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Sonography of Benign Conditions of the Anal Canal: An Update

¹ Department of Medical Imaging, Section of Ultrasound, Toronto General Hospital, University of Toronto, Toronto, ON M5G 2N2, Canada.
² Present address: Institute of Radiology, IRCCS S. Matteo Hospital, University of Pavia, Pavia, Italy.
³ Present address: Serves di Radiologia, Ospedale Civile di Voghera, Voghera, Pavia, Italy.
⁴ Present address: Department of Diagnostic Imaging, Foothills Medical Centre, 1403 29 St. NW, Calgary, AB T2N 2N9, Canada.

Received January 29, 2007; revised May 12, 2007;

 
Address correspondence to S. R. Wilson (stephanie.wilson{at}calgaryhealthregion.ca).

S. R. Wilson is an advisor to Philips Medical Systems.

CME

This article is available for CME credit. See www.arrs.org for more information.

Abstract

Top Abstract Introduction Anatomy of the Anal... Sonography Technique for the... Anal Incontinence Perianal Inflammatory Disease Summary References

 
OBJECTIVE. The objective of this article is to describe our experience with sonography for the study of benign conditions of the anal canal and perianal soft tissues.

CONCLUSION. Assessment of the anal sphincters in patients with fecal incontinence and documentation of perianal inflammatory masses and tracts in those with perianal inflammatory disease are the major indications for imaging the anal canal. We augment traditional transanal sonography with transperineal scanning in both sexes and transvaginal scanning in women to better show the anal canal in its quiet state to allow an accurate assessment of the integrity of the anal sphincters and of evidence of acute or chronic inflammatory involvement.

Keywords: anal canal • Crohn's disease • fecal incontinence • perianal inflammatory masses • sonography • transanal imaging • transperineal imaging • transvaginal imaging

Introduction

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Evaluation of the anal canal using sonography has unique features as compared with similar evaluation of other portions of the gut studied with intracavitary placement of an ultrasound probe. These include the stomach and rectum in particular, both of which have an easily distensible and accessible lumen allowing positioning of the ultrasound transducer crystal array at an appropriate location within the gut lumen to show the region of interest optimally. The anal canal, by comparison, in its resting state shows luminal apposition, and although the lumen may be distended, it does not allow free movement of the transducer array away from a region of interest related to the wall of the anal canal or from intraluminal or perianal pathology. In an effort to study diseases of the anal canal, the perianal soft tissues, and the integrity of the anal sphincters, we have introduced biplane transducer technology for our transanal sonographic procedures and have also augmented transanal sonography scans with both transperineal and transvaginal scans. Assessment of the anal sphincters in patients with fecal incontinence and documentation of perianal inflammatory masses and tracts in those with perianal inflammatory disease are the major indications for these techniques.

In this article, we describe our experience with sonography for the study of benign disease of the anal canal and perianal soft tissues.

Anatomy of the Anal Canal

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The anal canal is the most distal portion of the gastrointestinal tract, beginning at the anorectal line and ending at the anal verge. It is generally 3–4 cm in length and its lumen is divided into three parts: the colorectal zone, with columnar mucosa identical to the distal rectal mucosa; the transitional zone or pecten, which contains many epithelial variants; and the cutaneous zone, with squamous epithelium [1]. The dentate line refers to the mucocutaneous junction and is the location of the largest number of anal glands, which are implicated in the development of perianal sepsis.

The anal sphincter is composed of an involuntary smooth-muscle inner component, the internal anal sphincter, continuous proximally with the circular muscle fibers of the muscularis propria of the rectum, and a voluntary striated external anal sphincter. The internal sphincter is discontinuous in the outer third of the anal canal, whereas the external sphincter forms a slinglike band surrounding the canal in continuity with the levator ani and puborectalis muscles.

