Acquired cerebellar tonsillar ectopia

The presence of downward tonsilartonsillar descent in a child usually put Chiari malformation as the first possibility. But with the absence of the other findings of chiariChiari malformation such as small posterior fossa, together with the history of lumboperitoneal shunt and the absencelack of tonsillar herniation at the previous MRI study, Acquired Tonsillar Ectopiaacquired 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, rapidrapidly 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. The other less common procedure is Lumboperitoneallumboperitoneal shunt which is done 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. 
• Aa secondary arachnoid cyst (which is unlikely in the absence of  previous history of ventriculostomy or infection). 
• Anan 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.