Acquired tonsillar ectopia

Case contributed by Dr Mahmoud Yacout Alabd


Gradual onset weakness.

Patient Data

Age: 8
Gender: Male

Plain radiography showing the lumboperitoneal shunt catheter. There is also lower lumbar and sacral spinabifida together with pelvic bony deformity.


MRI study showing downward tonsillar descent with a right CP angle CSF intensity lesion.

Normal sized posterior fossa.

Engorged venous sinuses and cortical veins.


Three years old follow up MRI, prior to the current symptoms. Normal location of the cerebellar tonsils. Normal size of the posterior fossa. The right CP angle lesion is not seen.

Case Discussion

The presence of downward tonsillar descent in a child usually put Chiari malformation as the first possibility. But with the absence of the other findings of Chiari malformation such as small posterior fossa, together with the history of lumboperitoneal shunt and the lack of tonsillar herniation at the previous MRI study, acquired tonsillar ectopia  (previously known as acquired Chiari) can be diagnosed with confidence. 

It is a complication of reduced CSF pressure that leads to a downward descent of the cerebellar tonsils. It can occur in cases of lumbar puncture, rapidly relieve of hydrocephalus or due to an over-functioning CSF shunt as in this case. 

CSF peritoneal shunts can be either between the ventricular system and the peritoneal cavity (ventriculoperitoneal shunt) which is more common. The other less common procedure is lumboperitoneal shunt in which shunt between the spinal subarachnoid space (usually at the lumbar region) and the peritoneal cavity is done.

It is not clear whether this right CP cystic lesion is due to:

  • CSF accumulation as a result of mechanical obstruction and distorted posterior fossa anatomy
  • a secondary arachnoid cyst (which is unlikely in the absence of  previous history of ventriculostomy or infection)
  • an arachnoid cyst that was too small to be detected on the older MRI study, that 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 that is present since birth)

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


PlayAdd to Share

Case information

rID: 39950
Published: 15th Oct 2015
Last edited: 22nd Oct 2015
Inclusion in quiz mode: Included

Updating… Please wait.

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

 Thank you for updating your details.