Summary: Chiari 0 caused by cerebral thrombosis of transverse sinus (clot/collapse of anterior major vein in the brain).
Dr. B explained it like this: when one major vein is missing or not working, the others work harder to pump blood to the brain which then increases intracranial pressure which then pushes the cerebral tonsils down and backwards.
My treatment at this time is meds, diamox the most important one. It is supposed to reduce the amount of csf surrounding the brain and hopefully in doing so, alleviate some pressure on the cerebral tonsils. Dr. B said it will be a slow process. If the meds don't work, we'll go from there. I'll get a follow up MRI in a few months to see where my tonsils lie and assess symptoms again.
I admit, I never saw this coming. Pretty much - I had a stroke that went undiagnosed for 4 months by my neuro docs here.