Sonography Technique for the Study of the Anal Canal

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Transanal, Transperineal, and Transvaginal Imaging
The instrument used for transanal sonography consists of a handheld high-frequency 9-MHz probe inserted into the anal canal. Currently, using a single probe with biplane capability allowing placement of two transducer arrays is our technique of choice for the transanal component of the study. A curved array coupled with a linear array (HDI 5000, Philips Medical Systems) is better suited to the study of the anatomy of the anal canal than is an alternative design with two curved arrays, one on the end of the probe and the other on the transducer side, to provide scanning in perpendicular planes. The latter requires angulation and elevation of the examining hand for the study of the anal canal, whereas the former allows perpendicular scanning with the examining hand held in a more comfortable neutral position (Fig. 1). These probes provide state-of-the-art resolution and also have sensitive Doppler capability, providing useful information concerning morphology and blood flow. The disadvantages of biplane probes include the lack of a complete axial view of the anal canal and perianal soft tissues. Transanal sonography performed with a hard-tipped cone covering a rotating crystal mechanism is no longer used in our department.

View larger version (34K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1 —Diagram of transvaginal sonography of anal canal performed with biplane transducer with curved and linear array on transducer side. This technique requires no angulation or elevation of examining hand for study of anal canal and shows easily cross-sectional and long-axis views. Regardless of transducer selection, crystal array must be directed at anal canal. (Courtesy of Popovic G, Toronto, ON, Canada)

Despite the obvious advantages of resolution and versatility for study of the anal canal itself, transanal sonography is limited for the assessment of perianal inflammatory disease because patient pain may restrict transducer placement; and the rigid probe, positioned within the anal canal or rectum, may be inappropriately placed for the assessment of the perianal soft tissues because the pathologic process, frequently involving the perineum and the buttocks, is on a more caudal plane than the ultrasound beam [2].

View larger version (16K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A —Schematics of transperineal sonography. Long-axis (A) and cross-sectional (B) views. Transducer is placed on perineum between introitus and anal canal in women and between scrotum and anal canal in men. (Courtesy of Popovic G, Toronto, ON, Canada)

View larger version (16K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B —Schematics of transperineal sonography. Long-axis (A) and cross-sectional (B) views. Transducer is placed on perineum between introitus and anal canal in women and between scrotum and anal canal in men. (Courtesy of Popovic G, Toronto, ON, Canada)

Transvaginal sonography of the anal canal is performed by directing the ultrasound beam of a vaginally placed transducer toward the anal canal and scanning in both long-axis and crosssectional views. Traditional end-fired probes may be used by observing the rectum and anal canal in cross section as the probe is slowly withdrawn from the vagina while elevating the examining hand. Rotation of the hand will then show the canal in the long axis from the anorectal junction to the anal verge. Alternately, the biplane probe with both linear and curved components on the side of the transducer shows the cross-sectional and long-axis views with ease. For both assessment of perianal inflammatory conditions and determination of the integrity of the anal canal, we have found transvaginal scanning, which shows the anal canal easily in its entirety, to be of great value with accurate results [2]. Frudinger et al. [4], however, refute the value of vaginally placed probes for study of the integrity of the anal canal.

Recent technical developments include the addition of 3D transanal sonography or transvaginal sonography. The anal canal, similar to other organs and body anatomy having one long and two short dimensions, is ideally suited to study with 3D techniques. The advantages of 3D sonography include evaluation in arbitrary planes not available with 2D sonography to improve assessment of complex anatomic situations by 3D display. Three-dimensional sonography is valuable not only for showing the integrity of the sphincter muscle but also for displaying complex perianal fistulas.

