| disease | Anal Fistula |
| alias | Anorectal Fistula, Anal Fistula |
Anorectal fistula primarily affects the anal canal and rarely involves the rectum, hence it is commonly referred to as anal fistula. It is a granulomatous tract connecting to the perineal skin, with the internal opening usually located near the dentate line and the external opening situated around the anus or on the skin. The entire fistula wall consists of thickened fibrous tissue lined with granulation tissue, making it difficult to heal. Its incidence is second only to hemorrhoids and is more common in young adult males, possibly due to the vigorous secretion of sebaceous glands, one of the target organs of male sex hormones.
bubble_chart Etiology
There are two main types of perianal and perirectal abscesses: one is related to the anal glands and anal fistulas, referred to as "primary acute anal gland intermuscular fistulous abscess," abbreviated as "fistulous abscess," which is more common; the other is unrelated to the anal glands and anal fistulas, referred to as "acute non-anal gland non-fistulous abscess," abbreviated as "non-fistulous abscess," which is less common.
Most anal fistulas are caused by general pyogenic infections, while a few are due to specific infections, such as subcutaneous nodules, Crohn's disease, and ulcerative colitis, which are even rarer. Secondary infections from trauma to the rectum and anus can also lead to anal fistulas, and malignant tumors of the rectum and anus can ulcerate and form fistulas, though these cases are uncommon and distinctly different from general pyogenic anal fistulas.
Some speculate that the influence of sex hormones is the primary cause of anal fistulas. During puberty, the body's own sex hormones become active, prompting the development and proliferation of certain sebaceous glands, particularly the anal glands, with more pronounced growth in males than in females. Due to the vigorous secretion of anal glands, if combined with poor drainage or blockage of the anal gland ducts, infection can easily lead to anal gland inflammation. This explains why the incidence of anal fistulas is higher in young adult males. In females, the anal ducts are straighter and less curved than in males, making it less likely for secretions to accumulate, hence the lower incidence of anal fistulas in women. In old age, as other sebaceous glands generally atrophy, the anal glands also atrophy, which is why anal fistulas are rare in the elderly.
bubble_chart Pathological ChangesAn anal fistula has a primary internal opening, a fistula tract, branch tracts, and secondary external openings. The internal opening, which is the entry point of the infection, is mostly located within or near the anal crypts, commonly on either side of the posterior midline, but it can also occur anywhere in the lower rectum or anal canal. The fistula tract may be straight or curved, with few branches. The external opening is where the abscess ruptures or is incised for drainage, usually located on the skin around the anal canal. Because the primary infection source continuously enters the tract through the internal opening, and the tract winds its way near the internal and external sphincters, with the tract wall composed of fibrous tissue and granulation tissue inside, the condition tends to persist chronically.
Generally, a simple anal fistula has only one internal opening and one external opening, which is the most common type. If the external opening temporarily closes, causing poor local drainage, redness and swelling may gradually recur, leading to abscess formation again. The closed external opening may rupture again or form another external opening elsewhere. Repeated episodes like this can expand the scope of the lesion or sometimes create multiple external openings connected to the internal opening. Such an anal fistula is called a complex anal fistula, meaning it has one internal opening and multiple external openings. However, some believe that a complex anal fistula should not be defined by the number of external openings but rather by whether the main tract involves the anorectal ring or above. Even if such a fistula has only one external and one internal opening, its treatment is more complicated, hence termed a complex anal fistula. Antagonism, sometimes an anal fistula may have multiple external openings, but its treatment is not necessarily complex.
Anal fistula often has a history of perianal abscess spontaneously rupturing or being incised {|###|}expelling pus{|###|}, after which the wound fails to heal over time, becoming the external opening of the anal fistula. The main symptoms are recurrent discharge of small amounts of pus from the external opening, which stains underwear; sometimes the pus irritates the perianal skin, causing a sensation of {|###|}cutaneous pruritus{|###|}. If the external opening temporarily closes, pus accumulates, and the local area becomes red and swollen, leading to {|###|}distending pain{|###|}. The closed external opening may rupture again or form another new opening nearby. Repeated episodes like this can result in multiple external openings that communicate with each other. If the fistula tract drains well, there is no local pain, only mild discomfort or a feeling of fullness, which patients often disregard.
