I don't know much about cats but I have to agree that it does sound like a nerve problem and possibly some back issue. I'm really sorry you're going through this with Luna, I know your cats mean so much to you. Could you boost her bed higher for when she does decide to jump down so it won't be so far for her to jump? I hope you'll be able to convince Lily that chasing Luna is a bad idea...the water bottle shpuld help with that. Have a special birthday with her tomorrow-- hopefully she'll have another one next year.