Dear WRUMBLE,
Low blood pressure is one of my most obnoxious post-surgery symptoms that sill disables me. And, I am 6-years post surgery. If I stand up too fast - I’ll pass out. My neurosurgeon says I have permanent “orthostatic intolerance.” That means if I stand still in one place for over 10-seconds without moving any muscles, - especially leg muscles - my blood pressure will bottom out. When I did the tilt table test, my bottoming out blood pressure numbers were something like 60-over-45. I think that’s not too good - because I heard the tilt table techs saying they were about to go ‘get the paddles’ to revive me - once they leveled out the table. Fortunately, my pressure went up to 70-over-50 once I got some salt in me. But, I do have to be so careful.
Most of what I’ve learned about “why” and “how” symptoms happened - and keep happening - has been from my own study on brain anatomy. Like you, I’ve had to be ‘my own doctor!’ I think most of Chiarians have had to be quite proactive in trying to figure ourselves out. That’s what I love about this site - I am learning more and more every day.
All I know is that with me - my Chiari surgery involved about 1mm of cerebellum “ooze” that my neurosurgeon actually cut out & removed. (My neuro-doc jokingly said I wouldn’t miss that piece of brain) But, sometimes I DO wonder if I am missing that piece of cerebellum that got cut out! 
In my research on the brain - it is the cerebellum that is in charge of muscle coordination. A big part of my returning symptoms involves total loss of muscle coordination all over my body! I can’t walk straight. I stagger around like a drunk. I can’t pour water into a cup, tie a shoe, button a button, etc. I can get massively frustrated when I lose this muscle functioning. But, at least I have a 24-hour predictable ‘relapse’ every 4 days where my muscle coordination simply disappears. I’m learning to do nothing during those 24 hours. Just lay on my back and watch TV. In 24 hours, the muscle function returns. (So far - by the grace of God!) I just have to be patient and wait.
Same with the blood pressure problem. From researching the function of brain parts, I’ve learned that the brain stem is in charge of everything automatic - everything you don’t have to think about: sweating, shivering, eye focal muscles, intestinal movement, gag reflexes, ALL reflexes, cardiac function, diaphram breathing, etc. The list is enormous! When your brain stem gets “The Squish” from Chiaris, everything automatic can go wonky. For me, I had years of cardiac problems; palpitations for weeks, blood pressure skyrocketing followed by blood pressure plummeting - along with irregular heart beats. For years I went to various cardiologists. All of them tested my heart, and kept saying, “Your heart is normal. Don’t find anything wrong.” Grrr.
Well, the cardiologists were right. There was nothing with my heart. The problem was with my body’s ‘electrical system’ - which is the brain stem and its Central Nervous System. My CNS was short-circuiting all the time from Chiari ‘squish.’ Since my decompression surgery, I have very few of those old cardiac problems EXCEPT for the low blood pressure. Low pressure still has been my bane. Ugh.
I have asked my neurosurgeon about these symptoms - the “Why” and “How,” and I have not been given a definite answer. I’m beginning to wonder if even doctors who specialize in neurology don’t even know what specific parts of the brain and Central Nervous System all do! Personally, I think they need to study US. They need to follow all of us post-Chiari decompression surgery folk - and learn from us how our brains recover - or not recover - and why. 
I can’t believe how many of my acquaintances perpetually ask me “why does your blood pressure keep dropping? Why do you keep having symptoms after your surgery? How come your surgery didn’t fix you totally? Why? Why? Why?” It is frustrating. I get a lot of my answers from this website - not so much more from the medical professionals. Brains. Can’ live with 'em, can’t live without 'em!
Hope 2018 helps you ‘balance the pressure.’ 
BethR