Examination: The external opening appears as a papillary protrusion or a raised mass of granulation tissue, and pressing it yields a small amount of pus. Low anal fistulas usually have only one external opening. If the fistula tract is superficial, a hard cord-like structure can be palpated subcutaneously, extending from the external opening to the anal canal. High anal fistulas are often deeper in location, making the fistula tract difficult to palpate, but they frequently have multiple external openings. Due to irritation from secretions, the perianal skin often becomes thickened and reddened.
Currently, anal fistulas are classified into four categories based on their relationship with the anal canal and sphincter muscles.
2. **Transsphincteric anal fistula**: Can be either low or high-level, accounting for about 25%. It results from ischiorectal fossa abscesses. The fistula tract passes between the internal sphincter and the superficial and deep layers of the external sphincter. There are often multiple external openings with interconnected branches. The external openings are usually about 5 cm from the anal margin. In rare cases, the fistula tract may extend upward through the levator ani muscle into the perirectal connective tissue, forming a pelvirectal fistula.
3. **Suprasphincteric anal fistula**: A high-level fistula, rare, accounting for 5%. The fistula tract extends upward through the levator ani muscle and then descends to penetrate the skin in the ischiorectal fossa. Since the fistula often involves the anorectal ring, treatment is more challenging and usually requires staged surgery.
4. **Extrasphincteric anal fistula**: The rarest, accounting for 1%, resulting from pelvirectal abscesses combined with ischiorectal fossa abscesses. The fistula tract penetrates the levator ani muscle and directly connects to the rectum. This type of fistula is often caused by Crohn's disease, intestinal cancer, or trauma, and treatment must address the primary condition. The above classification is detailed in terms of high and low levels, aiding in surgical method selection.
If external openings are present on both sides of the anal canal, a **"horseshoe" anal fistula** should be considered. This is a special type of transsphincteric fistula and also a high-level curved fistula. The fistula tract encircles the anal canal, extending from one ischiorectal fossa to the opposite side, forming a semicircular shape resembling a horseshoe. There is usually one internal opening near the dentate line, with multiple external openings distributed on both sides of the anus and numerous branching tracts spreading outward. Horseshoe fistulas are further divided into anterior and posterior types, with the latter being more common due to the looser tissue in the posterior anal canal, making infection more likely to spread.
**Is there a pattern to the locations of the external and internal openings of anal fistulas?** Goodsall (1900) proposed that if a transverse line is drawn across the midpoint of the anus, an external opening anterior to this line usually indicates a straight fistula tract leading to the anal canal, with the internal opening corresponding to the external opening. If the external opening is posterior to the line, the fistula tract is often curved, with the internal opening usually located at the posterior midline of the anal canal. This is generally referred to as **Goodsall's rule**. While most fistulas conform to this rule, exceptions exist—e.g., anterior high-level horseshoe fistulas may be curved, while posterior low-level perianal abscesses may be straight. Clinically, the straight or curved nature of a fistula is not only related to its anterior or posterior position but also to its high or low level and the distance of the external opening from the anal margin. Cirocco (1992) conducted a retrospective analysis of a group of fistula cases to test the accuracy of Goodsall's rule in predicting fistula paths. The study found that the rule is highly accurate for predicting posterior external openings, especially in female patients (97% of internal openings were at the posterior midline crypt). However, it was less accurate for anterior external openings—only 49% had radial fistulas conforming to the rule, as Goodsall did not recognize that 9% of anterior fistulas originate from the anterior midline crypt.
**Digital rectal examination**: Grade I tenderness is present at the internal opening, and a hard nodule may occasionally be palpated. **Probe examination** is only used during treatment and generally not for diagnosis, to avoid perforating the fistula wall and creating a false internal opening. **X-ray contrast imaging** involves injecting 30–40% iodized oil into the external opening to visualize the fistula tract distribution, mainly used for high-level and horseshoe fistulas.
