Fistula-in-Ano 

A fistula-in-ano is a hollow tract lined with granulation tissue, connecting a primary opening inside the anal canal to a secondary opening in the perianal skin. Secondary tracts may be multiple and can extend from the same primary opening.

References to fistula-in-ano date to antiquity. Hippocrates made reference to surgical therapy for fistulous disease. In 1376, the English surgeon John Arderne (1307-1390) wrote Treatises of Fistula in Ano; Haemmorhoids, and Clysters, which described fistulotomy and seton use. Historical references indicate that Louis XIV was treated for an anal fistula in the 18th century.

In the late 19th and early 20th centuries, prominent physician/surgeons, such as Goodsall and Miles, Milligan and Morgan, Thompson, and Lockhart-Mummery, made substantial contributions to the treatment of anal fistula. These physicians offered theories on pathogenesis and classification systems for fistula-in-ano.^{[1, 2]}

Since this early progress, little has changed in the understanding of the disease process. In 1976, Parks refined the classification system that is still in widespread use. Over the last 30 years, many authors have presented new techniques and case series in an effort to minimize recurrence rates and incontinence complications, but despite 2500 years of experience, fistula-in-ano remains a perplexing surgical disease.

Frequency

The prevalence rate of fistula-in-ano is 8.6 cases per 100,000 population. The prevalence in men is 12.3 cases per 100,000 population, and in women, 5.6 cases per 100,000 population. The male-to-female ratio is 1.8:1. The mean age of patients is 38.3 years.^[3]

Differential diagnoses

The following do not communicate with the anal canal:

• Hidradenitis suppurativa
• Infected inclusion cysts
• Pilonidal disease
• Bartholin gland abscess in females

Treatment

Fistula-in-ano is treated surgically. No definitive medical therapy is available for this condition; however, long-term antibiotic prophylaxis and infliximab may have a role in recurrent fistulas in patients with Crohn disease.

Patient education

For patient education information, see the Digestive Disorders Center, as well as Anal Abscess, Rectal Pain, and Rectal Bleeding.

Fistula-in-ano is nearly always caused by a previous anorectal abscess. Anal canal glands situated at the dentate line afford a path for infecting organisms to reach the intramuscular spaces. The cryptoglandular hypothesis states that an infection begins in the anal gland and progresses into the muscular wall of the anal sphincters to cause an anorectal abscess. Following surgical or spontaneous drainage in the perianal skin, occasionally a granulation tissue–lined tract is left behind, causing recurrent symptoms. Multiple series have shown that the formation of a fistula tract following anorectal abscess occurs in 7-40% of cases.^{[4, 5]}

Other fistulas develop secondary to trauma, Crohn disease, anal fissures, carcinoma, radiation therapy, actinomycoses, tuberculosis, and chlamydial infections.

A thorough understanding of the pelvic floor and sphincter anatomy is a prerequisite for clearly understanding the classification system for fistulous disease. (See the image below.)

The external sphincter muscle is a striated muscle under voluntary control by 3 components: submucosal, superficial, and deep muscle. Its deep segment is continuous with the puborectalis muscle and forms the anorectal ring, which is palpable upon digital examination.

The internal sphincter muscle is a smooth muscle under autonomic control and is an extension of the circular muscle of the rectum.

In simple cases, the Goodsall rule can help to anticipate the anatomy of a fistula-in-ano. The rule states that fistulas with an external opening anterior to a plane passing transversely through the center of the anus will follow a straight radial course to the dentate line. Fistulas with their openings posterior to this line will follow a curved course to the posterior midline (see image below). Exceptions to this rule are external openings more than 3cm from the anal verge. These almost always originate as a primary or secondary tract from the posterior midline, consistent with a previous horseshoe abscess.^{[6, 7]}

The Parks classification system, demonstrated in the image below, defines 4 types of fistula-in-ano that result from cryptoglandular infections: intersphincteric, transsphincteric, suprasphincteric, and extrasphincteric.^[8]

An intersphincteric fistula-in-ano is characterized as follows:

• Common course - Via internal sphincter to the intersphincteric space and then to the perineum
• Incidence - 70% of all anal fistulas
• Other possible tracts - No perineal opening; high blind tract; high tract to lower rectum or pelvis

A transsphincteric fistula-in-ano is characterized as follows:

• Common course - Low via internal and external sphincters into the ischiorectal fossa and then to the perineum
• Incidence - 25% of all anal fistulas
• Other possible tracts - High tract with perineal opening; high blind tract

