What is the cause of "idiopathic syringomyelia"?

Idiopathic. Unfortunately, idiopathic means "unknown cause". While the cause may not have been adequately worked up yet (in which case it may be premature to call it idiopathic), if you have had an extensive workup and very smart people have looked at the studies, then the cause will likely remain a mystery for now.
Idiopathic. Idiopathic means we do not have a specific cause. Probably related to development of the neural tube, there is variability in the size of the central canal.
Idiopathic=dont know. Unfortunately, the term "idiopathic" means that there is no clear underlying cause. Once a cause has been identified, it is no longer an idiopathic disease. (as a side note, i was told by a mentor that the term "idiopathic" means that the doctors are idiots because they can't figure it out yet.).