I was diagnosed with OI in 2006 (the neurally mediated hypotension type), and in the last few years I thought that I had also developed POTS...but, I was wrong. Rather, the heart's SA node was misbehaving and creating what is informally called sick sinus syndrome where you move from bradycardia to tachycardia, experience lightheadedness, etc. I confused the tachycardia with POTS. I am just posting this because there is a slight chance that others who assume they have POTS might have also misidentified it, and sick sinus syndrome can be effectively treated. I just had a pacemaker implanted. It is a huge relief and I feel so much better. I had thought that my ME/CFS had progressed as I began losing function a few years ago. So now I am hoping to regain the lost function which would put me at a more liveable level of functionality. While it is slightly weird to know that a computer is controlling my heart rate, it is doing a much better job than I was!