Sonography of the Normal Anal Canal
The anal canal has a different appearance than the adjacent rectum when seen on sonography and is characterized by its round shape in cross section as compared with the collapsed rectum, which is generally more oval, and by the broad continuous circumferential hypoechoic band of muscle in its upper two thirds, which represents the internal anal sphincter (Fig. 3A, 3B, 3C, 3D, 3E, 3F). The upper anal canal can be recognized by identifying the puborectalis muscle as it sweeps around the rectum posteriorly, contributing fibers to the external anal sphincter and appearing as a circumferential hyperechoic band of striated muscle around the remainder of the anal canal. In the lowest portion of the canal, the internal anal sphincter cannot be seen, so only the hyperechoic external anal sphincter is visible. In females, the internal anal sphincter, if visualized, may be attenuated, possibly due to occult injury or denervation. The thickness of the internal anal sphincter ranges between 2 and 4 mm [5]. With age, there is increased thickness of the internal anal sphincter associated with thinning of the external anal sphincter [6]. This is an important factor for the diagnosis of atrophy of the external anal sphincter to be established.

View larger version (133K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A —Sonography of normal anal canal in three patients. A and B are transanal scans obtained with biplane 9-MHz transducer, C and D were obtained on curved transducer, and E and F were obtained at anorectal junction with traditional end-fired transvaginal probe. Axial image of healthy 20-year-old woman shows anterior portion of anal canal. Internal anal sphincter (IS) is prominent hypoechoic layer.

View larger version (109K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B —Sonography of normal anal canal in three patients. A and B are transanal scans obtained with biplane 9-MHz transducer, C and D were obtained on curved transducer, and E and F were obtained at anorectal junction with traditional end-fired transvaginal probe. Long-axis view corresponding to A shows internal sphincter (IS) in its entirety. Arrow marks anorectal junction, where thin muscularis propria thickens and becomes more round-appearing internal anal sphincter. There is no sphincter defect. Magnification on two crystal arrays is never exactly same.

View larger version (165K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3C —Sonography of normal anal canal in three patients. A and B are transanal scans obtained with biplane 9-MHz transducer, C and D were obtained on curved transducer, and E and F were obtained at anorectal junction with traditional end-fired transvaginal probe. Axial image of healthy 40-year-old woman shows prominent hypoechoic ring of internal anal sphincter (IS).

View larger version (140K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3D —Sonography of normal anal canal in three patients. A and B are transanal scans obtained with biplane 9-MHz transducer, C and D were obtained on curved transducer, and E and F were obtained at anorectal junction with traditional end-fired transvaginal probe. Rotating transducer by 90° from position in C shows anal sphincter in longitudinal view. Anal verge is on right and anorectal junction (arrow) on left of image. Internal anal sphincter (IS) appears as two longitudinal substantial hypoechoic bands.

View larger version (138K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3E —Sonography of normal anal canal in three patients. A and B are transanal scans obtained with biplane 9-MHz transducer, C and D were obtained on curved transducer, and E and F were obtained at anorectal junction with traditional end-fired transvaginal probe. Axial image of lower rectum of healthy 56-year-old woman shows prominence of submucosa as broad echogenic layer. Muscularis propria (arrows) surrounds gut and is thin and hypoechoic.

View larger version (144K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3F —Sonography of normal anal canal in three patients. A and B are transanal scans obtained with biplane 9-MHz transducer, C and D were obtained on curved transducer, and E and F were obtained at anorectal junction with traditional end-fired transvaginal probe. At anorectal junction of same patient as in E, thin muscularis propria thickens and becomes more roundappearing internal anal sphincter (IS). External anal sphincter (arrowheads) appears at this level as echogenic, rather poorly marginated sling-like structure around posterior aspect of anal canal. There is no sphincter defect.

Anal Incontinence

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Anal incontinence, the involuntary loss of gas, liquid, or solid stool, is a distressing condition with a substantial impact on quality of life [7]. Affected patients are often reluctant to discuss their problem, thereby providing inaccurate and misleading information [8]. Proper clinical assessment is essential and requires completion of a bowel history questionnaire to exclude specific causes such as irritable bowel disease, laxative abuse, celiac disease, and other conditions that might present as fecal incontinence [9]. Scoring systems based on descriptive measures, severity measures, and impact measures have been established and are shown to correlate well with physicians' clinical impressions and are necessary for comparative evaluations of treatment measures [10].

