The opening of the anal sinus is upward, and the sinus is normally closed, so feces and bacteria cannot easily enter, but when diarrhea occurs, loose stool can easily enter the anal sinus and cause sinusitis. Dry stools or foreign bodies in the stool can also cause sinusitis when they pass through the anal canal and damage the sinus or anal flap. Inflammation of the anal sinus can often cause inflammation, resulting in swollen and enlarged anal papillae. In addition, inflammation of the anal sinus can spread through the anal ducts and glands, causing perianorectal abscesses.