List of Abbreviations
Computed tomography enterography
Examination under anesthesia
Magnetic resonance enterography
Fistulae occur in 14%–50% of all patients with Crohn’s disease (CD). In a population-based study from Olmsted County, Minnesota, 35% of CD patients had at least one fistula. Perianal fistulae comprise at least half of all cases, enteroenteric fistula (EEF) 25%, rectal/anovaginal 10%, and others such as enterocutaneous fistula (ECF) and enterovesical account for 10%–15% of the fistulas. Over a 20-year follow-up, two-thirds of patients experience only one fistula episode whereas the remainder will have two or more related episodes. Of note, the presence of a fistula is an indicator of more aggressive disease that may require more frequent hospitalizations, higher incidence of surgery, and increased utilization of corticosteroid treatment. The diagnosis, assessment, and the treatment of fistulizing CD is complex and mandates a multidisciplinary approach involving gastroenterologists, surgeons, and radiologists at a specialized referral center.
A fistula is defined as a pathological connection adjoining two epithelialized surfaces. They can connect a portion of the intestine to the outer surface (e.g., enteroatmospheric or anus to outer skin) or to another inner surface (e.g., enteroenteric, enterovesicular).
Despite the prevalence of fistulae in patients with CD, the pathophysiology remains largely unknown. The first step in the formation of a fistula is thought to be tissue destruction from transmural inflammation. The impaired ability of mucosal fibroblasts to migrate toward the area of tissue injury, and the inability of fibroblasts to repair mucosal defects may stimulate the migration of epithelial cells toward the defect. For faster migration, epithelial cells undergo epithelial-mesenchymal cell transition and develop a mesenchymal cell-like phenotype with loose cell to cell contacts, invasive potential, and downregulated apoptotic pathways which may penetrate into the bowel and lead to fistula formation. However, the exact mechanism has yet to be defined.
Diagnosis is most often initiated by the presence of drainage on physical exam, air or stool in the urine in the setting of fistulisation to the bladder, or new onset of high output liquid stool in the setting of an internal fistula. Following a careful history and physical exam, imaging is of paramount importance in diagnosis and classification of fistulae. Of the currently used methods for mapping fistula anatomy, examination under anesthesia (EUA), MRI of the pelvis, and endoanal ultrasound (EUS) all show similar accuracies. While the gold standard for diagnosis remains undefined, when combining EUA and MRI, diagnostic accuracy approaches 100%. And, an international consensus report has recommended the use of MRI and clinical examination to assess fistula closure in clinical trials.
Treatment Goals and Classification of Healing
The primary goal for the patient is to reduce or eliminate fistula secretion and abscess risk. In addition, avoidance of a stoma and fecal incontinence are critical to consider when treating a fistula.
The results of treatment can be classified as closure, improvement, remission, or definitive fistula closure. Closure of the individual fistula is considered the cessation of drainage when light manual compression is applied. Improvement refers to a reduction in the open or secreting fistula by greater than 50% compared with the baseline level on at least two consecutive examinations at least 1 month following fistula treatment. Remission refers to the closure of all fistulas in relation to the baseline level on at least two consecutive examinations, at least 1 month following the treatment intervention for the fistula. And, definitive fistula closure refers to complete closure of the fistula, where probing to open the tract is not feasible and the fistula cannot be visualized on MRI.
Another simplified and commonly used classification of treatment success in perianal fistulas is clinical and radiographic healing. Clinical resolution refers to decreased drainage, or cessation of drainage in the case of complete clinical resolution. Radiographic healing refers to scarring or resolution of the fistula tract identified on initial imaging. This most commonly, and most accurately, is determined by MRI.
Classification of Perianal Fistula From CD (Expansion of the Park’s Classification)
In approximately 10% of the patients, a perianal fistula is the initial manifestation of CD. In fact, the formation of perianal fistulae may precede the onset of intestinal CD by several years. Patients with colonic CD, particularly those with active proctitis, have a significantly higher incidence of perianal fistulae than patients without colorectal disease. Perianal fistulae can be classified as low, high, simple, or complex. Low refers to perianal fistulas originating below the dentate line, whereas high originate above the dentate line. Simple fistulas are those that are low and painless with a single external opening and no evidence of rectovaginal involvement or anorectal stricture. The definition of a complex fistula is not standardized but most agree that any fistula that is high transsphincteric or when a fistulotomy would result in incontinence, the fistula should be considered to be complex. The definition also includes all those caused by CD, those associated with pain, multiple external openings, those involving a rectovaginal component, or those with anorectal stricture or active proctitis.