View larger version (155K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4A —Classic sphincter defect from obstetric trauma in 28-year-old woman. Axial image taken from transvaginal approach shows full-thickness anterior defect from 11to 1-o'clock positions.

View larger version (138K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4B —Classic sphincter defect from obstetric trauma in 28-year-old woman. Sagittal transperineal image shows that defect is full-length given that no internal anal sphincter can be seen anteriorly. Residual internal anal sphincter (IS) is normal posteriorly and appears as dominant linear hypoechoic band.

Although it can affect all ages and both sexes and has many causes, fecal incontinence is most common in women after injury to the anal sphincter, usually its anterior portion, during vaginal delivery [11, 12]. It is also increasingly recognized in elderly patients of either sex. The integrity of the external sphincter is thought to be the most important component of continence. Assessment, therefore, should include documentation of any disruption or change in the morphology of either sphincter, with specific attention to the detection of atrophy of the external anal sphincter.

Sonography of the Anal Sphincters
Transanal sonography is safe, is easy to perform, requires no preparation or sedation for the patient, and has been the gold standard in the morphologic diagnosis of the anal canal because of its higher sensitivity (100%) than other diagnostic tools for the evaluation of sphincter defects [13, 14].

During transanal sonography, the internal anal sphincter appears as a well-defined ring of low reflectivity with homogeneous hypoechogenicity that lies immediately subjacent to the hyperechoic submucosal layer, whereas the external anal sphincter usually appears hyperechoic with a heterogeneous appearance related to the different orientation of some fibers and lies immediately outside the internal sphincter (Fig. 3A, 3B, 3C, 3D, 3E, 3F). The external anal sphincter is not nearly as conspicuous on a sonographic scan as the internal anal sphincter.

A sonography examination may confirm the presence or absence of sphincter defects and residual defects after surgery (Fig. 4A, 4B). Defects should be documented using a clock-face orientation with anterior at 12 o'clock; the length of the remaining intact sphincter muscle should also be documented. Transanal sonography can further evaluate indirectly the neuropathic anal sphincter by allowing the thickness of the internal anal sphincter to be measured [15].

An external sphincter defect appears as a break, usually hypoechoic, in the normal texture of the echogenic muscle ring, and an internal sphincter defect as an interruption in the hypoechoic ring (Fig. 5A, 5B, 5C, 5D). Anovaginal fistulas are infrequent but observed associations of sphincter defects (Figs. 5C and 5D). They often show as either a hypoechoic or an aircontaining tract running from the anal canal posteriorly to the vagina anteriorly.

View larger version (154K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5A —Anal sphincter defects shown in four patients. Axial transperineal image in 27-year-old woman with prior obstetric trauma shows defect in hypoechoic ring of internal anal sphincter from 10- to 1-o'clock positions. Smaller hypoechoic defect is seen in echogenic tissue of external anal sphincter from 11- to 12-o'clock positions. This very clear defect in both sphincters suggests that surgical repair would be potentially possible.

View larger version (147K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5B —Anal sphincter defects shown in four patients. Axial transvaginal image obtained with probe in 22-year-old woman with fecal incontinence after severe previous injury from boat propeller shows unusual disruption of anal sphincters. There are two defects: one anteriorly in external anal sphincter, showing hypoechoic defect from 12- to 2-o'clock positions, and second full-thickness defect of internal anal sphincter and external anal sphincter from about 6- to 9-o'clock positions. These multifocal defects suggest difficulty for surgical repair.

View larger version (159K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5C —Anal sphincter defects shown in four patients. Axial transperineal image in 30-year-old woman with previous obstetric trauma and large episiotomy shows extensive anterior disruption with full-thickness sphincter disruption from 8- to 4-o'clock positions. Multiple echogenic bubbles of air are noted in secondary anovaginal fistula.

