Keywords

1 Introduction

A combination of several factors produces fecal continence. These factors include fecal composition, functional aspects, such as intestinal motility and filling awareness, and morphological aspects related to the anatomy of the rectum, anal canal, and sphincter apparatus [1]. Many causes can interfere with the balance between two or more of these factors, resulting in fecal incontinence. Therefore, each of these elements must be thoroughly evaluated to understand the genesis of fecal incontinence and choose the most appropriate treatment, whether medical, surgical, or a combination of these [2]. The most common causes include traumatic (obstetric, surgical) defects of the sphincter complex, pudendal nerve dysfunction, and rectal prolapse. In women, fecal incontinence is most frequently caused by childbirth, where the muscular sphincter fibers are commonly damaged [3]. Digital rectal examination is the first step in the clinical evaluation of patients with fecal incontinence, giving an initial qualitative assessment of the external sphincter tone at rest and during contraction. Functional competence of the sphincter apparatus can subsequently be tested by anorectal manometry, electromyography, and pudendal nerve terminal motor latency test [4]. Finally, diagnostic imaging is crucial for the morphological evaluation of the anatomical structures involved in fecal continence and for the assessment of functional abnormalities of the defecatory act. The role of endoanal ultrasound (EAUS) is consolidated in clinical practice as the surgeon usually performs it during the physical examination [5]. In addition, X-ray and magnetic resonance defecography combine high spatial resolution, which allows detailed assessment of all structures involved in the genesis of incontinence, with functional assessment, to identify the most appropriate therapeutic approach [6].

2 Imaging Techniques

2.1 X-Ray Defecography

X-ray defecography is a cost-effective imaging technique for the evaluation of the defecatory act in the physiological sitting position [7].

2.1.1 Execution Protocol

The rectal ampulla is filled with a high-density barium sulfate paste, usually 150–200 mL. In some cases, this is followed by distension of the vaginal canal, using 45 mL of the same paste, and the bladder, through the injection of 400 cm3 of iodinated contrast medium. The intestinal loops can be identified by administering 200 mL of 60% barium sulphate orally, 1 h before the examination.

The examination is conducted with the patient sitting on an appropriate radiolucent support to evaluate the defecatory act in the physiological sitting position. First, anteroposterior and laterolateral scans are taken at rest, followed by the dynamic phases of the study, in particular contraction, straining, and evacuation, which are acquired in the laterolateral projection. Eventually, a final acquisition is made after evacuation.

2.1.2 Image Analysis

The analysis is focused on assessing the position of three specific landmarks (the anorectal junction, the posterior vaginal fornix, and the bladder base) from the reference line drawn between the two ischial tuberosities.

2.1.3 Imaging Findings

Due to poor spatial resolution, X-ray defecography fails to identify slight abnormalities of the anal canal and sphincter apparatus; thus, it may only show indirect signs of fecal incontinence. The most important findings are listed below [8]:

  1. 1.

    Barium leakage, due to the patient’s inability to retain the contrast medium.

  2. 2.

    Distension of the anal canal by the barium paste: during the resting phase, the walls of the anal canal are not juxtaposed and have an average transverse diameter ≥10 mm.

  3. 3.

    Rectal hyperdistention, with a maximum transverse diameter >7.5 cm.

  4. 4.

    Ineffective squeezing due to an impaired anal sphincter contraction.

  5. 5.

    Anorectal angle >116° under resting conditions, implying puborectalis sling hypotonia.

  6. 6.

    Rectal prolapse, fall of the anorectal junction >4 cm below the bisischiatic line.

  7. 7.

    Rectoanal intussusception, manifesting as an annular filling defect extending into the anal canal.

  8. 8.

    Anterior rectocele, protrusion of the anterior wall of the rectum >3 cm beyond a reference line drawn upwards from the anterior margin of the anal canal (Fig. 6.1).

Fig. 6.1
The radiographic image of the rectum and anal canal with a contrast. In part a, the lateral view of the patient is taken with contrast. In part b, the front view is taken to check for incomplete expulsion.

X-ray defecography in the evacuation (a) and post-evacuation phase (b). Images show the presence of an anterior rectocele with incomplete expulsion of contrast medium

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2.2 Magnetic Resonance Defecography

Due to its spatial resolution, magnetic resonance (MR) imaging provides a detailed anatomical representation of the anal canal, sphincter apparatus, and all muscles and ligaments involved in fecal continence, as well as a broad overview of the pelvic organs [8,9,10]. This imaging technique is also able to give crucial functional information through dynamic evaluation of the defecatory act [11, 12]. Using rapid sequences, MR defecography can assess the defecatory phases of rest, squeezing, straining and evacuation in a more accurate way than X-ray, to identify both anatomical and functional causes of fecal incontinence [13]. In addition to increased accuracy and reproducibility of measurements, MR defecography is also superior to the X-ray technique as it does not use ionizing radiation. Performance of the examination in the supine position is one disadvantage of this technique because the defecatory act is less physiological [14].

