I think you should bring a urine sample in to your veterinarian. Perhaps you should also have your cat examined and get some blood work done. At 16 years there may need to be some things that you need to do to support the liver and kidneys. Derma-IonX has helped many types of skin problems but I would like to make sure you are not dealing with some other diseases too. Besides erma-IonX I also like to use probiotics and Omega 3,6,9. These supplements help to improve general body condition.
Please let us know what you find out and I can make some other suggestions if necessary.