What are possible causes of arnold-chiari malformation?

Acquired or congenit: There are both acquired and congenital forms of Chiari. In a 43 yo, it is likely acquired. The acquired form, almost always a Chiari I, may be caused by increased pressure in the head, or more likely the cause will be unknown. Chiari II is always a congenital condition and is much more severe than Chiari I (in general). Read more here: http://surgeonwriter.com/chiarimonograph/

Arnold-Chiari: There are acquired Chiari malformations but these are mostly due to a device placed in the lower back. Most cases are congenital meaning they are due to a malformation at the base of the skull that has been present since birth.