I have a cousin on my dad's side who was diagnosed with IST at the same time I was. My sister, my dad, and my cousin's mom (dad's sister) have all been to the doctor about arrhythmias and episodes of tachycardia, but none of them have symptoms that are particularly disruptive. I thought this was unusual, as at the time of diagnosis, I was told by my electrophysiologist that IST was fairly rare.