It may be the drug that is causing the problem. From Marvistavet's
website on the side efects of Methimazole:

"Facial itching is a more serious side effect. This side effect also
resolves with anti-itch medication and discontinuation of methimazole.
Cats who have this side effect can be expected to have it again if
medication is restarted and another form of treatment should be
instituted. Facial itching occurs in less than 4% of cats on
methimazole."

I had a cat that had a similar reaction to Tapazole and developed
serious facial sores that made him miserable. I elected to stop the drug
and get him radioactive iodine therapy as that was really the only
option that would not only cure his condition, but give him some much
needed relief. I realize your cat is 16, but Zach was probably 12-13
when he received the treatment and there were 16 and 17 year old cats
getting treatment at the clinic I used. We had 3 good years together
because of the treatment. It was worth every penny and then some.

Megan

                                   

It sounds like a flea problem to me, Dave. Our little Franchester
scratched to high heavens behind his little ear a few months back. He
is an inside cat too, and for the life of us, we couldn't understand
how he possibly could have picked up fleas. Especially since we had
previously eraticated them all.
At first, we couldn't find any sign of the vermin filth, but then we
decided to check under his chin. Sure enough, there they were.
Scurrying to-and-fro. It turned out that we had taken him to a
pet-friendly hotel with us on vacation a few weeks before. That, we
figure, is how he picked them up.
We recently aquired new beanbag furniture. Vinyl it is. We've had
'chessie's front claws out since he was a baby, all to good ends. We
decided to go ahead and have his rear ones removed as well, as he
immediately began to climb on it and leave little holes everywhere.
Now, no more can he tear up the new furniture OR rip at his cheeks
when going after fleas and the like. It was a good investment, as it
killed two birds with one stone.
'Chessie' is now about two years old, so we were a wee bit concerned
that he might have long term problems with pain from the surgery. But
he came thru with flying colours. It might do you good to check into
this (still) inexpensive and readly available process. Hope this
helps.

I wish to take this moment to say hello to all my krazy old friends
and fellow kat lovers in here. I know it's been a while since I
posted, but I sometimes get a tear in my eye when thinking about all
the good times and good people I met in this place in the past. I will
try not to offend those here who didn't like me. I have lurked here
without comment for a few months, but my little kitty gets me so happy
that I just had to share. I will be here for the foreseeable future,
eager to share and lend my opinions when they are called upon.