A suprasphincteric fistula-in-ano is characterized as follows:

• Common course - Via intersphincteric space superiorly to above puborectalis muscle into ischiorectal fossa and then to perineum
• Incidence - 5% percent of all anal fistulas
• Other possible tracts - High blind tract (ie, palpable through rectal wall above dentate line)

An extrasphincteric fistula-in-ano is characterized as follows:

• Common course - From perianal skin through levator ani muscles to the rectal wall completely outside sphincter mechanism
• Incidence - 1% of all anal fistulas

Current procedural terminology codes classification

This includes the following:

• Subcutaneous
• Submuscular (intersphincteric, low transsphincteric)
• Complex, recurrent (high transsphincteric, suprasphincteric and extrasphincteric, multiple tracts, recurrent)
• Second stage

Unlike the current procedural terminology coding, the Parks classification system does not include the subcutaneous fistula. These fistulas are not of cryptoglandular origin but are usually caused by unhealed anal fissures or anorectal procedures, such as hemorrhoidectomy or sphincterotomy.

Patients often provide a reliable history of previous pain, swelling, and spontaneous or planned surgical drainage of an anorectal abscess. Signs and symptoms of fistula-in-ano, in order of prevalence, include the following:

• Perianal discharge
• Pain
• Swelling
• Bleeding
• Diarrhea
• Skin excoriation
• External opening

Important points in the patient’s history that may suggest a complex fistula include the following:

• Inflammatory bowel disease
• Diverticulitis
• Previous radiation therapy for prostate or rectal cancer
• Tuberculosis
• Steroid therapy
• Human immunodeficiency virus (HIV) infection

A review of symptoms may reveal the following in patients with a fistula-in-ano:

• Abdominal pain
• Weight loss
• Change in bowel habits

No specific laboratory studies are required in the diagnosis of fistula-in-ano (although the normal preoperative studies are performed, based on age and comorbidities). Instead, physical examination findings remain the mainstay of diagnosis. The examiner should observe the entire perineum, looking for an external opening that appears as an open sinus or elevation of granulation tissue. Spontaneous discharge via the external opening may be apparent or expressible on digital rectal examination.

Digital rectal examination may reveal a fibrous tract or cord beneath the skin. It also helps to delineate any further acute inflammation that is not yet drained. Lateral or posterior induration suggests deep postanal or ischiorectal extension.

The examiner should determine the relationship between the anorectal ring and the position of the tract before the patient is relaxed by anesthesia. The sphincter tone and voluntary squeeze pressures should be assessed before any surgical intervention, to delineate whether preoperative manometry is indicated. Anoscopy is usually required to identify the internal opening.

Radiologic studies are not performed for routine fistula evaluation. However, they can be helpful when the primary opening is difficult to identify. In the case of recurrent or multiple fistulas, such studies can be used to identify secondary tracts or missed primary openings.^[9]

Fistulography

This involves injection of contrast via the internal opening, which is followed by anteroposterior, lateral, and oblique radiographic images to outline the course of the fistula tract. The accuracy rate is 16-48%.^[10]

Fistulography is well tolerated but requires the ability to visualize the internal opening. Except in the case of recurrent disease, fistulography may be slightly more useful than a careful examination under anesthesia.

Endoanal/endorectal ultrasonography

These studies involve passage of a 7- or 10MHz transducer into the anal canal to help define muscular anatomy differentiating intersphincteric from transsphincteric lesions. A standard water-filled balloon transducer can help to evaluate the rectal wall for any suprasphincteric extension.

Investigations have shown that the addition of hydrogen peroxide via the external opening can help to outline the fistula tract course. This may be useful to help delineate missed internal openings.

These studies are reported to be 50% better than physical examination alone to help find an internal opening that is difficult to localize. This modality has not been used widely for routine clinical fistula evaluation.

MRI

Findings on magnetic resonance imaging (MRI) scans show 80-90% concordance with operative findings when a primary tract course and secondary extensions are observed. MRI is becoming the study of choice when evaluating complex fistulas and recurrent fistulas. It has been shown to reduce recurrence rates by providing information on otherwise unknown extensions.^{[11, 12]}

CT scan

A computed tomography (CT) scan is more helpful in the setting of perirectal inflammatory disease than in the setting of small fistulas because it is better for delineating fluid pockets that require drainage than for delineating small fistulas. CT scanning requires administration of oral and rectal contrast. Muscular anatomy is not well delineated.

Barium enema/small bowel series

This is useful for patients with multiple fistulas or recurrent disease to help rule out inflammatory bowel disease.

