Anal glands have an opening in the anal crypts that are found at the dentate line. Infection in these glands spreads into the intersphincteric space, and from there it passes between or though the sphincters to reach the skin. The infection usually presents as an abscess, which drains spontaneously or by incision, leading to the potential for a fistula. The fistula connects the internal opening (the gland opening at the dentate line) with the site of external drainage (the external opening). Aside from anal cryptoglandular infection, other causes of anal fistulas are Crohn disease, anorectal neoplasms, trauma, and mycobacterial or fungal infections.
Anal fistulas are classified according to their relationship to the anal sphincter complex ( Table 5-1 ). Persistence of the tract results in a chronic cycle of contamination through the internal opening, seeding the tract with pus, which drains intermittently from the external opening. The tendency of the external opening to seal results in inadequate drainage with recurrent abscesses and even the development of multiple external openings. It also contributes to the chronicity of the process.
Few patients with anal fistulas are truly asymptomatic; however, the risks of treatment should be balanced against the severity of symptoms. Identification of a fistula does not mandate repair, and a long-term indwelling seton is an option to minimize symptoms in patients for whom definitive repair may be risky or result in poor function. However, in most cases, when exacerbations of infection occur, repair should be recommended. The decision about how to treat a fistula should take into consideration the patient’s comorbid conditions, the potential underlying cause of the fistula, and the morbidity of treatment.
Diagnosis and Evaluation
Careful attention to the patient’s history of perianal infections and symptoms can often suggest the diagnosis of a fistula, which is usually confirmed by examination. Symptoms include intermittent pain, blood-tinged perianal drainage, skin irritation, moisture, pruritus, soilage or seepage, and recurrent abscesses. It is critical to assess continence and obtain a thorough history, including obstetric experiences and bowel function. Patients with pre-existing anorectal dysfunction, chronic diarrhea, or inflammatory bowel disease require particularly cautious treatment to avoid incontinence and recurrence.
The differential diagnosis of anal fistulas includes hidradenitis suppurativa (usually distinguished by the multiple superficial scars in the area and the lack of an internal opening at the dentate line), a simple epidermal inclusion cyst, a skin infection such as a boil, and pilonidal disease. In women, drainage from a Bartholin gland infection may mimic the external opening of an anal fistula.
An external opening that appears to be chronic is suggestive of a fistula. When palpable induration leading toward the anal canal is noted, a fistula is highly likely. Discomfort during examination and deep anal crypts may preclude identification of the internal opening in the office, but purulent drainage from a visible internal or external opening often confirms the diagnosis. Imaging studies are not routinely used because in the vast majority of cases, examination under anesthesia allows identification of the tract and treatment without the extra expense and delay entailed in obtaining imaging studies.
Routine antibiotic therapy has little role in the management of an anal fistula. Antibiotics are indicated when a patient has associated cellulitis, or for patients with immunosuppression, diabetes, recent implants, valvular heart disease, or prostheses. Metronidazole is often used for patients with perianal Crohn disease, although data to support this practice are limited. The use of postoperative antibiotics after fistula repair is common, but this practice has not been shown to affect outcome.
Preparation and Examination
No routine preoperative laboratory evaluations are mandatory except for an electrocardiogram in patients older than 45 years; consideration may be given to obtaining a blood cell count or blood chemistry panel in patients with comorbidities. Preparation of the anal area with use of an iodine or chlorhexidine solution is routine but does not sterilize the fistula tract or the rectum. Use of a Foley catheter is not necessary. The initial examination after induction of anesthesia can be performed with the patient in the lithotomy position. Definitive treatment of posterior tracts is easiest with the patient in the lithotomy position, whereas in patients with anterior internal openings, treatment is easiest with the patient in the prone position.
After the patient has been anesthetized, the external opening is gently probed with an appropriately sized blunt probe, taking care to avoid creation of false passages. Having a finger in the anal canal helps define the direction of the tract. If the probe does not exit through the internal opening, anoscopy during injection of the tract with saline solution, methylene blue, or hydrogen peroxide can help identify a patent internal opening. The type of anoscope used (e.g., Bivalve or Hill-Ferguson) depends on the surgeon’s preference, but adequate lighting is facilitated by scopes with an integral fiber-optic light source or use of a headlight. With use of these maneuvers, an instrument can almost always be guided through the fistula tract. Creation of false openings or tracts should be avoided. Once the tract is identified, its anatomy can be defined and it can be categorized according to Parks classification (see Table 5-1 ). When the anatomy is unclear or if a probe cannot be passed through an irregular tract, magnetic resonance imaging (MRI) or ultrasound can be helpful. Management decisions can then be made with these data in light of the patient’s specific comorbidities and clinical presentation.
For recurrent fistulas or complex tracts, particularly in patients with Crohn disease, or when a supralevator source is suspected based on office examination, high-resolution MRI or computed tomography (CT) may be considered preoperatively to assist in surgical planning, patient counseling, and intraoperative guidance. Fistulography offers a less complete evaluation but may help in localizing the connection of perineal fistulas to abdominopelvic processes or may assist the surgeon when the tract cannot be identified intraoperatively.
A general management algorithm for anal fistulas is presented in Figure 5-1 .
Intersphincteric Anal Fistula
An intersphincteric fistula ( Fig. 5-2 ) typically has an external opening relatively close to the intersphincteric groove, near the anal verge. This feature can often be helpful in providing preoperative counseling about the type of procedure most likely to be performed before the diagnosis is definitively made at the time of surgery. The tract will lie parallel to the long axis of the anal canal before traversing the internal sphincter near the dentate line, typically deep in a crypt. Palpation of the tract should reveal that it consists of skin and anoderm, hemorrhoidal tissue, and the lower end of the internal sphincter.
Treatment consists of laying open the fistula tract, usually after injection of a local anesthetic for postoperative analgesia and hemostasis. Minimizing cautery minimizes delay in healing. The fibrous base of the tract is curetted but left intact and the edges are shelved to help healing occur by secondary intention. All tissue is sent for histologic evaluation. Any superficial extensions of the fistula tract should be unroofed at the time of the fistulotomy.
In a related though less common scenario, a patient has no visible external opening and the intersphincteric abscess has extended proximally toward the supralevator location. In such cases it would be reasonable to place a drain transanally in the defect and obtain cross-sectional imaging with CT or MRI to identify any supralevator abscess. This uncommon condition can then later be treated with an endoanal advancement flap and drainage through the intersphincteric plane.
In most cases, performing a fistulotomy to treat an intersphincteric fistula results in a cure and does not affect continence. However, in a recent series of patients treated with a fistulotomy for intersphincteric fistula, changes in continence occurred in 30 of 148 patients (20%). Incontinence was minor in most patients, with flatus incontinence accounting for nearly three fourths of the cases. This possibility should be discussed preoperatively, and patients with recurrent disease and poor sphincter tone may have a better outcome with an alternative procedure (such as a cutting seton or an advancement flap).