View larger version (168K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5D —Anal sphincter defects shown in four patients. Axial linear transperineal image obtained in 34-year-old woman with first-degree tear after obstetric delivery shows anovaginal air-containing fistula as echogenic air-containing tract running directly anterior from anal canal to vagina. There is large anterior full-thickness defect in anal sphincter from 10- to 2-o'clock positions.

MRI of the Anal Sphincters
MRI may show with high resolution the normal anatomy and the defects of the sphincter complex [17]. MRI is comparable to transanal sonography in the characterization of damage to the internal sphincter, but it seems to be superior to transanal sonography for the evaluation of external sphincter atrophy [7, 18–21]. MRI can differentiate between muscles, scars, and fat tissue, so it can be used to detect more easily the thickness and the fat content of the external sphincter [21], making it more sensitive than sonography for this indication.

Perianal Inflammatory Disease

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Primary Perianal Inflammatory Disease
Perianal abscess and fistula in ano are thought to arise from infection in small intersphincteric anal glands, with abscess representing the acute manifestation and fistula in ano, the chronic form. The cryptoglandular theory of pathogenesis of spontaneous perianal inflammation suggests that small intersphincteric anal glands, usually at the level of the dentate line, are obstructed as a result of fecal material, foreign bodies, or trauma, with subsequent stasis and infection. From their origin, fistulas then track into the sphincter mechanism, with a variation of outcome, but commonly form abscesses in the ischiorectal fossa or descend to an external opening on the skin of the perineum or buttock. Two distinct populations develop perianal inflammatory disease: patients affected by Crohn's disease and patients, usually young males, who develop a perianal abscess or fistula as a spontaneous event [22].

Fistulous tracts are classified according to Parks et al. [23] into four types: intersphincteric (between the internal and external sphincter), transsphincteric (crossing both the internal and external anal sphincters into the ischiorectal or ischioanal fossa), suprasphincteric (passing upward in the intersphincteric plane to a point above the puborectalis muscle where it tracks laterally and caudally into the ischioanal fossa), and extrasphincteric (passing directly from the perineal skin to the rectum outside both sphincters) (Fig. 6). Their reported incidence in the population is as follows: intersphincteric, 70%; transsphincteric, 23%; suprasphincteric, 5%; and extrasphincteric, 2% [23].

View larger version (63K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6 —Diagram shows different types of fistulous tracts as classified according to Parks et al. [23]: On left of image, cephalad line represents extrasphincteric tract running in suprasphincteric plane before descending to perineum. Middle line shows transsphincteric tract, and caudal line shows short extrasphincteric tract running directly from anal canal to external opening on perianal skin. On right of image are two intersphincteric tracts: Cephalad tract ascends in intersphincteric plane before running suprasphincteric to descend in extrasphincteric plane. More caudal tract on right descends in intersphincteric plane to skin. (Adapted with permission from Parks AG, Gordon PH, Hardcastle JD. A classification of fistula-in-ano. Br J Surg 1976; 63:1–12 [23]. Permission granted by John Wiley & Sons Ltd. on behalf of BJSS Ltd.)

The treatment of fistulas is surgical, and knowledge of the exact relationship of every fistula to the perianal anatomic structures and spaces is essential to reduce recurrence and postoperative fecal incontinence. The Parks et al. [23] classification is used as a guide to operative treatment.

Perianal fistulas may also be caused by other conditions, including tuberculosis, trauma during childbirth, pelvic infection, pelvic malignancy, rectal duplication cysts, surgical trauma [24], and radiation therapy [25]. Idiopathic fistulas are believed to represent the chronic phase of anal gland sepsis. Although less common, another cause of perianal sepsis is pilonidal sinus disease. Pilonidal sinus and fistula in ano are two kinds of fistula, the differential diagnosis of which is fundamental because both tend to recur. The most important difference is that pilonidal sinus disease originates subcutaneously from a chronic infection of hair follicles with subsequent formation of an abscess and fistulous track, whereas anal fistula has an enteric communication within the anal canal or rectum [26].