2.2.1 Acquisition Protocol

Patient compliance is crucial for a successful examination. The rectal ampulla should be distended with 150 mL of ultrasound gel administered via a syringe through the anus. An additional 30–50 mL of ultrasound gel should also be administered through the vaginal canal to properly visualize the posterior vaginal fornix, an essential landmark in the diagnosis of rectal prolapse [15].

The MR imaging protocol starts with a T1-weighted (T1W) localizer sequence with a wide field of view to identify the sagittal midline section, including the pubic symphysis, bladder neck, vagina, rectum, and coccyx. Subsequently, T2-weighted (T2W) turbo spin-echo sequences are acquired in the sagittal and axial planes.

By accurately depicting muscular structures and supporting ligaments, the T2W static sequences show muscular and fascial asymmetries, defects, pathological thickening or thinning, and irregularity of the contours, as well as the physiological thickness and integrity of the sphincter apparatus and the puborectalis sling [16].

The T2W static sequences are followed by T2W dynamic acquisitions (TRUE FISP, FIESTA, balanced FFE) in the midsagittal plane during rest, squeezing, straining and evacuation to define the position of the pelvic organs and detect perineal descent and pelvic prolapse.

2.3 Magnetic Resonance Anatomy of the Anal Canal

The anal canal is the terminal part of the large intestine located between the anal verge below and the rectum above. Its upper limit is the anorectal junction (ARJ), where the puborectalis muscle forms a U-shaped sling posteriorly [17]. On MR imaging, the mean length of the anal canal from the ARJ to the caudal tip of the subcutaneous external anal sphincter (EAS) is approximately 4.4 cm. The dentate (pectinate) line marks the transition between columnar epithelium (intestinal mucosa) and squamous epithelium, but it is not distinguishable at MR [16].

The anal canal shows a cylindrical morphology, with a three-layered wall of mucosa, submucosa, and muscle. The muscular layer is composed of an inner layer, the internal anal sphincter (IAS), and an outer layer, the external anal sphincter (EAS) [18]. On T2W images, the IAS appears as a moderately hyperintense circular smooth layer in the axial plane and as a longitudinal band in the coronal plane, with a mean thickness of 3.5 mm. Similarly, the EAS complex appears as a hypointense circular skeletal layer, with a mean thickness of 4 mm (Fig. 6.2) [18, 19].

Fig. 6.2
The M R I findings of the rectum, and anal canal. In part a, the coronal view of the abdomen with contrast in the rectum, an arrow shows the E A S, and the asterisk represents the I A S. In part b, the sagittal view of the abdomen with contrast visible in the rectum, pubic symphysis, and coccyx is visualized.

Normal anatomy of the anal canal in the coronal (a) and sagittal (b) plane. The external layer corresponds to the external anal sphincter (arrowhead), the inner one corresponds to the internal anal sphincter (asterisk). The intersphincteric space is shown as a thin hyperintense line in the middle

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The EAS complex is made up of three components (Fig. 6.3):

  • a deep part surrounding the upper portion of the anal canal and merging with the puborectalis sling

  • a superficial part surrounding the whole anal canal and continuing posteriorly with the anococcygeal ligament

  • a subcutaneous portion, the outermost layer, surrounding the lower side of the IAS just beneath the perianal skin.

A T2W hyperintense intermsphincteric space lies between the IAS and EAS. The best anatomical assessment of these counterparts is made on images taken in the midsection of the anal canal, where the two halves of the external sphincter join.

Fig. 6.3
The M R I findings of the anal canal in 3 different views, part a, the deep part of the canal, and its sphincters. Part b, the superficial part of the canal, and part c, the subcutaneous plane and the soft tissue around the canal are seen.

Anal canal anatomy along its long axis, deep (a), superficial (b), and subcutaneous plane (c)

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The midsagittal plane helps depict the supporting structures, particularly the anococcygeal ligament, posteriorly connecting the external sphincter to the coccygeal bone, and the puborectalis and bulbocavernosus muscles, both anterior, which support the EAS complex [20].

2.4 Morphologic Diagnostic Criteria

In MR imaging, the morphological findings of fecal incontinence concern primarily the sphincter complex, particularly the presence of defects or atrophy within the sphincteric ring, both internal and external.

A sphincteric defect is defined as a minimum of 30° discontinuity of the muscle ring (isolated or combined IAS and EAS; anatomical defect) (Fig. 6.4). At MR imaging, it appears as a low signal deformation of the muscle ring due to the replacement of muscular fibers by fibrous tissue [21] (Fig. 6.5).

Fig. 6.4
The M R I image of the anal canal along the 3 axis, the asterisk shows the posterior lesion of the external sphincter, and soft tissue is visualized.

Axial T2w fat saturation sequence shows a posterior lesion of the external anal sphincter (asterisk)

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Fig. 6.5
The M R I of the dense area in the inner anal sphincter, as indicated by the asterisk and other soft tissues are visualized. The veins and 2 bulb like structures on either side of the anal canal can be visualized.

