INTRODUCTION — Perianal fistulas and abscesses are among the most serious manifestations of Crohn disease and non-Crohn related anorectal disease (picture 1 and picture 2). Complications can lead to difficulties with recurrent or non-healing fistulas or abscesses. In addition, these patients are at risk of incontinence as a result of the destructive nature of the fistulizing process and/or inadvertent damage to the anal sphincters during surgical exploration.
The lifetime risk for developing a fistula in patients with Crohn disease is 20 to 40 percent [1-4]. The frequency of perianal fistulas/abscesses in patients without Crohn disease has not been well established, but in a telephone survey of 102 randomly selected individuals, 20 percent of the individuals contacted had perianal symptoms (hemorrhoids, fistulas, etc) [5]. Despite the significant prevalence of perianal disease, the evaluation of this problem was, in the past, largely limited to digital rectal examination.
The inability of the clinician to directly visualize the fistula or abscess makes it difficult to assess the lesions. The clinician must essentially discern the perianal anatomy by touch. This task is made even more problematic by the induration and inflammation that is usually present in these patients. Even surgical evaluation is only 35 to 85 percent accurate when compared with the results of other diagnostic tests and clinical evaluation [6-9].
The importance of accurately characterizing the perianal process prior to embarking on therapy cannot be overemphasized. The risk of incomplete healing, a recurrent fistula, or even inadvertent sphincter injury is increased if fistula anatomy is incorrectly delineated or an occult abscess missed. An imaging modality should ideally provide a virtual road map that the clinician can use to plan therapy. This is especially true with fistulas that involve a significant portion of the anal sphincter complex. Such patients are at the greatest risk of developing incontinence from the destructive fistulizing process or from overly aggressive surgical treatment.
Thus, patients with simple fistulas that only involve a small portion or none of the external anal sphincter generally do well with either medical or surgical treatment. Imaging of the fistula is helpful in determining the type of fistula to guide treatment but is not always needed. By contrast, for patients with a complex fistula (ie, one that involves a significant portion of the sphincter complex), preoperative imaging is mandatory.
Several imaging modalities are available to evaluate perianal fistulas and abscesses. These include fistulography, computed tomography, magnetic resonance imaging, and ultrasonography (both transrectal and endoscopic). The efficacy of each modality (with emphasis on endoscopic ultrasonography) will be reviewed here.
FISTULOGRAPHY — Fistulography has traditionally been the primary method of evaluating perianal fistulas. It involves insertion of a small caliber catheter into the external opening of a fistula and injection of radiographic contrast material directly into the fistula track. Radiographs from different angles are then obtained.
Fistulography has several drawbacks. The crucial determination of the fistula's course in relation to the sphincter complex must be inferred because the musculature of the anorectum cannot be visualized. In addition, instillation of the contrast material can be painful and can lead to the theoretical dissemination of septic fistulous contents.
Furthermore, the accuracy of fistulography has been questioned. In one retrospective study of 25 patients, the findings of fistulography were compared with the operative findings [10]. Fistulograms were correct in only four patients (16 percent). In addition, false-positive results that could have led to unnecessary complications occurred in three patients (12 percent).
As a result of these limitations, fistulography should generally be reserved for select patients in whom there is concern about a fistulous connection between the rectum and an adjacent organ such as the bladder.
COMPUTED TOMOGRAPHY — Computed tomography (CT) permits structures outside of the bowel lumen to be visualized, an advantage compared with fistulography. It is valuable for evaluating suspected perianal abscesses and inflammation [11-15]. In one study, for example, CT (with intravenous and, when possible, rectal contrast) was useful in differentiating perirectal abscesses from severe perirectal cellulitis and correctly identified 13 surgical proven abscesses in 10 patients [16].
The utility of CT for perianal fistulas is less clear. Fistulas are identified on CT when either a linear track containing air or contrast material is demonstrated extending from the bowel. The limited resolution of CT makes it difficult to differentiate between inflammatory soft tissue streaking and a fistula tract [15]. One study of 25 patients with suspected perianal Crohn disease compared the efficacy of endoscopic ultrasound (EUS) and CT [17]. EUS was conducted using a 5 MHz radial scanning scope. CT was performed using both intravenous and rectal contrast. Results were compared with findings at surgery and/or clinical course. EUS was found to be more accurate than CT in the evaluation of perianal fistulas (82 versus 24 percent).
Another limitation of CT is that the classification of fistulas can be difficult because CT scans in the transverse plane make the identification of the levator ani unreliable [16]. Because of these reasons, and because CT exposes the patient to ionizing radiation, the use of CT in these patients is limited.