Sonography of Perianal Inflammatory Disease
The motivation for imaging patients with perianal inflammatory disease is documentation of internal and external openings; the presence, course, and nature of fistulous tracts, either intersphincteric or transsphincteric; and the presence of any abscess [27]. Transanal and endorectal sonography and other sonographic techniques may evaluate the involvement of the sphincter complex, which may become more heterogeneous and less well defined. With the use of high-frequency linear or curved array probes on the perineum in both transverse and longitudinal planes, fistulous and sinus tracts (Fig. 7A, 7B), collections in the perineum, buttocks, scrotum, and labia can be assessed and followed in a retrograde direction to their connection with the anal canal. A high sensitivity of 96% has been shown for the detection of tracts, with a negative predictive value of close to 100%, on perianal sonography [28].

View larger version (163K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7A —Spontaneous perianal inflammatory disease in two patients. Axial transperineal image in 32-year-old man shows internal opening at 6-o'clock position posteriorly (arrowhead). Transsphincteric hypoechoic tract runs through sphincter and then shows extension to both right and left (arrows). Tract could be followed to external opening on right buttock (not shown).

View larger version (151K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7B —Spontaneous perianal inflammatory disease in two patients. Transperineal sagittal image obtained with linear probe placed on skin immediately lateral to anal canal in 50-year-old man who presented with perianal pain and no external opening shows anal canal (AC) in median view as hypoechoic band. Air-containing abscess (A) communicates with anal canal at its mid point posteriorly. There was no tract to skin.

Sonography can identify the internal opening in the upper anal canal with a high accuracy in more than 90% of patients [30]. Further, 3D imaging may better map the course of a fistula than 2D imaging (Fig. 8). However, for some authors, hydrogen peroxide [31, 32] or SH U 508A (Levovist, Schering) [33] injection of the external opening and a 3D reconstruction during transrectal sonography may better demarcate small fistulous tracts, internal openings, and secondary extensions and may facilitate differentiation between an active fistulous tract and fibrotic tissue, thereby providing a dynamic depiction of perianal fistulas [34], although with some limits and possible pitfalls [31, 35].

View larger version (129K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8 —Sonogram shows contribution of 3D volume acquisition to assessment of transsphincteric fistula in 44-year-old woman with no history of inflammatory bowel disease. Three-dimensional volume was achieved in transverse plane (not shown). Coronal reconstruction image shows entire course of fistulous tract (arrows), which ascends inside of sphincter before crossing through sphincter to descend outside of sphincter to perineum.

View larger version (156K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 9 —Thickening of rectal wall shown in 29-year-old woman with symptomatic Crohn's disease. Axial transvaginal image shows thick rectal wall in cross section. Outer mixed echogenic layer of soft tissue surrounds entire rectum.

Perianal manifestations of Crohn's disease reflect the transmural nature of the disease. In these patients, an anorectal fistula can manifest externally (enterocutaneous) or internally as enteroenteric, enterovesical, and enterovaginal fistulas. Perianal Crohn's disease may manifest as thickening of the rectal wall (Fig. 9), inflammation of perirectal fat, enlargement of perirectal lymph nodes, fissures, fistulas or sinus tracts (or both) and rectovaginal tracts, ischiorectal abscesses, skin tags due to lymphatic obstruction, and anal and rectal ulcers and strictures [36]. Treatment for perianal Crohn's disease is most often conservative and includes medical therapy with infliximab [3]. Surgical treatment is infrequent and reserved for patients with abscess formation.