Park’s classification of perianal CD classifies anal fistulas based on their relationship to the anal sphincter complex. The nomenclature reflects the fistula’s relationship to the external sphincter: intersphincteric, transsphincteric, suprasphincteric, and extrasphincteric ( Table 9.1 ).
|Simple low intersphincteric|
|High blind tract|
|High tract with an opening in the rectum|
|High tract with rectal opening, no perineal opening|
|Secondary to pelvic disease|
|High blind tract|
|Secondary to anal fistula|
|Inflammatory bowel disease or other anal disease|
An intersphincteric fistula occurs in 20%–45% of cases and does not penetrate the external sphincter. Parks described seven subtypes of intersphincteric fistula ( Table 9.1 ) of which a high blind tract with an extension in the intersphincteric groove cephalad toward the rectum was the most common. A transsphincteric fistula occurs in 30%–60% of cases and penetrates the external sphincter below the level of the puborectalis muscle, exiting into varying levels within the ischiorectal fossa. A suprasphincteric fistula occurs in 20% of the cases and describes a fistala tract that is over the top of the puborectalis, then downward again through the levator plate to the ischiorectal fossa, and finally out the skin. As this tract passes over the puborectalis, it is in the supralevator space, and abscess formation in this space can result in horseshoe extension around the rectum. Extrasphincteric fistula, the least common occurring in 2%–5% of cases, passes from the perineal skin through the ischiorectal fat and levator muscles into the rectum ( Fig. 9.1 ).
Classification of Enterocutaneous Fistula
Enterocutaneous fistulae, also known as enteroatmospheric fistulae, are a devastating manifestation of CD with significant morbidity and distress to the patient. A distinction should be made between postoperative fistulae (discussed later) and primary ECF as the majority are comprised of postoperative fistulas.
Primary ECF originate from actively inflamed intestine. The lifetime risk of developing ECF in CD patients ranges from 20% to 40%. In a recent population-based study, the cumulative risk of developing ECF was 12% and 24% after 10 and 20 years, respectively. Conservative management with local wound control, medical management, and intravenous nutritional support should always be attempted first. This is especially true to patients at risk of short bowel following surgery or those with significant comorbidities. Those patients with an increased probability of spontaneous fistula closure with conservative management include patients with a low daily output (<500 mL/day), distal anatomic location, or single opening, and patients without malnutrition (serum albumin >3 g/dL) and those with intact intestinal continuity.
With the introduction of biologic therapy, there has been an improved rate of fistula healing with medical management alone. A recent study suggests that infliximab therapy can close ECF in one-third of patients and maintain this in one-fifth. Although promising and an improvement in medical therapy, the majority of patients will still require surgical intervention for resolution of symptoms. Again this should not be performed until sources of sepsis are controlled and a patient’s nutritional status is optimized.
The less commonly noted peristomal ECF may be conservatively treated with local wound care and protection of the surrounding skin. If this is not effective, surgical intervention includes relocation of the stoma and resection of affected bowel.
Classification of Enteroenteric Fistula
Enteroenteric fistulae (e.g., ileocolonic, ileoileal, jejunoileal, duodenocolonic) are classified by two anatomic locations joined by the epithelialized fistula tract. For example if a fistula connects ileum to colon, it is called ileocolonic; if a fistula connects duodenum to colon, it is called duodenocolonic; and if a fistula connects ileum to ileum, it is called ileoileal. The first named anatomic location often refers to the origin of the fistula, which is most often in a location of active disease. The second named anatomic location is the receiving segment, often free of disease. Half the time, diagnosis is made preoperatively by magnetic resonance enterography or computed tomography enterography. The other half of the cases are diagnosed intraoperatively, at which time one-third of the patients are found to have internal fistulae. Since most fistulas are asymptomatic, the presence of fistula alone is not an indication for surgery. For those patients who are symptomatic, resection of involved segment of bowel is the most definitive treatment.
Ileocolic fistulae from the distal ileum or ascending colon are often asymptomatic. In one series of 64 patients, the fistulae were diagnosed by radiographic studies in 75% of the patients and at surgery in 25% of the patients. They create a very short bypass of intestine and therefore do not have any nutritional repercussions. Therefore, these fistulas do not mandate surgical intervention. However, this site of enterenteric fistula is often associated with active inflammatory disease that requires medical or surgical attention, and which time the fistulae may be removed when removing the terminal ileal disease ( Fig. 9.2 ).