I'm pretty sure my grandmother had it, undiagnosed. My mom said she was fatigued a lot, prone to fainting or feeling faint and had no tolerance for heat in her 30s and 40s especially. She had periods in her life where she was lying down during the day, daily. Never diagnosed. For some reason, if it is genetic in my case, my mom was never at all affected. I have 3 sisters but I am the only one with POTS.