I have no idea what brought it on, I had no warning that it was coming. I was standing with him in the paddock, he was chilled out, we were snuggling. Without warning, he stumbled a bit to the right and fell. He hit the ground hard enough that it flipped him over. When he got up his hindquarters and back legs were visibly shaking. The shaking stopped after a few seconds and he walked off like nothing had happened.
It was literally about 30 seconds from the stumble, to him being back on his feet and fine again.
I was in shock, so didn't think to take his vitals straight away. Rang my vet and he prompted me to check his heart rate, etc. Heart rate was 36ish, capillary refill was normal, gums normal healthy pink. Ran him through basic at home neuro tests (tight circles, crossing legs, flexing) and all seemed normal there too.
Just looking for any thoughts on what may have caused this. I have spoken with my vet and his first thought is a heart condition. His advice at this stage is to observe him, take vital signs, etc, as well as doing lunge work daily to see whether he has any reaction to exercise. He has been off work for a while due to battling greasy heel. Obviously, I can't ride him, at least until we have it figured out, as there is the risk of it happening again.
Please feel free to ask questions, I may have forgotten something as I am quite stressed.