Anorectal fistula mainly invades the anal canal and rarely involves the rectum, so it is often referred to as anal fistula, a granulomatous canal around the anus that consists of three parts: the internal opening, the fistula, and the external opening. The internal opening is often located in the lower rectum or anal canal and is mostly one; the external opening is in the perianal skin and can be one or more, and is persistent or intermittently recurrent. Etiology Most fistulas are caused by perirectal abscesses, so the internal opening is mostly at the sinus on the dentate line, and the external opening is formed where the abscess breaks down on its own or is incised and drained, and is located on the perianal skin. Because of the rapid growth of the external opening, the abscess often heals pseudo-evidently, resulting in recurrent abscess rupture or incision and the formation of multiple fistulas and external openings, making simple anal fistulas complex. The fistula is surrounded by reactive dense fibrous tissue, with inflammatory granulation tissue near the lumen, and the lumen may become epithelialized in later stages. Most fistulas are the result of generalized purulent infections and, rarely, tuberculous infections. Other specific infections such as ulcerative colitis, Crohn’s disease and other specific inflammatory conditions, malignant tumors, and traumatic anal canal infections can also cause anal fistulae, but are less common. This classification method is more commonly used in clinical practice. (1) Low anal fistula: the fistula is located below the deep part of the external sphincter. It can be divided into low simple anal fistula (only 1 fistula) and low complex anal fistula (multiple fistulas and fistulas). (2) High anal fistula: the fistula is located deeper than the external sphincter. It can be divided into high simple anal fistula (only 1 fistula) and high complex anal fistula (multiple fistulas and fistulas). (1) Inter-anal sphincter type: About 70% of fistulas are caused by perianal abscesses. The fistula is located between the internal and external sphincters, with the internal mouth near the dentate line and the external mouth mostly near the anal verge, and is a low-level anal fistula. (2) Transanal sphincter type: about 25%, mostly caused by abscesses in the sciatic anal canal space, and can be low or high anal fistula. The fistula passes through the external sphincter, the sciatic-rectal space, and opens on the perianal skin. (3) Supra-anal sphincter type: A high anal fistula, less common, accounting for about 4% of cases, in which the fistula extends upward between the sphincters, crosses the puborectalis muscle, and penetrates the perianal skin downward through the colorectal space. (4) External sphincter of the anal canal: the least common, accounting for only 1%. It is mostly a consequence of a pelvic rectal gap abscess combined with a sciatic anal canal gap abscess. The fistula is located from the skin of the perineum up through the colorectal space and the anal raphe, and then penetrates into the pelvis or rectum. These fistulas are often caused by trauma, intestinal malignancy, or Crohn’s disease and are more difficult to treat. Clinical manifestations The main symptom is the repeated discharge of small amounts of purulent, bloody, or mucus discharge from the external orifice of the fistula. In larger high anal fistulas, because the fistula is located outside the sphincter and is not controlled by the sphincter, there is often fecal and gas discharge. Due to the stimulation of the discharge, the anal area is moist and itchy, and sometimes eczema is formed. When the external opening heals and an abscess is formed in the fistula, it can be significantly painful and can be accompanied by systemic infection symptoms such as fever, chills, and malaise, which are relieved by abscess penetration or incision and drainage. Recurrent attacks of the above symptoms are the clinical characteristics of fistula. On examination, single or multiple external openings can be seen on the perianal skin as red papillary elevations, with pus or purulent blood discharge when squeezed. The number of external orifices and their location in relation to the anus are helpful in diagnosing anal fistulae: the greater the number of external orifices and the more distant they are from the anal verge, the more complicated the fistula. 1, rectal finger examination Determining the location of the internal orifice is very important for a clear diagnosis of anal fistula. In the case of anal fistula, there is mild pressure pain at the internal orifice, and sometimes a hard node-like internal orifice and a cord-like fistula can be found. The anoscopy can sometimes reveal the internal orifice, and it is advisable to use a soft probe when probing the fistula from the external orifice because of the possibility of false passage. If the above methods are not sure, you can also inject 1~2ml of methylene blue solution from the external mouth and observe the stained area of the white wet gauze strip filled into the anal canal and lower rectum to determine the location of the internal mouth. 3.Imaging examination Iodine oil fistulography is a routine clinical examination method. Anal canal ultrasound and magnetic resonance examination can improve the diagnosis rate of anal fistula. For patients with complex, multiple surgeries and anal fistulas of unknown etiology, barium enema or colonoscopy should be done to exclude the presence of Crohn’s disease, ulcerative colitis and other diseases. Treatment Anal fistulas do not heal on their own. Untreated, they can recur as perirectal abscesses and therefore must be treated surgically. The principle of treatment is to incise the fistula, create an open wound, and induce healing. There are many surgical approaches, and the surgery should be selected based on the height of the internal opening and the relationship between the fistula and the anal sphincter. The key to surgery is to minimize damage to the anal sphincter to prevent anal incontinence and to avoid recurrence of anal fistula. 1, fistulotomy is a method to open up the fistula and heal the wound by the growth of granulation tissue. It is suitable for low anal fistula, because the fistula is below the deep part of the external sphincter, and only the subcutaneous and superficial part of the external sphincter is damaged after the incision, so there is no postoperative anal incontinence. 2, hanging thread therapy is the use of rubber bands or corrosive thread mechanical compression, slow incision of anal fistula method. This method also has the advantages of simple operation, less bleeding, convenient drug changes, and no skin incision adhesion before the rubber band falls off. 3.Anal fistula excision Cut open the fistula and remove all the fistula wall to healthy tissue without suturing the wound; if the wound is large, it can be partially sutured and partially opened, filled with oil gauze, so that the wound grows from bottom to outside until it heals. It is suitable for low level simple anal fistula. It should also be noted that good or bad wound dressing changes after anal fistula surgery is a key issue to ensure that the surgery is successful.