Same with our Tigger. He could not walk in a straight line.
As far as heart worm medz go. I think if you stick to cat food only and avoid giving people food then maybe you do not need them.
We flea treat our cats but we never got them heart worm meds.
Your little Izzy will be okay. I know you do not want to share her real name and I respect that so here is her forum name or whatever you choose.
Oh and if she gets a taste of the outside you will never hear the end of it.