An anal fistula is a granulomatous canal that connects the rectal canal to the perianal (or perineal) skin and consists of three parts: an internal opening, a fistula, and an external opening. The fistula wall is composed of thickened fibrous tissue with a layer of granulation tissue attached. The entire wall consists of thickened fibrous tissue with a layer of granulation tissue. It can develop at any age and is most common in young men. It is mostly caused by perirectal abscesses. The abscess breaks down on its own or forms an external opening at the site of incision and drainage.
Because of the rapid growth of the external opening, the abscess often heals pseudo-evidently, resulting in recurrent abscess rupture or incision, forming multiple fistulas and external openings, making simple 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. Atopic inflammatory diseases such as tuberculosis, ulcerative colitis, Crohn’s disease, malignant tumors, and traumatic anal canal infections can also cause anal fistulae, but they are less common.
I. Clinical manifestations and classification
The main symptom is a small amount of purulent, bloody or mucus discharge from the external orifice of anal fistula. The fistula may be accompanied by systemic infection symptoms such as fever, chills, and fatigue, and the recurrence of these symptoms is a clinical characteristic of the fistula. In the examination, a single or multiple external orifices can be seen on the perianal skin, and a small amount of pus repeatedly flows from the external orifices, polluting the underwear, or itching due to pus irritation of the perianal skin, and thickening and reddening of the skin over time; if the external orifices are closed and pus accumulates, local redness, swelling and pain can occur, which can be ulcerated again, or new external orifices can be formed nearby, and so on. The fistula may be recurrent. There is mild tenderness at the internal orifice during rectal palpation, and sometimes a nodular internal orifice and a fistula with strips can be found.
There are many ways to classify anal fistulas, and the following two are briefly described.
1. Classification of fistulas according to their location
(1) Low anal fistula: The fistula is located below the deep external sphincter. It can be divided into low simple anal fistula (only one fistula) and low complex anal fistula.
(2) High anal fistula: the fistula is located above the deep external sphincter. It can be divided into high simple anal fistula (only one fistula) and high complex anal fistula. This classification method is more commonly used in clinical practice.
2. Classification according to the relationship between the fistula and the sphincter
(1) Inter-anal sphincter type: It accounts for about 70% of anal fistulas and is mostly 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 anal sphincter: the least common, accounting for only 1%. This type of anal fistula is often caused by trauma, intestinal malignancy, Crohn’s disease, and is more difficult to treat.
Diagnosis
1. Medical history
Pain in the perianal area, with external orifice (redness, swelling, heat, pain, abscess manifestation and general weakness, etc.) for a long time, intermittent perianal abscess formation, constant discharge of a small amount of pus from the external orifice causing skin itching and eczema, etc. It is important to identify the location of the internal orifice of the anal fistula to clarify the diagnosis of anal fistula.
The location of the internal orifice can be determined by following Goodsall’s (1900) rule: a horizontal line is drawn at the midpoint of the anus. If the external orifice of the fistula is in front of the line, the fistula is often straight towards the anal canal and the internal orifice is often on the sinus opposite the external orifice. Most fistulas conform to this rule, but there are exceptions.
Cirocco (1992) performed a retrospective analysis of a group of fistula cases to test the accuracy of Goodsall’s rule in predicting the course of fistulae and concluded that the rule was quite accurate in predicting the course of posterior external fistulae, especially in women, with 97% of patients meeting the rule, but not in predicting the course of anterior external fistulae, with only 49% of patients having radiolucent fistulae meeting the rule Only 49% of the patients had radial fistulae.
2. Recommended tests
(1) Rectal palpation: a fistula in the form of cords can be visualized.
(2) Proctoscopy: sometimes an internal opening can be found.
(3) Melan staining: inject l to 2 ml of Melan solution into the external orifice 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 orifice.
(4) Iodine oil fistulography is a routine clinical examination method.
3.Optional examinations
(1)Endorectal ultrasound: it can distinguish the relationship between the anal fistula and the surrounding tissues, and can distinguish the location of the internal and external openings of most fistulas.
(2) Barium enema or colonoscopy: it is suitable for patients with complex, multiple surgeries and anal fistula of unknown etiology to exclude the presence of Crohn’s disease, ulcerative colitis, etc.
III. Treatment
1.Drug treatment
Same as perianal abscess. It is only used in the early stage of formation of recurrent abscesses and in preparation for surgery.
(1) Sitz bath (l/5000 potassium permanganate solution).
(2) Antibiotics.
(3) Local physiotherapy, etc.
2.Surgical treatment of anal fistula
(1) Indications
Most difficult to heal on their own, still recurring after treatment and the formation of perirectal abscesses.
(2) Surgical methods
The purpose is to open the fistula with surgery or thread to achieve gradual healing; the key is to prevent damage to the sphincter muscle causing dysfunction.
(1) Conventional surgery: wire hanging therapy, commonly used for high level simple anal fistula. Advantages: it does not cause incontinence of the anal sphincter. After the above treatment, most of the patients are ligated and the tissue breaks on its own 10-14 days after surgery. The trauma gradually heals from the inside out.
②Other treatments: fistulotomy (for low anal fistula), fistulotomy (for low simple fistula only).
③Complications of surgical treatment: anal incontinence, anal stricture, etc.
Determining the location of the internal orifice is very important to clarify the diagnosis of anal fistula. There is mild pressure pain at the internal orifice during rectal palpation, and sometimes a hard node-like internal orifice and a cord-like fistula can be felt. Anoscopy can sometimes reveal the internal orifice, and the external orifice may cause pseudo-access when exploring the fistula, so it is advisable to use a soft probe.
If the above methods are not sure about the internal orifice, 1 to 2 ml of methylene blue solution can be injected into the external orifice and the stained area of the white wet gauze filled into the anal canal and lower rectum can be observed to determine the location of the internal orifice. The fistula can appear as a hypoechoic and mixed echogenic area, and the inflammatory hyperplastic area can be seen as a colored blood flow number with a disrupted mucosal continuity or limited bulging changes.
For patients with complex, multiple surgeries and unknown etiology of anal fistula, a barium enema or colonoscopy should be performed to exclude the presence of Crohn’s disease, ulcerative colitis, and other diseases.
The majority of fistulas require surgery because they are difficult to heal on their own and can recur and form perirectal abscesses without treatment. The principle of treatment is to incise the fistula to create an open wound and promote healing. There are many surgical approaches, and the choice of surgery should be 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 the fistula.