An anal fistula refers to a fistula that communicates with the anal canal or rectal cavity. It is a sequela of perianal abscess rupture. One end of the fistula connects to the infected site of the anal sinus and anal gland inside the anal canal, known as the internal opening; the other end leads to the perianal skin, called the external opening. In folk terms, based on its main symptoms - continuous discharge of pus and pus-blood from the sore on the perianal skin, resembling leaks from a broken house - it was named "anal fistula" by ancient people. In severe cases of anal fistula, feces may also be discharged, hence the more vivid metaphor of "rats stealing feces". A fistula differs from a sinus. A fistula is a channel with two openings connecting the inside and outside of the body, allowing secretions and pus to leak out. A sinus has a bottom and only one external opening for discharging secretions and pus-blood. Clinically, there is a distinction between a fistula and a sinus, and they should not be confused. The formation of an anal fistula is the inevitable result of the development of perianal abscesses around the anus and rectum, and generally goes through four stages:
The first stage: infection and inflammation of the anal crypts and anal valves. Initially, it is limited to local inflammation, but if not treated in time, the inflammation can spread to the perianal area.
The second stage: inflammation starts from the local anal crypts and anal valves, gradually spreading and forming perirectal inflammation. If the inflammation cannot be controlled, it may invade tissues with low resistance.
The third stage: due to the decreased resistance of the perirectal tissue spaces, they become places where bacteria invade, spread, and multiply, making these tissues susceptible to infection and inflammation. When the body's defenses cannot overcome the pathogens, perianal abscesses form. If perianal abscesses are properly handled in their early stages, they often resolve without leaving sequelae. However, if treatment is delayed or improperly managed, tissue necrosis and pus can spread along local spaces, complicating the condition. Therefore, early surgical incision and drainage are recommended to discharge the pus and control the inflammation.
The fourth stage: after spontaneous rupture of the perianal abscess or treatment by incision and drainage, although the abscess cavity gradually shrinks, the ulcer fails to heal for a long time. At this point, the cavity walls have formed a hard duct wall due to connective tissue proliferation, leaving a hollow space in the middle, which is the fistula tract. Pus frequently flows out along the fistula tract, causing repeated infections and recurrences, and fails to heal for a long time, eventually becoming a fistula.