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.