Odd that you username is shunt.
No,I am not a cat.
No,I have never had a shunt fitted.
Yes I do have limited knowledge of such shunts.
Both in all animals including humans, there can be a build up of fluid in the brain due to many causes and if this fluid is not released then death occurs quite quickly.
In animals it is due mainly to tumours and in humans similar causing Hydrocephalus {fluid on the brain}
A shunt can be inserted into the brain and the fluid drained directly into the heart or even abdominal cavity by a tube,
This tube can even have a valve to control the flow.
Such a device is used in children with congenital Hydrocephalus called Spitz-Holter valve named after two American paediatric surgeons.
No, your cat will not feel a thing.