Sonography of Perianal Crohn's Disease
Abdominal sonography and endorectal sonography play a well-defined role in evaluation of Crohn's disease patients [37]. The aim of a sonographic examination for anorectal complication includes documentation of the classic features shown elsewhere (rectal wall thickening > 4 mm, creeping fat, hyperemia, perienteric lymphadenopathy) and of the complications frequently associated with the disease (fistulas, phlegmon, or abscess) (Fig. 10A, 10B). Further, sonography has been used to study response to treatment with infliximab [3], to identify patients in whom the possibility of surgery or percutaneous intervention would justify other means of investigation, and to detect postoperative recurrence of Crohn's disease.

View larger version (172K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 10A —Complex and extensive perianal abscesses in 35-year-old woman with known Crohn's disease. Transvaginal axial image of anal canal shows large internal opening (arrow) at 6-o'clock position posteriorly. Transsphincteric tract runs to bilobed horseshoe abscess with components to right and left of anal canal.

View larger version (159K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 10B —Complex and extensive perianal abscesses in 35-year-old woman with known Crohn's disease. Obtained at slightly different location, image shows additional deep, lobulated fluid-containing abscess on left side.

View larger version (149K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 11A —Adenocarcinoma complicating chronic fistula in 61-year-old man with Crohn's disease. Transperineal low axial image shows anal canal in cross section. Posterior to canal, well-defined and solidappearing mass (arrows) is seen.

View larger version (109K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 11B —Adenocarcinoma complicating chronic fistula in 61-year-old man with Crohn's disease. Transanal transverse color Doppler sonogram confirms solid and vascular mass totally engulfs the seton, which shows here as echogenic focus with shadowing (arrow). Tumor arises from mucosa, and complete destruction of wall layers is shown.

View larger version (150K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 12A —Biopsy-proven chronic inflammatory mass in symptomatic 47-year-old man with rectal pain, unchanged over 3-year interval. No evidence of cancer or Crohn's disease was seen either clinically or on biopsy. Endorectal axial sonogram shows heterogeneous hypoechoic mass mimicking rectal cancer. Wall layers are destroyed.

View larger version (109K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 12B —Biopsy-proven chronic inflammatory mass in symptomatic 47-year-old man with rectal pain, unchanged over 3-year interval. No evidence of cancer or Crohn's disease was seen either clinically or on biopsy. Addition of color Doppler sonogram shows mild hypervascularity of mass.

The activity of inflammatory change correlates with marked hypoechogenicity and hyperemia, as seen on color and power Doppler evaluations. Contrast-enhanced sonography has also been shown to be useful in Crohn's disease in the evaluation of disease activity and in the follow-up of patients to evaluate response to therapy and to differentiate inflammatory strictures from those of fibrotic origin [38].

Ileal Pouch-Anal Anastomosis
Ileal pouch-anal anastomosis is the procedure of choice for patients who require surgery for ulcerative colitis to avoid permanent ileostomy and preserve the anal sphincter mechanism. With this procedure, an ileal reservoir is constructed after total colectomy and anastomosed to the anus. The functional outcome after surgery depends to a large extent on postoperative function of both sphincters, which can be well studied using transanal sonography. It is, in fact, possible to evaluate both anal sphincters and to document their defects or changes in volume, length, and average thickness [39].

Unfortunately, ileoanal pouch complications include surgery-related complications and mechanical complications, inflammatory or infectious disorders, functional disorders, dysplasia or neoplasia, and systemic or metabolic disorders [40]. Pouchitis, the most common longterm complication of surgery, occurring in up to 46% of patients, is an idiopathic inflammatory disease of the ileal reservoir that is a form of recurrent inflammatory bowel disease. On sonography, pouchitis may show subtle wall thickening in the pouch and an unexplained large volume of fluid feces within its lumen (Fig. 13A, 13B, 13C). It can occur in an acute form, which responds rapidly to medical therapy, and in a chronic form, which does not respond completely to medical therapy and requires longterm suppressive therapy [41].

View larger version (137K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 13A —Pouchitis and anastomotic inflammatory mass in 48-year-old woman with total colectomy for ulcerative colitis and ileoanal anastomosis. Transvaginal image of pouch (P) shows that it is thick walled, distended with liquid stool, and surrounded by excessive echogenic inflammatory fat (F).