MAGNETIC RESONANCE IMAGING — Because of the limitations of fistulography and computed tomography (CT), attention has turned to the newer technologies, such as magnetic resonance imaging (MRI) (image 1 and image 2) and ultrasound, as a better means of imaging perianal fistulas. Several prospective studies have looked at MRI in the evaluation of fistula-in-ano in patients without Crohn disease. Most of these studies used examination under anesthesia as the gold standard [8,18,19]. Two of the largest studies had widely differing results. In one, the results of MRI were compared with surgical evaluation in 35 patients [20]. The overall concordance between MRI and surgery was 85 percent. In contrast, using similar methodology, the group from University Hospital Nottingham reported accuracy of MRI in 33 patients to be only 42 percent [18].
However, at least three studies have questioned the use of surgical evaluation as a gold standard [7,9,21]. Patients were followed postoperatively to allow time for a missed fistula or abscess to declare itself, thereby permitting determination of the predictive value of MRI and examination under anesthesia. In one study of 42 patients MRI was found to be more sensitive than examination under anesthesia (95 versus 76 percent) [21]. Another series of 40 patients who were followed for 14 months postoperatively found a sensitivity and specificity of 89 and 69 percent, respectively, for MRI compared with 73 and 47 percent, respectively, for surgical exploration [7]. An updated report of 52 patients from the same group found MRI to be slightly more sensitive than surgical assessment in determining disease severity (81 versus 77 percent), although these results were not statistically significant [9].
Only a few studies have looked at MRI utility specifically in patients with Crohn disease with perianal fistulas:
●In a pilot study, MRI was able to identify eight out of nine (89 percent) perianal fistulas [22]. However, this study used unreliable imaging modalities such as CT and fistulography as the gold standard. Five years later, the same group found MRI to be 86 percent sensitive in delineating perianal fistula anatomy in 34 Crohn patients using surgical findings as the gold standard [23]. MRI seemed to have more difficulty demonstrating the shorter, more superficial tracks.
●One of the largest studies included 54 patients with suspected perianal Crohn disease [24]. The authors reviewed the proctological, MRI, and intraoperative findings to determine a consensus gold standard that they used as their benchmark. A total of 90 fistulas and 83 abscesses were found in these patients. MRI was 82 percent accurate for determining fistula anatomy, and, as in the previous report [23], tended to miss the short or superficial fistula tracks.
●Another report focused on 18 patients who were studied before and after treatment with infliximab [25]. A fistula track with signs of active inflammation was visualized with MRI in all patients prior to therapy. The fistula track remained visible in 8 of 11 patients who responded clinically to infliximab. After long-term treatment (46 weeks), MRI signs of active track inflammation had resolved in three of six patients. These findings suggest that despite closure of draining external orifices following infliximab therapy, fistula tracks can persist with varying degrees of inflammation. Similar findings have been described by others using both MRI and endosonography [26-29].
ENDOSONOGRAPHY — Endosonography, both blind transrectal (TRUS) and endoscopic ultrasound (EUS), has also been used to evaluate perianal fistulas. Similar to the magnetic resonance imaging (MRI) literature, the vast majority of the ultrasound studies have focused on patients without Crohn disease.
Technical aspects — Two discrete rings of tissue can be seen when using a radial scanning echoendoscope to examine the anorectum (image 3). The inner hypoechoic ring of tissue represents the internal anal sphincter, which is formed by the thickened continuation of the circular smooth muscle of the rectum. It is usually approximately 3 cm in length. The outer hyperechoic ring of tissue represents the external anal sphincter, which is formed by the downward extension of the skeletal muscle of the puborectalis. It is generally 4 cm in length.
The initial endosonographic studies used 7 MHz radial probes that were placed blindly into the rectum. These early pilot studies yielded promising results with the sensitivity for visualizing anal fistulas greater than 90 percent [30,31]. Several centers have tried to increase the sensitivity of ultrasound by instilling hydrogen peroxide into the fistula tracks [32,33]. Hydrogen peroxide acts as a contrast medium for ultrasound, creating echo-rich bubbles within the fistula track. This method is limited to fistulas with cutaneous openings. A limitation of this approach is that hydrogen peroxide can cause acoustic shadowing that may lead to misinterpretation of the fistula track. In our experience, a 7 MHz linear scanning ultrasound probe is able to clearly demonstrate the air within a fistula tract, thus making instillation of hydrogen peroxide unnecessary (image 4). Frequently, by applying gentle pressure to the fistula tract with the linear probe we can clearly visualize the air bubble moving within the tract itself (movie 1).
Accuracy — In one of the more commonly quoted TRUS studies in the literature, the group from St. Mark's reported disappointing results using TRUS to evaluate fistula-in-ano [6]. In this prospective study of 38 patients with suspected fistula-in-ano, digital rectal examination by an experienced consultant was compared with TRUS. Surgical findings were considered the gold standard. Digital rectal exam (DRE) was found to be more accurate than ultrasound in determining the course of the primary fistula track (85 percent versus 72 percent).
