Most anal fistulas occur as a result of an anal abscess originating from anal glands located between the 2 sphincters (muscles) of the anus. As a result of infection and inflammation of the glands, an abscess develops.
Due to the progressing infection, the swelling increases and the abscess may press against the skin around the anus. It may empty spontaneously or after surgery.
When the abscess heals, an open connection may remain between the anal canal and the skin; this is referred to as an anal fistula.
Not every patient develops an anal fistula after an abscess.
+/- About 50% of patients develop a fistula after drainage of an abscess.