Imaging examination: Yang (1993) reported 17 cases clinically suspected of having anorectal abscesses or fistulas. Among them, 6 cases were clinically suspected of having abscesses, and anal ultrasound (AUS) also showed abscess manifestations. Additionally, 82% (9/11) of cases where AUS detected fistulas were missed by routine clinical examinations. Lunniss (1994) compared AUS and magnetic resonance imaging (MRI), concluding that the former sometimes has diagnostic value for intersphincteric fistulas but cannot definitively diagnose extrasphincteric or transsphincteric fistulas. In contrast, the latter demonstrates absolute superiority and accuracy for complex high anal fistulas, horseshoe-type anal fistulas, and clinically difficult-to-diagnose cases.
bubble_chart Treatment Measures
Anal fistulas cannot heal on their own and must be treated surgically. The principle of surgical treatment is to completely incise the fistula tract, and if necessary, remove the surrounding scar tissue simultaneously, allowing the wound to heal gradually from the base upward. Depending on the depth and curvature of the fistula, options include thread-drawing therapy, fistulotomy, or fistulectomy. In rare cases, initial stage [first stage] suturing or free skin grafting may be performed after fistulectomy.
(1) Thread-Drawing Therapy
This is a slow incision of the fistula tract. It utilizes the mechanical effect of a rubber band or medicated thread (the medicated thread also has a corrosive effect), causing ischemia in the ligated tissue and gradual compression. Meanwhile, the ligature serves as a drainage device for the fistula, allowing exudate to drain and preventing acute infection. As the superficial tissue is cut, the wound at the base begins to heal gradually. The greatest advantage of this gradual cutting method is that even if the anal sphincter is severed, it does not retract excessively or shift position, generally avoiding fecal incontinence.
This method is suitable for low or high simple straight fistulas with internal and external openings within 3–5 cm of the anus, or as an adjunct to the incision or excision of complex anal fistulas:
1. Method
(1) Place the patient in the lateral position. First, tie a rubber band to the tail end of a probe, then gently insert the probe head from the external opening of the fistula inward until the internal opening is found near the dentate line of the anal canal. Next, insert a finger into the anal canal to locate the probe head, bend it, and pull it out through the anal opening. Care must be taken to avoid forceful insertion of the probe to prevent creating a false passage.
(2) Pull the probe head completely out of the internal opening of the fistula, allowing the rubber band to pass through the external opening into the fistula tract.
(3) Lift the rubber band, incise the skin layer between the internal and external openings of the fistula, tighten the rubber band, and clamp it close to the subcutaneous tissue with a hemostat. Below the hemostat, tighten the rubber band with a thick silk thread and perform a double ligation, then release the hemostat. Apply a Vaseline gauze to the incision. Postoperatively, perform sitz baths daily with a 1:5000 potassium permanganate solution and change the dressing. Generally, the fistula tissue is incised by the rubber band around 10 days postoperatively, and the wound heals in 2–3 weeks.
2. Advantages of this method:
(1) The procedure is simple, quick, and causes minimal bleeding.
(2) Before the rubber band falls off, the skin incision usually does not "bridge."
(3) Dressing changes are convenient.
3. Key points for successful thread-drawing:
(1) Accurately locate the internal opening. If no bleeding occurs when the probe exits the internal opening, the position is likely correct.
(2) The wound must heal from the base first, allowing the internal damage of the anal canal to heal before the superficial skin adheres prematurely. Typically, the rubber band falls off in 7–10 days. If it remains after 10 days, the silk thread ligating the rubber band may be too loose and require retightening.
(2) Fistulotomy
The surgical principle is to completely incise the fistula tract and sufficiently excise the scar tissue at the edges of the incision to ensure drainage and gradual healing. This method is only suitable for low straight or curved anal fistulas. The procedure is as follows.
1. Correctly explore the internal opening: The method for locating the internal opening is the same as in thread-drawing therapy. After locating the internal opening, withdraw the probe from the anus. If the fistula is curved or branched and the probe cannot reach the internal opening, inject a small amount of 1% methylene blue solution through the external opening to identify the internal opening. Then, use a grooved probe to gradually incise and explore the tract until the internal opening is found. If the internal opening cannot be located despite careful exploration, treat the suspected diseased anal crypt as the internal opening.
