What causes deafness in cats?

I think my cat has become deaf. I don't know how old he is, he was a rescue cat. He was probably 8 years old when I got him according to the vet, and I've had him now for five years. He has always had a propensity to get a head cold or sinus infection, and this winter's been really bad. He's been off and on chlopheniramine since October. The medication is given transdermally on his ear, but not internally. I've had him to the vet at least twice in the past year because he just couldn't shake it. Didn't run a fever, either, but really congested, and they thought there was a sinus infection. Antibiotic injections seemed to clear it up for awhile.

What I've noticed now  is that he doesn't come when I call his name, I can snap my fingers by his ears and I don't get a reaction. I haven't tried the vacuum trick but will next time I get it out. I have been trying to figure out how long this has been going on, and I can't put my finger on anything more than six months.

My question, and I've tried to find an answer online but haven't, could the use of chlorpheniramine transdermally cause deafness? If so, will discontinuing the medication restore hearing? Or should I look for other things like damage from inner ear infections? Or is it just due to old age and the fact that he's always had some medical issues being from a hoarding case?

Deafness can be caused by lots of things.  It could by old age, it can be caused by chronic ear infections or a severe ear infection.   I have never heard of chorlampheniramine causing deafness before and I have used it long term in one of my cats

I took in a 13 year old kitty last year, so I've only had him a year and didn't know him that well.  It was clear to me, though, that his hearing wasn't really good and now over the last year, it's pretty much gone.  He has IBD and I know he has been on several medications over his lifetime but when I took him to the vet she said in all likelihood it's just age related.  She checked his ears and found nothing wrong and then she tested by closing the curtain to the exam room he was in and had her assistant drop a loud can (sounded like a can with coins or rock or something in it) outside the curtain, from two different directions.  He didn't react at all.  :-(  

So I"m learning to use hand motions, leave on more lights so he's not disoriented, and things like that.  Poor boy.  

The repeat/ongoing infections may have damaged the middle/inner ear.