I think I have a good answer to this topic. For starters I have heart failure, autonomic dysfuction (POTS exactly), and a headache and blurry vision in one eye. All of this started after a flu shot. So for the longest time I had no idea what was causing the headache or blurry vision until a Dentist showed me a medical book called Myofascial Pain and Dysfunction. I got the book and read the whole boring thing (well it's not all boring). It's essentially about trigger points, which are knots in muscles. I'm thinking my headache is caused by some really strange knots that I have. They're not rounded or ball like they're more like firm hard wires. Well the section of this book covering the Sternocleidomastoid muscle which is in the neck says that trigger points in that muscle can cause autonomic symptoms. In the book they describe a symptom that sounds a lot like POTS but maybe the authors weren't completely familiar with it. They also say the full extent of autonomic symptoms that can occur aren't known. Knots can be in different parts of the muscle and from person to person there's a variety of different ways the nerves could have grown through the muscle (just as we grew and developed). With all of the cranial nerves running through the neck it certainly seems possible that trigger points in the neck could cause POTS. I have a PDF copy of that book too if anyone wants it.