A fistula-in-ano is an epithelial-lined tract connecting the anal canal to the perianal skin. Anal fistulas can have many causes but are most commonly the result of an anorectal abscess. Classification of the fistula is determined in relation to the anal sphincters. Although benign, the condition can cause significant distress and embarrassment to the patient. Treatment focuses on control of the infection and maintaining fecal continence. Many treatment options are available, and novel treatments are steadily proposed and tested. This article will review the fundamental principles of fistula-in-ano diagnosis and treatment.
Fistula-in-ano often occurs following anorectal abscess. An anorectal abscess occurs when an anal gland becomes obstructed, resulting in infection and abscess formation. The infection is located near the sphincter complex, and therefore the fistula can traverse the sphincters. One-third of patients undergoing incision and drainage of an anorectal abscess will later develop a fistula. Thirty to 70% of patients diagnosed with an anorectal abscess will already have a fistula present on exam.
Fistula-in-ano is one of the most common anorectal diseases. The prevalence is greater in men than women, with a rate of 12.3 cases per 100,000 and 5.6 cases per 100,000, respectively. The average age at diagnosis is 38 years, with most occurring between 20 to 40 years of age. Risk factors for fistula development include obesity, diabetes, smoking, hyperlipidemia, and a sedentary lifestyle.
Simple anal fistulas are thought to be due to glandular obstruction resulting in an anorectal abscess and, ultimately, a fistula. Characteristics of a simple fistula include a single tract, subcutaneous tract, and those that involve less than 30% of the external sphincter. A simple fistula is the easiest to treat and has the lowest recurrence and complication rates.
Complex fistulas include those that involve more than 30% of the external sphincter, fistulas with multiple tracts, recurrent fistulas, and those associated with other predisposing factors, including Crohn disease and radiation treatment. Due to the large involvement of the external sphincter, a simple fistulotomy should not be performed due to the risk of postoperative fecal incontinence. Complex repair or staged repair is preferred to preserve sphincter function.
Patients presenting with an anorectal abscess often experience significant perineal pain and occasionally drainage. Systemic signs, such as fever, are rare in immunocompetent patients. The pain experienced is often exquisite, preventing a patient from sitting or tolerating a physical exam. Deeper abscesses may cause low back pain or pressure.
A thorough history and physical examination are necessary to classify the fistula and create a treatment plan appropriately. Previous diagnoses of inflammatory bowel disease, rectal or anal neoplasm, previous anorectal disease, and prior surgeries are important in diagnosis and treatment planning. Baseline fecal continence should be assessed and documented. In the presence of multiple fistulas or ongoing recurrence, a diagnosis of inflammatory bowel disease should be considered.
A complete rectal exam is essential for the accurate diagnosis of fistula-in-ano. It is important to rule out other anorectal pathology and confirm the diagnosis of a fistula. Due to patient discomfort, this exam is often performed under anesthesia in the operating room. This allows for a complete examination as well as treatment in the same setting. The rectal exam should include inspection of the perianal skin and any external openings. A digital rectal exam should be performed to evaluate for any masses as well as sphincter tone. The internal opening may also be palpable within the anal canal. Anoscopy may be used to locate and evaluate the internal opening. Fistula probes help identify the tract in most cases, but if unsuccessful, injection of hydrogen peroxide may lead to the identification of the internal opening.
Classification of anal fistulas is based on anatomy, specifically in relation to the sphincter complex. In 1976, Dr. Parks published a paper describing a classification system for anal fistulas that is still widely used today. Four types were described: intersphincteric, transsphincteric, suprasphincteric, and extrasphincteric. Appropriate classification is essential for adequate treatment and conservation of the external anal sphincter to preserve fecal continence.
The classification of the fistula is often diagnosed during an exam under anesthesia in the setting of a simple fistula. Evaluation of complex fistulas, including recurrent disease and fistulas in the setting of perianal Crohn disease, may benefit from pre-operative imaging. MRI, endoanal ultrasound, fistulography, and CT are all proven to contribute to an accurate diagnosis, with MRI being the most sensitive (>90%). A combination of two imaging modalities increases the diagnostic accuracy to 100%.
The treatment of an anal fistula depends on the location of the fistula as well as precipitating factors. Most fistulas are treated surgically, through a variety of different procedures depending on external and internal sphincter involvement. Complex fistulas, particularly those caused by Crohn disease, are treated medically. The most common options will be discussed in detail.
Fistula-in-ano is not a life-threatening condition but can have a significant impact on a patient’s quality of life. Treatment is focused on the destruction of the fistula tract with the preservation of the sphincter complex. Complications of treatment can be life-altering for a patient and include fecal incontinence, fistula recurrence, and anal stenosis. The risk of recurrence varies between 3% to 57% depending on the anatomic classification of the fistula and treatment modality.
The greatest risk of surgical treatment involves disruption of the external sphincter resulting in fecal incontinence. The preoperative evaluation must include the patient’s preoperative continence status and, if possible, documentation of sphincter tone prior to anesthesia and paralysis. The anatomic classification of the fistula, as well as the surgical approach, are the most important factors when determining the risk of post-operative fecal incontinence. For this reason, the surgical approach depends on the anatomic considerations to reduce the risk of incontinence. Complex fistulas, including suprasphincteric and extrasphincteric fistulas, have a higher risk of incontinence due to sphincter involvement.
Recurrence of fistula-in-ano is higher in obese patients, smokers, and those with a history of anal surgery. Anatomical factors including multiple fistula tracts, high transphincteric fistula, inability to determine an internal opening, seton placement, and horseshoe abscesses were also associated with a higher rate of recurrence. Recurrence of the fistula often occurs within the first year of treatment.
A diagnosis of fistula-in-ano should be considered for any patient with purulent or feculent anal drainage, especially with a history of an anorectal abscess, Crohn’s disease, or prior radiation. Diagnosis is often confirmed on physical exam, but more complex fistulas may require imaging for identification. Classification of the fistula is based on anatomy, which is best assessed during the exam. Due to high levels of pain with the exam, an exam under anesthesia may be necessary for an accurate diagnosis.
The treatment options for fistula-in-ano vary widely depending on external sphincter involvement. The development of new and novel treatments aimed at sphincter conservation continues. Although medical treatment is appropriate for some cases, most fistulas require a surgical procedure. Diagnosis or suspicion of an anal fistula should be followed by surgical consult. In the setting of a complex fistula, colorectal surgical consult may be necessary.
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