Axial T2w sequence: a hypointense area, relatable to fibrous scar tissue, may be appreciated in the context of the inner anal sphincter at 11 o’clock (asterisk)

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Fecal incontinence may be caused by sphincter disruptions sustained during childbirth [22]. There is a correlation between anal sphincter atrophy, which might cause late-onset incontinence without evidence of any disorders within the sphincteric complex, and pudendal neuropathy, a condition favored by stretching during vaginal delivery [23] (Fig. 6.6). In particular, the pudendal nerve is the main nerve of the anorectal region, supplying both sensory and motor innervation [24].

Fig. 6.6
The M R I findings of the abdomen with an axial view, the pudendal nerve is indicated by an asterisk and a puborectalis sling is indicated by the arrows. In part c, a nonequal puborectalis sling is seen below the anal canal, soft tissue can be visualized.

Signal hyperintensity and increased thickness of the left pudendal nerve (a, asterisk), causing asymmetry of the puborectalis sling (b and c, arrows)

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Vaginal delivery, neurologic disorders, and diabetes may induce degeneration of the external anal sphincter with loss of muscle fibers and fatty infiltration and/or loss of anal sensation leading to fecal incontinence [25].

Anal sphincter atrophy is a condition sustained by a severe thinning of the sphincteric ring, responsible for passive fecal incontinence. When concerning the IAS, atrophy implies muscle degeneration characterized by a thickness <2 mm. EAS atrophy might indicate severe sphincter thinning or a regular consistency, with fatty replacement of skeletal muscle fibers.

EAS atrophy can be stratified using a grading system proposed by Terra et al. [25]. This score considers the percentage of fat content of the EAS and measurements of the area of the remaining EAS:

  • no atrophy: no thinning or replacement of the sphincter muscle by fat

  • mild atrophy: <50% thinning or replacement of sphincter muscle by fat

  • severe atrophy: ≥50% thinning or replacement of sphincter muscle by fat

The clinical relevance of correctly diagnosing anal sphincter atrophy in the setting of fecal incontinence is high, as atrophy is a negative predictive factor for the outcome of secondary sphincter repair.

2.5 Functional Diagnostic Criteria

On dynamic MR acquisitions, the pelvic floor shows a physiological craniocaudal excursion of 2 cm. When assessing pelvic floor function on MR imaging, the most relevant anatomical landmark is the pubococcygeal line (PCL), drawn between the inferior margin of the pubic symphysis and the upper limit of the coccyx.

In patients without pelvic prolapse, during the straining and evacuation phases, the bladder base, the upper third of the vagina (posterior vaginal fornix), and the ARJ are located above the PCL [14].

During contraction, the pelvic organs show an upward displacement of 1–2 cm from the PCL. During straining and evacuation, they are displaced downwards within a range of 2–3 cm from the PCL, compared to the resting position [15].

The most frequent functional MR findings in fecal incontinence are described in the following sections.

2.5.1 Rectal Prolapse

According to the distance between the PCL and the lowest point of the prolapsed organ during evacuation, rectal prolapse is graded as:

  • mild, if the distance is less than 3 cm (Baden-Walker grade 1–2),

  • moderate, if the distance is between 3 and 6 cm (Baden-Walker grade 3)

  • severe, if the distance is more than 6 cm (Baden-Walker grade 3–4).

2.5.2 Rectocele

Rectocele is an anatomical defect of the anorectal region manifesting as bulging of the anterior rectal wall within the posterior vaginal wall.

There are three different degrees of rectocele:

  • grade 1: point of maximum convexity up to 2 cm from to the axis of the ARJ (non-pathological)

  • grade 2: point of maximum convexity between 2 and 4 cm from to the axis of the ARJ

  • grade 3: point of maximum convexity >4 cm from the axis of the ARJ.

2.5.3 Rectoanal Intussusception

Rectal intussusception is defined as the invagination of the rectal wall into the rectal lumen; at first, it may be only located on one side and is referred to as rectal wall inversion [26]. Rectoanal intussusception is classified as:

  • intra-rectal when the invagination is confined to the rectum.

  • intra-anal or procidentia if it extends into the anal canal.

  • extra-anal when it extends beyond the anal sphincter.

3 Conclusions

MR defecography and 3D endoanal ultrasound (EAUS) are useful techniques for portraying morphologic changes in the anal sphincters that may result in fecal incontinence.

EAUS is usually the first imaging study to be performed, as it provides information about the morphology of a potentially damaged sphincter [27].

In selecting patients for sphincteroplasty, MR imaging is subsequently performed to detect areas of atrophy and fatty replacement of the anal sphincter complex that EAUS fails to identify [28]. MR defecography can also provide additional functional information and depict abnormalities associated with fecal incontinence, such as pelvic organ prolapse, rectoanal intussusception, rectocele, pelvic floor weakness, and pudendal nerve neuropathy [29]. Preoperative measurement of sphincteric thickness during both EAUS and MR imaging may represent a potential predictor of surgical outcome. EAUS can also depict any residual sphincter defect when performed in the postsurgical setting. In short, MR imaging qualifies as the recommended imaging technique in the preoperative evaluation, while EAUS is the preferred study in post-surgical evaluation.

Finally, although the role of X-ray defecography is limited nowadays, mostly because of radiation protection issues, the technique is still useful in selected cases, to depict fecal overflow incontinence and/or overactive, noncompliant rectal ampulla.