2. Incise the fistula and adequately excise the marginal tissue. Incise all superficial tissues of the fistula, from the external opening to the internal opening and the corresponding anal sphincter muscle fibers. After the fistula is incised, check for any branch tracts, and if found, they should also be incised. Once the entire fistula is incised, thoroughly scrape away the necrotic granulation tissue. Generally, there is no need to excise the entire fistula to avoid creating an excessively large wound. Finally, trim the wound edges to form a "V" shape with a small base and a wide opening, facilitating healing from the deep part of the wound first.
3. Anal sphincterotomy During the operation, the relationship between the position of the probe and the anorectal ring should be carefully palpated. If the probe enters below the anorectal ring, even if the entire fistula and most of the external sphincter and corresponding internal sphincter are incised, fecal incontinence will not occur due to the preservation of the puborectalis muscle. If the probe enters above the anorectal ring into the rectum (e.g., suprasphincteric or extrasphincteric fistula), the fistula should not be incised; instead, seton therapy or staged seton surgery should be performed. In the initial stage [first stage], the fistula below the ring is incised or excised, and the fistula above the ring is threaded with a thick silk suture and tightly ligated. In the intermediate stage [second stage], after most of the external wound has healed and the anorectal ring has adhered and fixed, the anorectal ring is incised along the seton.
After the fistula is incised, the granulation tissue on its posterior wall can be scraped off with a curette. Generally, excision is unnecessary to minimize bleeding and avoid injury to the sphincter muscle on the posterior wall. The excised fistula tissue should be sent for pathological examination.
4. Wound management Postoperative wound management is often crucial to the success of the surgery. The key is to ensure the wound heals gradually from the base to the surface. The dressing should be changed daily, preferably after defecation. The packing in the wound should be gradually reduced until the wound inside the anal canal heals completely. Digital rectal examination every few days can dilate the anal canal and prevent bridging adhesions, avoiding false healing.
(III) Fistulectomy
Unlike fistulotomy, this procedure involves complete excision of the fistula down to healthy tissue. It is also suitable for low anal fistulas with more fibrous tracts.
Method: First, inject 1% methylene blue into the external opening of the fistula, then gently insert a probe from the external opening and pass it through the internal opening. Grasp the skin around the external opening with tissue forceps, incise the skin and subcutaneous tissue around the external opening, and then use an electrocautery or scissors to excise the skin, subcutaneous tissue, methylene blue-stained fistula wall, internal opening, and all surrounding scar tissue along the probe's direction, leaving the wound completely open. After careful hemostasis, pack the wound with iodoform gauze or Vaseline gauze.
(IV) Initial stage [first stage] suture after fistulectomy
This method was first introduced by Tuttle (1903) but did not gain widespread adoption, possibly due to theoretical inadequacies, unsatisfactory surgical outcomes, and opposition from many colorectal surgery experts. In 1949, Starr reintroduced this method and proposed some effective measures, achieving satisfactory results and leading to its broader application. This method is only suitable for simple or complex low straight anal fistulas. If the fistula feels like a hard cord on palpation, the results are even better. Key surgical points: ① Preoperative bowel preparation is required, antibiotics should be administered before and after surgery, and bowel movements should be controlled for 5–6 days postoperatively. ② The entire fistula must be excised, leaving a fresh wound surface free of any granulation or scar tissue. ③ Excessive removal of skin and subcutaneous fat should be avoided to facilitate wound suturing. Therefore, high curved fistulas are not suitable for suturing due to their multiple branches, which often require excessive tissue removal for complete excision. ④ All layers of the wound must be sutured and aligned without leaving dead space. ⑤ Strict aseptic techniques must be followed during surgery to prevent contamination, such as accidental incision of the fistula. According to domestic literature, among 1,064 cases of fistulectomy with primary suture, the initial stage [first stage] healing rate was 73.4%–97.6%, with a healing time of 20–22 days. Lower initial stage [first stage] healing rates were mostly observed in complex high anal fistulas.
