White blisters in cat's ears.

My cat has a number of small white "blisters" on her ears. She repeatedly scratches and shakes her head, more than is normal for a cat. What is wrong with her? She's 16 years old with kidney disease. The local vet checked her for parasites and found none. They're pretty baffled as to what it could be. We give her Epakitin for kidney disease and the occasional hairball ointment. We have given both to her for about two years without any adverse reaction.

I just sent two photos. I hope you can tell something from them. They're very hard to photograph, particularly since there's a lot of contrast between her skin and fur. From what I can tell, they're mostly on the backs of her ears. Our local vet also said she noticed one or two around the mouth, which we haven't been able to detect. Each one is about two or three millimeters wide and feels hard to the touch. I should also note that her fur is matted because we've been applying some ointment our vet gave us. That ear in particular is rather bare from scratching. When she scratches, it's above the eyes and behind the ears. She also loves to have the inside of her ears rubbed, but we've been trying not to do that ever since this became an issue- just in case. Also the red marks are from a scraping the vet did to check for parasites- she nicked some of the lumps.

Overall Isis is getting by very well for her age and considering she has KD. She eats well and even plays now and then. But her poor ears have really had it hard. I should also note that she's strictly an indoor cat, if that matters.