 years
Gender: Male

Plain radiography showing a lumboperitoneal shunt catheter. One free end of the shunt catheter is seen in the peritoneal cavity, with the other end seen in place in the spinal canal at the upper lumbar spine.

Lower lumbar and sacral spina bifida, along with pelvic bony deformity.

Normal sized posterior fossa.

Engorgement of the venous sinuses and cortical veins as a result of negative CSF pressure.

Herniated cerebellar tonsils through the foramen magnum by around 3 cm. 
Right cerebellopontine angle (CPA) lesion with fluid instensity on T2WI indenting the pons and right cerebellar hemisphere. The pons is displaced to the left side and the left CPA cistern is obliterated. On FLAIR, the lesion's fluid signal is suppressed, confirming its CSF content. The lesion shows a bright signal on DWI, but high ADC values denote no diffusion restriction, ruling out epidermoid.

MR study done 3 years prior


Normal location of the cerebellar tonsils. Normal size of the posterior fossa. The right CPA lesion is not visible.

Case Discussion

The presence of downward tonsillar descent in a child usually put Chiari I malformation as the first possibility. But given a history of a lumboperitoneal shunt and a lack of tonsillar herniation on the previous MRI study, acquired tonsillar ectopia (previously known as acquired Chiari) can be diagnosed with confidence.

Acquired tonsillar ectopia is a complication of reduced CSF pressure, whereby the cerebellar tonsils are pulled downward. It can occur as a result of lumbar puncture, rapid relief of hydrocephalus or due to an over-functioning CSF shunt, as in this case. 

CSF-peritoneal shunts are commonly inserted between the ventricular system and the peritoneal cavity (ventriculoperitoneal shunt) which is more common. The other less common procedure is the lumboperitoneal shunt in which a shunt is inserted between the spinal subarachnoid space (usually at the lumbar region) and the peritoneal cavity.

Regarding the right cerebellopontine angle cystic lesion, it not clear whether it is due to:

  • CSF accumulation as a result of mechanical obstruction and distorted posterior fossa anatomy
  • a secondary arachnoid cyst (unlikely in the absence of a previous history of ventriculostomy or infection)
  • an arachnoid cyst that was too small to be detected on the older MRI study and has increased in size over the period of three years, augmented by the over-functioning shunt and negative CSF pressure. (Arachnoid cysts in children are usually a congenital finding present since birth.)

The patient's symptoms were probably caused by the combined effect of the pressure caused by the downward tonsillar herniation compressing the brain stem and the right CP angle cyst.

How to use cases

You can use Radiopaedia cases in a variety of ways to help you learn and teach.

Creating your own cases is easy.

Updating… Please wait.

 Unable to process the form. Check for errors and try again.

 Thank you for updating your details.