Pressure evaluation of the sphincter mechanism is helpful in certain patients, including the following:

• Patients in whom decreased tone is observed during preoperative evaluation
• Patients with a history of previous fistulotomy
• Patients with a history of obstetrical trauma
• Patients with a high transsphincteric or suprasphincteric fistula (if known)
• Very elderly patients

If a decrease in pressure is found, surgical division of any portion of the sphincter mechanism should be avoided.

Examination under anesthesia

An examination of the perineum, digital rectal examination, and anoscopy are performed after the anesthesia of choice is administered. This examination is necessary before surgical intervention, especially if outpatient evaluation causes discomfort or has not helped to delineate the course of the fistulous process.

Several techniques have been described to help locate the course of the fistula and, more importantly, identify the internal opening. They include the following:

• Inject hydrogen peroxide, milk, or dilute methylene blue into the external opening and watch for egress at the dentate line; in the authors' experience, methylene blue often obscures the field more than it helps to identify the opening
• Traction (pulling or pushing) on the external opening may also cause a dimpling or protrusion of the involved crypt
• Insertion of a blunt-tipped crypt probe via the external opening may help to outline the direction of the tract; if it approaches the dentate line within a few millimeters, a direct extension likely existed (care should be taken to not use excessive force and create false passages)

Proctosigmoidoscopy/colonoscopy

Rigid sigmoidoscopy can be performed at the initial evaluation to help rule out any associated disease process in the rectum. Further colonic evaluation is performed only as indicated.

Indications

Therapeutic intervention is indicated for symptomatic patients. Symptoms usually involve recurrent episodes of anorectal sepsis. An abscess develops easily if the external opening on the perianal skin seals itself.

Crohn disease of the perineum with multiple and often complex fistulas requires careful surgical treatment. Acute perianal abscess requires incision and drainage. Definitive repair of fistulas in these patients requires that the intra-abdominal disease be under control with medical therapy. If controlled, routine therapy is warranted. Recurrent fistulous disease to the rectum and perineum with persistent anorectal sepsis is an indication for panproctocolectomy. Studies have identified a role in Crohn disease for fistula therapy with infliximab, the monoclonal antibody to tumor necrosis factor, with 50-60% response rates for perianal fistulas.^{[13, 14]}

Contraindications

If patients are without symptoms and a fistula is found during a routine examination, no therapy is required.

Surgery for fistula-in-ano should not be performed for definitive repair of the fistula in the setting of anorectal abscess (unless the fistula is superficial and the tract is obvious). In the acute phase, simple incision and drainage of the abscess are sufficient.^[15] Only 7-40% of patients will develop a fistula. Recurrent anal sepsis and fistula formation are 2-fold higher after an abscess in patients younger than 40 years and are almost 3-fold higher in nondiabetics.

The laying-open technique (fistulotomy) is useful for 85-95% of primary fistulas (ie, submucosal, intersphincteric, low transsphincteric). (See the image below.)^{[16, 17, 18]}

A probe is passed into the tract through the external and internal openings. The overlying skin, subcutaneous tissue, and internal sphincter muscle are divided with a knife or electrocautery, thereby opening the entire fibrous tract.

At low levels in the anus, the internal sphincter and subcutaneous external sphincter can be divided at right angles to the underlying fibers without affecting continence. This is not the case if the fistulotomy is performed anteriorly in female patients. If the fistula tract courses higher into the sphincter mechanism, seton placement should be performed. Curettage is performed to remove granulation tissue in the tract base.

Opening the wound out on the perianal skin for 1-2cm adjacent to the external opening with local excision of skin promotes internal healing before external closure. Some advocate marsupialization of the edges to improve healing times. Perform a biopsy on any firm, suggestive tissue.

Complete fistulectomy creates larger wounds that take longer to heal and offers no recurrence advantage over fistulotomy.

A seton can be placed alone, combined with fistulotomy, or in a staged fashion. This technique is useful in patients with the following conditions^{[19, 20, 21]} :

• Complex fistulas (ie, high transsphincteric, suprasphincteric, extrasphincteric) or multiple fistulas
• Recurrent fistulas after previous fistulotomy
• Anterior fistulas in female patients
• Poor preoperative sphincter pressures
• Patients with Crohn disease or patients who are immunosuppressed

Beyond giving a visual identification of the amount of sphincter muscle involved, the purposes of setons are to drain, to promote fibrosis, and to cut through the fistula. Setons can be made from large silk suture, silastic vessel markers, or rubber bands that are threaded through the fistula tract.

