It's possible there is a connection between your sister's Chiari malformation (CM) and her appendectomy. But it's also possible the two are separate, unrelated problems.
And without a before and after MRI, it may not be possible to find out what happened. The MRI would show changes in the position of the cerebellum and the amount and location of cerebrospinal fluid (CSF).
In CM, the cerebellum (part of the brain at the base of the skull) slides down through the opening for the spinal cord. Pressure on the cerebellum can cause serious neurologic symptoms. The CSF around the brain normally moves in gentle waves with every heartbeat.
Anything that blocks CSF movement can cause it to build up in tube-shaped pockets along the spine. These pockets of CSF are called syrinxes. Pressure from the syrinx can damage and even destroy the spinal cord.
Problems could have occurred if your sister had a spinal injection of anesthetic for her appendectomy. The procedure could have caused a tear in the lining around the spinal cord. CSF could have leaked out, collecting in a little pool around the base of the spinal canal.
Pressure on the spinal cord from the built up fluid could have caused her symptoms. Careful study of MRIs may help identify the cause of the problem. The treatment is likely to be decompression of the spinal cord.
The surgeon will remove a piece of bone from around the cerebellum. Symptoms usually resolve quickly after this operation. In the future, your sister should make sure her doctors and surgeons know she has a CM before planning any treatment for her.