Heat and epilepsy aren't fun. I've lived in the desert my entire life, but started having seizures about 9 years old.
I've lived in the more moderate heat, mostly - where it gets between 90F - 100F in the summer, but I've also lived where it gets to be 120F+ in the summer and let me tell you...those 120 degree days could cause me a lot of issues and we moved away from that after doing two summers out there (not in a row, we went one year, took a break one year, went back a different year - yeah, nope).
When I am dehydrated, my seizures are the worst ever. Last time I had a bad seizure, I was dehydrated, but I wasn't aware enough to drink anything safely and had 2-3 more and since they weren't wanting to stop - I was sent to the hospital by ambulance. I had COVID as well. They pumped me full of 3 1/2 IV fluid bags, and I was barely getting hydrated. It was bad. What did they recommend? Electrolytes such as Gatorade.
Electrolytes are insanely important because our bodies are mostly electrolytes. When they say our bodies are mostly water, that's not exactly true - it's electrolytes! Electrolytes have insanely important minerals that our bodies can't function without like potassium, sodium, etc. etc. There was a great electrolyte powder I loved, but it is too expensive and one of my kids has sensitivities to it's ingredients so I'd prefer not to have it in the house (it's called Ultima). Even if I have to, Gatorade or Powerade are helpful, but it's so much sugar and I'm not fond of the ingredients, but if it is a dire need, it will still help. Body Armor is great, but I don't handle the no sugar (Erythritol) one well, unfortunately, so I will only get the original if I need it.
What works the best and has less sugar is Coconut Water (get a good organic brand, have it on hand for those days you feel dehydrated).
If you get to know the feeling of getting dehydrated, you can help yourself better. If your throat feels like sand paper or it's sticking together, you're dehydrated. If you can't sweat or cry, you're dehydrated. Dry mouth can be a good indication of dehydration, but not always. If you sweat, you should replenish those fluids with mineralized or electrolyte drinks. Keep in mind if you're dehydrated too much, you probably won't feel thirsty and a drink is going to be the last thing you want.
Low electrolytes/Dehydration are a common seizure trigger.