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There are a few main reasons kitty's get skin problems. Allergies and skin parasites are the major ones, and it sounds like your vet has been over most of this with you.
The number one cause for allergies is fleas. Is he on she on flea control? Number 2, inhalant allergies, just like those you and i are allergic too. Of course not too much we can do there. If you have air filters/ purifies great! And 3rd is food. Have you tried a hypoallergenic diet? A food try does take up to 6 weeks, and it can be the only thing she eats.
For really bad skin allergies, the feline doctor I work for prefers to use dexamethizone. It is a type of steroid but one that has shown more effective then pred. We use it until the skin is completely clear of any signs and work them off the steroid. The hardest part of controlling allergies is stooping the itching long enough for the skin to heal.
Here in Oregon we have a few dermatologist who can preform lab tests to see what she is allergic too. Then they can formulate desensitization injections they teach you how to give. If this sounds like something you'd like to pursue, I would call your vet and see if anyone in your area offers animal dermatology.
I hope this helps and she can get some relief! Let me know if you have any other questions!
The fleas could have started the snowball effect. If she has one allergy, aka fleas, she probably has more. One of the newer diets that cats seem to really like is royal canine HP23. It is a prescription diet, so you can only get it through a vet, or online vet pharmacy. It only comes in a dry form. Best way to think of allergies is a cup.. Your kitty can tolerate some irritants to her system, but once the cup is full, she starts to show signs and its hard to get back down to a tolerable level. So once she shows signs we need to break the cycle. So, we try to control what we can, fleas and food. and help her when you see signs. Something to try. Chlortrimaton-trade name, chlorpheniramine is the drug name. get the 4mg, and give her 1/2 a tablet once a day, see if that gives her some relief. I would also talk to your vet about dexamethizone. Here is a site, with lots of good information.
Let me know if there is anything else! God luck!