My Maine Coon Cat - Creature's Eye

ON CREATURES LEFT EYE A FILM STARTS FORMING ACROSS FROM THE OUTER CORNER TOWARDS THE CENTER. ABOUT EVERY TWO TO THREE MONTHS HE HAS TO GET A STEROID SHOT WHICH IMPROVES THE CONDITION TEMPORARILY. HAS ANYONE HAD THIS HAPPEN TO THEIR PET AND WHAT WAS THE TREATMENT? MY VET SAID HE HAD SEEN IT HAPPEN IN DOGS BUT NEVER A CAT. IT MAY BE AN ALLERGY BUT HE WAS NOT SURE WHAT THE CAUSE WAS AND I WAS JUST CURIOUS ABOUT ANYONE WHO HAS A PET WITH THIS CONDITION. THANK YOU FOR YOUR INTEREST - CREATURE THANKS YOU ALSO - JAN