At the Sanctuary cats with suspected sinus infection will always have a culture taken to identify the infection so appropriate therapy can be used. Most veterinarians also have access to labs that can do PCR testing to rule out the most common respiratory infections that fail to culture. Allergies can also play a role, if all of these diagnostics fail to identify an infection. The lining of the sinus can undergo chronic changes if the condition is not cleared up quickly, leading to a chronic problem. Therefor, I urge you and your treating veterinarian to be as aggressive as required to resolve this condition before it becomes a chronic problem.