My boyfriend's cat has had this issue for over a year and the vet cannot figure it out. (Hundreds of dollars wasted)
He won't stop grooming until he bleeds, ripping out fur and itching all around. Worst at the rear. Then there's him practically tearing out his claws, like grabbing onto them with his teeth and jerking. He does the same to his paw pads.
We've changed everything, food, litter, food dishes, nothing seems to work I checked one of the claw shells he tore out and there is some thick black clumps on the inside. (may be dried blood though)
He's fully indoor!
EDIT: We've changed his food to sensitive stomach & then tried sensitive skin when he started to throw it up towards the second bag (both from a vet approved websites) Steroid shots work for a week in a half and then we tried these steroid pills (less than effective) Changed his litter to organic Plastic dishes to metal ones
This didn't seem to start happening until he got his first case of flees (they're long gone now)