I've got two w/it; one actually has vestibular hypoplasia which affects the hearing and caused Creamsicle to go deaf. She's the worst case. You'd never know Coco Puff has it now. He just goes into a sort of daze like seizure every once in a while. As for how common it is, I have no idea. My old vet thought that it can be caused by the feline herpes virus. I didn't even know what it was called until I became a member of PT, which is why I like it here so much. We learn and teach, we comfort and are comforted.