(V) Skin grafting after fistulectomy
After anal fistula excision, if the wound is too large, superficial, and without special complications, free skin grafting may be considered. The preoperative and postoperative requirements are the same as those for the initial stage [first stage] suture of anal fistula excision. Key surgical points: ① The wound surface should be flat, and hemostasis must be complete. ② The suturing of the free skin graft area must be thorough, and pressure fixation and bandaging should be applied to prevent the retention of gas or blood under the wound surface. This is one of the important measures for surgical success. ③ If the wound has significant oozing, delayed skin grafting is necessary—first covering the wound with Vaseline gauze and performing free skin grafting 2–3 days later. Hughes (1953) reported 40 cases, with 30 achieving complete success in skin grafting, while the majority of the rest survived. Goligher (1975) reported 22 cases, all involving low anal fistulas, with poorer results—only 13 cases achieved complete survival.
(VI) Treatment of Horseshoe Anal Fistula
Fistulotomy combined with seton therapy should be employed. For posterior horseshoe anal fistulas, a grooved probe is first inserted from both external openings, and the fistula tract is gradually incised until the two tracts meet near the posterior midline. Then, the grooved probe is carefully used to explore the internal opening. The internal opening is often located near the dentate line in the posterior midline of the anal canal. If the fistula tract passes below the anorectal ring, the entire fistula tract and the subcutaneous and superficial portions of the external sphincter can be incised in one stage. If the internal opening is too high and the fistula tract passes above the anorectal ring, seton therapy must be used. Specifically, the subcutaneous and superficial portions of the external sphincter and the fistula tract below them are incised, and then a rubber band is inserted through the remaining tract opening and brought out through the internal opening, tied around the anorectal ring. This avoids fecal incontinence caused by a one-time division of the anorectal ring. The skin and subcutaneous tissue at the wound edges are then trimmed to keep the wound open, and granulation tissue on the fistula wall is curetted. The wound is packed with iodoform or Vaseline gauze.
(VII) Advancement of Sliding Mucosal Flap to Close the Internal Opening
After complete excision of the fistula tract and internal opening, a mucosal flap is advanced to repair the rectal defect. This flap actually includes a partial-thickness portion of the rectal wall to enhance its strength.
Advantages of this method: ① Most of the sphincter is preserved, making it suitable for rectovaginal fistulas and high transsphincteric anal fistulas; ② Minimal scar formation; ③ Avoids anatomical deformities; ④ No need for protective intestinal diversion. Aquilar et al. (1985) used this method to treat 189 cases of high transsphincteric anal fistulas with good results, reporting a recurrence rate of only 2%, but with undergarment soiling and stenosis at 8%, grade I flatus incontinence at 7%, and liquid stool incontinence at 6%. Wedell et al. (1987) reported 30 cases, with good results in 29. Jones et al. (1987) applied this method to treat anal fistulas caused by Crohn's disease, achieving a success rate of only 57%, while the success rate was higher for non-Crohn's cases. However, some authors have adopted the method of directly suturing the internal opening.
Postoperative Care
Proper dressing changes for anal fistula wounds postoperatively are a critical factor in the success of the surgery. Even if the surgery is successful, neglecting wound dressing changes can often lead to failure. Therefore, the treating physician must personally change the dressings or at least regularly inspect the wound. Key points for dressing changes: ① Sitz baths and irrigation: Postoperative sitz baths should be performed daily, especially after bowel movements, and must not be overlooked. Ensuring wound cleanliness accelerates healing. For large wounds, irrigation should be performed, first using hydrogen peroxide solution, followed by warm saline or antibiotic solution. Irrigation should be done with sufficient pressure to ensure the cleaning solution reaches every corner of the wound. ② Dressings: Internal wound dressings prevent surface adhesion (skin bridging), so the wound should be wider at the top than the base to allow healing from the bottom up. If pus is found in the wound upon dressing removal, it indicates a residual abscess cavity, and drainage should be immediately expanded; otherwise, the wound will not heal. ③ Digital rectal examination: This can detect dead spaces or retained pus in the wound and also identify any tendency toward stricture of the anus. If present, regular anal dilation should be performed. Therefore, regular digital rectal examinations are necessary.
