How strong is the link between being rem sleep disorder and developing parkinson's/lewy body dementia? is this something to be very worried about?

Worry never helps: That said, there is great interest in the relationships. It's often the best early sign of the diseases. So, do talk w/ your Dr and pursue early treatment. That's been the best way to delay and arrest progress and minimize symptoms. Worry is costly and can make symptoms worse. Just be smart and do good research by asking good ?s and working w/ Dr on plans. I wish you the best!

Association: There is an association between the two, but causation is another matter. However, it is clear that anyone with these should be followed by a neurologist who might be able to treat both, as well as any other neurologic issues that may arise.