View larger version (155K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 13B —Pouchitis and anastomotic inflammatory mass in 48-year-old woman with total colectomy for ulcerative colitis and ileoanal anastomosis. Axial image taken with transvaginal probe shows region of ileoanal anastomosis. There is hypoechoic masslike area within thickened anterior wall.

View larger version (110K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 13C —Pouchitis and anastomotic inflammatory mass in 48-year-old woman with total colectomy for ulcerative colitis and ileoanal anastomosis. Addition of color Doppler sonogram shows profuse hypervascularity in this inflammatory mass. This patient responded to conservative management. Pouchogram (not shown) obtained 1 month later did not show leak and follow-up sonogram (not shown) showed normal appearance.

Undiagnosed Crohn's disease is one of the main causes that lead to pouch excision, so it remains a relative contraindication to the ileal pouch-anal anastomosis; it can manifest as an anal fistula, an abscess, or chronic refractory pouchitis [42]. Crohn's disease of the ileoanal pouch shows the expected manifestations of Crohn's disease, including wall thickening, hyperemia, surrounding inflammatory fat, and perienteric lymphadenopathy. Any patient with ulcerative colitis treated with total colectomy and an ileoanal pouch who develops recurrent or new symptoms must be considered to have possible misdiagnosis of their colitis with possible Crohn's disease as an explanation.

Formation of fistulas from the ileal reservoir to various sites (vaginal, cutaneous, perineal, presacral) is an uncommon but distressing problem that occurs in 5–10% of patients with ileoanal pouch and may potentially lead to pouch failure with a significant morbidity [43]. Pouch–vaginal fistula is uncommon but does represent the most common type of fistula to occur after this procedure [44, 45], occurring in 3.3–16% of women who undergo ileal pouchanal anastomosis [46, 47]. Fistula diagnosis can often be confirmed by clinical examination or examination with the patient under anesthesia if sepsis and discomfort are present, but sometimes other tests are required, such as pouchography, CT, MRI, or endorectal sonography [31]. Cancer arising from the ileal pouch or from the anal transitional zone is rare and usually develops from 3 to 51 years after surgery [48, 49].

MRI
The role of MRI in the diagnosis of perianal inflammatory disease is well defined [50]. It is frequently performed in our institution in any patient for whom we have not achieved a successful endorectal sonography study of the anal canal. We routinely recommend MRI because we recognize that the activity of a fistula is sensitively shown by this technique and that MRI is also superior to transanal sonography for the evaluation of inflammatory extension to the supralevator region and for differentiation of carcinoma from a chronic inflammatory mass within a perianal fistula. Taylor et al. [27], moreover, have shown the fundamental role of MRI in the differential diagnosis between fistula in ano and pilonidal sinus.

Summary

Top Abstract Introduction Anatomy of the Anal... Sonography Technique for the... Anal Incontinence Perianal Inflammatory Disease Summary References

 
Transanal sonography augmented by transvaginal scanning in women and transperineal scanning in both sexes allows accurate evaluation of the anal canal and perianal inflammatory complications. The procedures are well tolerated by patients and are easy to perform after a necessary learning curve. These studies may be used to identify fistulous tracts and inflammatory masses and to show their relationship to the sphincter mechanism while also showing the integrity of the sphincter components. MRI is recommended to document further complex perianal inflammatory disease before surgery, to confirm the extent of abnormalities in patients with suspected supralevator extension, to differentiate cancer from large chronic inflammatory masses, and to assess atrophy or scarring of the external sphincter that may contribute to fecal incontinence.

Acknowledgments
 
We thank Gordana Popovic for her artistic talents in the production of the schematic drawings.

References

Top Abstract Introduction Anatomy of the Anal... Sonography Technique for the... Anal Incontinence Perianal Inflammatory Disease Summary References

 

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