However, these results must be interpreted with caution for several reasons. The difference between DRE and TRUS did not meet statistical significance. In addition, as the authors readily admit, the rigid nature of the probe prevented good acoustic coupling higher in the rectum, thus preventing the interpretation of higher fistula tracks. Furthermore, the focal length of the probe utilized for this study was only 3 cm, which limited scanning to no deeper than the external anal sphincter (EAS). To prevent these problems, we use an inflatable balloon probe on a flexible echoendoscope to permit better acoustic coupling throughout the rectum. In addition, we evaluate the fistulas with both radial and linear scanning instruments in order to achieve a greater depth of imaging and to more thoroughly characterize the fistulas.
The destructive and recurrent nature of perianal Crohn disease makes accurate imaging more difficult than with simple fistula-in-ano. However, several non-blinded studies have shown ultrasound to be a viable modality for examining the perianal manifestations of Crohn disease [34-39]. A prospective blinded study compared EUS with computed tomography (CT) in 25 Crohn patients with suspected perianal involvement [17]. A 5 MHz radial scanning probe was used to conduct the ultrasound examinations. Surgery or fistulography was used as the gold standard. EUS was found to be superior to CT with a sensitivity of 82 versus 24 percent, respectively.
A later, randomized prospective study randomly assigned 10 patients with EUS with examination under anesthesia, or examination under anesthesia alone [40]. Patients were managed medically or surgically based upon the finding. At the end of one year, patients randomized to the EUS arm were more likely to have complete cessation of drainage, suggesting that the EUS may have improved care.
At least four prospective studies have compared MRI with endosonography in the evaluation of perianal fistulas [20,41-43]. Two of these reports concentrated exclusively on patients with Crohn disease [42,43]. The two studies focusing primarily on non-Crohn patients using surgery as the gold standard found MRI to be superior to TRUS in imaging fistulas [20,41]. Both of these studies used 7 MHz rigid radial scanning ultrasound probes. The limitation of this equipment (see 'Technical aspects' above) most likely contributed to the lower ultrasound sensitivity seen in these studies.
In contrast, one of the prospective studies comparing MRI and EUS for Crohn perianal fistulas found endosonography to be the most sensitive modality for imaging fistulas [42]. In this pilot study of 22 patients, surgical evaluation was used as the gold standard. The agreement for fistulas with the surgical findings for endosonography and MRI was 82 percent and 50 percent, respectively. Ultrasound was performed with a 7 MHz linear scanning probe. Although not used in this study, we also use a radial scanning probe to provide complimentary information. This can reveal fistulas not apparent with the linear probe (image 5).
The poor sensitivity of MRI in this study (50 percent) may be secondary to the use of a body coil for imaging instead of a pelvic phased array coil. The pelvic phased array coil is a receive-only coil and provides better spatial resolution than is available with the body coil. A study using the phased array suggested that it was highly accurate in detecting primary tracks and secondary extensions and provided important additional information in 12 out of 56 patients (21 percent) enrolled in the study [44]. The benefit of MRI was most obvious in patients with fistulas related to Crohn disease and in patients with complex fistulas associated with a recurrence.
Similar conclusions were reached in another study in 34 patients with Crohn disease who were suspected of having perianal fistulas [43]. Patients underwent EUS and MRI within the same week followed by surgical examination under anesthesia (EUA). The gold standard anatomy was defined after reviewing data from all three modalities. All three methods demonstrated good agreement with the gold standard (EUS 91 percent, MRI 87 percent, and EUA 91 percent). The accuracy increased to 100 percent when EUA was combined with either EUS or MRI.
Several studies have evaluated EUS and MRI in monitoring the course of fistula healing in patients with perianal Crohn disease [25,45-48]. Two reports also suggested that treatment based upon EUS findings was helpful in determining the optimal time to remove a seton [40,47]. In addition, one prospective study utilizing MRI to monitor fistula healing in patients with Crohn perianal fistulas on anti-TNF treatment suggested that imaging may be useful in identifying patients who need continued medical therapy [49].
INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.
Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)
●Basics topics (see "Patient education: Anal abscess and fistula (The Basics)")
SUMMARY AND RECOMMENDATIONS
●Perianal fistulas and abscesses are a common problem; proper therapy depends upon the accurate assessment of the perianal anatomy. (See 'Introduction' above.)
●Fistulography is not an accurate means of evaluating perianal disease. (See 'Fistulography' above.)
●Computed tomography (CT) may be helpful when looking for large intrapelvic abscesses. CT is inaccurate for the detection and classification of perianal fistula tracks and small perianal abscess collections. (See 'Computed tomography' above.)
●Magnetic resonance imaging (MRI) and endosonography are an accurate means of evaluating perianal disease, with an accuracy of approximately 80 to 90 percent compared with surgical exam under anesthesia. Accuracy of MRI depends upon availability of MRI imaging with dedicated pelvic coils. The dedicated pelvic coil provides better spatial resolution in the pelvis. Similarly, the accuracy of ultrasound depends upon achievement of good acoustic coupling by using water-filled inflatable balloons. (See 'Magnetic resonance imaging' above and 'Endosonography' above.)
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