Sinusitis is an inflammation or swelling of the tissue lining the sinuses. Healthy sinuses are filled with air. But when they become blocked and filled with fluid, germs can grow and cause an infection. FESS or functional endoscopic sinusitis surgery is a minimally invasive surgical treatment that uses the nasal endoscope to enlarge the nasal drainage pathways of sinuses, which will improve sinus ventilation. The surgery is used to treat inflammatory sinus diseases that don’t respond to drugs.