welcome to TiMS, aj.
interesting, i didn't know anything about chiari malformations. what i learned so far: there are several types. yours, type i/'secondary' CM, is the only one that can be acquired after birth (vs primary/congenital CM which can be caused by genetic/nutritional issues during fetal development).
CM has several different causes. It can be caused by structural defects in the brain and spinal cord that occur during fetal development, whether caused by genetic mutations or lack of proper vitamins or nutrients in the maternal diet. This is called primary or congenital CM. It can also be caused later in life if spinal fluid is drained excessively from the lumbar or thoracic areas of the spine either due to injury, exposure to harmful substances, or infection. This is called acquired or secondary CM. Primary CM is much more common than secondary CM.
so. injury, exposure to harmful substances, infection. do these appear in your medical history to such an extent that the docs feel confident they're the source of your issues? also, do they know for sure that it developed later in life? is it possible a slowly developing issue has begun to manifest itself, but it was always there? based on this blurb about it, it seems like type I is actually primary, but it just doesn't get noticed as early as more severe primary cases...
"Type I involves the extension of the cerebellar tonsils (the lower part of the cerebellum) into the foramen magnum, without involving the brain stem. Normally, only the spinal cord passes through this opening. Type I — which may not cause symptoms — is the most common form of CM and is usually first noticed in adolescence or adulthood, often by accident during an examination for another condition. Type I is the only type of CM that can be acquired."
i will have a read, to try and find out how researchers think this problem is acquired when it manifests later in life.