the role of imaging tests in the evaluation of anal abscesses and fistulas-uptodate 5-7-2012

7/29/2019 The Role of Imaging Tests in the Evaluation of Anal Abscesses and Fistulas-uptodate 5-7-2012

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The role of imaging tests in the evaluation of anal abscesses and fistulas

Authors

David A Schwartz, MD

Maurits J Wiersema, MD

Section Editor

J Thomas LaMont, MDDeputy Editor

Shilpa Grover, MD, MPH

Disclosures

All topics are updated as new evidence becomes available and ourpeer review

process is complete.

Literature review current through: Feb 2013. | This topic last updated: 5, 2012.

INTRODUCTION Perianal fistulas and abscesses are among the most serious

manifestations of Crohn's disease and non-Crohn's related anorectal disease (picture 1

andpicture 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 resultof 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's disease is 20 to 40

percent [1-4]. The frequency of perianal fistulas/abscesses in patients without Crohn's

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 physician 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 to 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 isincorrectly delineated or an occult abscess missed. An imaging modality should

ideally provide a virtual road map that the physician 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 ismandatory.
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Several imaging modalities are available to evaluate perianal fistulas and abscesses.

These include fistulography, computed tomography (CT), magnetic resonance

imaging (MRI), 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 retrospectivestudy of 25 patients, the findings of fistulography were compared to 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 to 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's disease compared the efficacy of endoscopicultrasound (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 to 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 offistulography and computed tomography (CT), attention has turned to the newer
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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's

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 to surgical evaluation in 35 patients [20]. The overallconcordance 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 to 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's

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's 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's

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 fistulatracks.

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].
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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's disease.

Technical aspects Two discrete rings of tissue can be seen when using a radialscanning 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 about 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 instillinghydrogen peroxide intothe 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 to 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 acousticcoupling 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's 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 perianalmanifestations of Crohn's disease [34-39]. A prospective blinded study compared
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EUS to computed tomography (CT) in 25 Crohn's 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 withexamination 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 to endosonography in the

evaluation of perianal fistulas [20,41-43]. Two of these reports concentrated

exclusively on patients with Crohn's disease [42,43]. The two studies focusing

primarily on non-Crohn's 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 'Technicalaspects' 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's

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's disease

and in patients with complex fistulas associated with a recurrence.

Similar conclusions were reached in another study in 34 patients with Crohn's 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's disease [25,45-47]. Two reports also

suggested that treatment based upon EUS findings was helpful in determining theoptimal time to remove a seton [40,47]. In addition, one prospective study utilizing
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MRI to monitor fistula healing in patients with Crohns perianal fistulas on anti-TNF

treatment suggested that imaging may be useful in identifying patients who need

continued medical therapy [48].

INFORMATION FOR PATIENTS UpToDate offers two types of patient

education materials, The Basics and Beyond the Basics. The Basics patienteducation 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 thekeyword(s) of interest.)

Basics topics (see "Patient information: 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 accuracy of approximately 80 to 90

percent compared to 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.)

Use of UpToDate is subject to the Subscription and License Agreement.

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