A peri anal sinus (fistula) is an abnormal connection between the inside lining of the anus and the skin around the anus. It usually develops after chronic infection of the anus wall. In time, a small hole develops between the anus and the skin, which will leak pus and sometimes other anal fluids. Anal fistulas tend to cause repeated formation of abscess within the anal wall and are generally best dealt with surgically. Operations usually involve opening the sinus to heal from the inside, with resulting eventual closure of the connection to the skin. If left untreated, there is a chance that in time it may close naturally, but you are at risk of recurring infection.