Single-stage seton (cutting)

Pass the seton through the fistula tract around the deep external sphincter after opening the skin, subcutaneous tissue, internal sphincter muscle, and subcutaneous external sphincter muscle. The seton is tightened down and secured with a separate silk tie.

With time, fibrosis occurs above the seton as it gradually cuts through the sphincter muscles and essentially exteriorizes the tract. The seton is tightened on subsequent office visits until it is pulled through over 6-8 weeks. A cutting seton can also be used without associated fistulotomy. (See the image below).

Two-stage seton (draining/fibrosing)

Pass the seton around the deep portion of the external sphincter after opening the skin, subcutaneous tissue, internal sphincter muscle, and subcutaneous external sphincter muscle.

Unlike the cutting seton, the seton is left loose to drain the intersphincteric space and to promote fibrosis in the deep sphincter muscle. Once the superficial wound is healed completely (2-3mo later), the seton-bound sphincter muscle is divided.

Two studies (74 patients combined) supported the 2-stage approach with a 0-nylon seton. Once wound healing is complete, the seton is removed without division of the remaining encircled deep external sphincter muscle. The researchers reported eradication of the fistula tract in 60-78% of cases.

Mucosal advancement flap is reserved for use in patients with chronic high fistula but is indicated for the same disease process as seton use.^{[13, 22]} Advantages include a 1-stage procedure with no additional sphincter damage. A disadvantage is poor success in patients with Crohn disease or acute infection.

This procedure involves total fistulectomy, with removal of the primary and secondary tracts and complete excision of the internal opening.

A rectal mucomuscular flap with a wide proximal base (2 times the apex width) is raised. The internal muscle defect is closed with an absorbable suture, and the flap is sewn down over the internal opening so that its suture line does not overlap the muscular repair.

Preoperative

Preoperative details include the following:

• Rectal irrigation with enemas should be performed on the morning of the operation
• Anesthesia can be general, local with intravenous sedation, or a regional block
• Administer preoperative antibiotics
• The prone jackknife position with buttocks apart is the most advantageous position

Intraoperative Details

Intraoperative considerations include the following:

• Examine the patient under anesthesia to confirm the extent of the fistula
• Identifying the internal opening to prevent recurrence is imperative
• A local anesthetic block at the end of the procedure provides postoperative analgesia

Postoperative Details

Most patients can be treated in an ambulatory setting with discharge instructions and close follow-up care. Sitz baths, analgesics, and stool-bulking agents (eg, bran, psyllium products) are used in follow-up care. Frequent office visits within the first few weeks help to ensure proper healing and wound care.

Importantly, ensure that the internal wound does not close prematurely, causing a recurrent fistula. Digital examination findings can help to distinguish early fibrosis. Wound healing usually occurs within 6 weeks.

Advances in biotechnology have led to the development of many new tissue adhesives and biomaterials formed as fistula plugs. By their less-invasive nature, these therapies lead to decreased postoperative morbidity and risk of incontinence, but long-term data are lacking for eradication of disease, especially in complex fistulas, which carry high recurrence rates.^[23]

Reported series exist of fibrin glue treatment of fistula-in-ano, with 1-year follow-up showing recurrence rates approaching 40-80%.^{[24, 25, 26]} The Surgisis fistula plug has also had mixed long-term results in direct clinical trials.^{[27, 28, 29]}

Early success rates have been reported for newer materials, such as acellular dermal matrix and the bioabsorbable Gore Bio-A fistula plug, in low fistulas and good animal model data.^[30] Evidence regarding long-term success with plug techniques for complex disease awaits randomized trials.

The postsurgical prognosis in fistula-in-ano is as follows:

• Standard fistulotomy - The reported rate of recurrence is 0-18%, and the rate of any stool incontinence is 3-7%.
• Seton use - The reported rate of recurrence is 0-17%, and the rate of any incontinence of stool is 0-17%.
• Mucosal advancement flap - The reported rate of recurrence is 1-17%, and the rate of any incontinence of stool is 6-8%^[22]

Postoperative complications

Early postoperative complications may include the following:

• Urinary retention
• Bleeding
• Fecal impaction
• Thrombosed hemorrhoids

Delayed postoperative complications may include the following:

• Recurrence
• Incontinence (stool)
• Anal stenosis - The healing process causes fibrosis of the anal canal; bulking agents for stool help to prevent narrowing
• Delayed wound healing - Complete healing occurs by 12 weeks unless an underlying disease process is present (ie, recurrence, Crohn disease)