Role of magnetic resonance defecography in the assessment of obstructed defecation syndrome

Abstract

Obstructed defecation syndrome (ODS) represents an important cause of constipation, primarily arising from dysfunctions within the pelvic floor. Characterized by an inability to complete defecation or effectively evacuate fecal material despite the urge to defecate, ODS results in a persistent sensation of incomplete evacuation and often requires excessive straining during defecation. Conventional clinical examinations fail to adequately assess the complex dynamic dysfunctions of the pelvic floor and anorectal region. Magnetic resonance defecography (MRD), a sophisticated form of dynamic pelvic floor imaging, provides a comprehensive, non-invasive means of visualizing and quantifying various anorectal and pelvic floor abnormalities. By allowing detailed assessment of structural and functional deficits during the defecation process, MRD plays a crucial role in the diagnostic workup of ODS, enabling colorectal surgeons to formulate more precise and individualized treatment strategies. This manuscript highlights the important anatomical and functional disorders of pelvic floor that are associated with ODS.

Key Words: Magnetic resonance defecography; Obstructed defecation syndrome; anorectal junction descent; rectocele; intussusception; pelvic dyssynergia

Core Tip: Obstructed defecation syndrome (ODS) represents an important cause of chronic constipation, resulting from various functional and anatomical abnormalities of the pelvic floor. Magnetic resonance defecography (MRD) owing to its exceptional spatial resolution and dynamic imaging capabilities, facilitates an in-depth assessment of the pelvic floor's supporting structures. It provides direct visualization of the pelvic floor musculature and an indirect evaluation of the endopelvic fascia. MRD plays a pivotal role in the management of ODS by elucidating critical anatomical and functional impairments, thereby guiding therapeutic interventions.

INTRODUCTION

Defecation is an important physiological process that entails the precise coordination of various elements, including bowel motility, rectal sensation, the contraction of the rectum, and the relaxation of the anal sphincters, as well as the coordinated function of the pelvic floor muscles[1]. Functional constipation refers to a common condition where no physical obstruction exists in the defecation pathway. This disorder is frequently seen by physicians, gastroenterologists, and colorectal surgeons. It is estimated that 16% of the adult population worldwide is affected by functional constipation, with the prevalence rising to 33.5% among individuals aged 60 to 100 years[2,3]. Given its widespread occurrence, functional constipation places a significant burden on healthcare resources. Moreover, it is associated with considerable psychological distress and a marked reduction in quality of life for those affected[4]. Obstructed defecation syndrome (ODS) is a specific type of functional constipation, distinguished by difficulty or an inability to evacuate stool from the rectum[5,6]. Patients with this condition often report symptoms such as the sensation of incomplete evacuation, rectal blockage, the passage of hard stools, the need for manual rectal or vaginal intervention, excessive straining, and infrequent bowel movements[7,8]. It is estimated that ODS affects approximately 7% of the adult population[2].

Diagnosing the pelvic floor dysfunction associated with ODS through physical examination alone is challenging[5]. Studies show that physical assessments often fail to accurately identify pelvic floor abnormalities in 45%–90% of patients, leading to misdiagnosis. This diagnostic gap frequently results in inappropriate treatments and recurrent symptoms in 10%–30% of patients following surgery[7,9].

Magnetic resonance defecography (MRD) is a dynamic real time imaging of pelvic floor using fast imaging sequences to study the functional and structural disorders of various pelvic organs. It is particularly valuable due to its multiplanar acquisition, lack of ionising radiation, high soft tissue contrast resolution, and its capacity to concurrently assess abnormalities across different pelvic compartments[10,11]. Additionally it also assesses the anatomy of pelvic organs and important pelvic muscles in the same procedure. Studies have demonstrated that MRD can reveal additional pathologies that may be missed during a physical examination, with its findings influencing surgical intervention in up to 67% of cases[2,3]. The anterior and middle compartment abnormalities can often be detected through physical examination, however, posterior compartment abnormalities particularly peritoneoceles, enteroceles and recto-rectal intussusception are not easily detected by physical examination alone, but can be accurately evaluated with MRD[12]. In patients with ODS, MRD is invaluable for diagnosis and treatment planning. The American College of Radiology Appropriateness Criteria has rated MR defecography with rectal contrast 9 for defecation disorder indicating it is "appropriate" (scores between 7 and 9 are considered appropriate in most cases)[2].

MRD has been investigated in the assessment of ODS, with evidence suggesting that a substantial proportion of patients with ODS display various structural abnormalities. In the study conducted by RB Thapar et al[7], only 28 (14.5%) of the 192 patients with ODS exhibited normal MRD, while the remaining 164 (85.5%) demonstrated one or more abnormalities, including dysfunction in one or more compartments.

BRIEF ANATOMY OF PELVIC FLOOR

The pelvic is anatomically divided into three compartments: The anterior, middle, and posterior compartments[1,2]. The anterior compartment includes the urinary bladder and urethra, the middle compartment contains the uterus and vagina, and the posterior compartment houses the rectum and anal canal (Figure 1). Pelvic floor is a complex structure, made up of fascia, ligaments, and muscles, organized into three distinct layers, each contributing to the support and proper function of the pelvic organs[13,14]. These layers, from top to bottom, are as follows:

Figure 1 T2-weighted image in sagittal plane shows the three compartments of pelvis. The anterior compartment (yellow) contains the urinary bladder (B) and urethra, middle compartment (green) contains the uterus, cervix and vagina (V) and the posterior compartment (red) contains the rectum (R) and anal canal.

Endopelvic fascia

This is the most superior layer, composed of connective tissue, and wraps around the pelvic organs and levatorani muscles, providing structural support. It is not visible on magnetic resonance imaging (MRI), but the alignment of the pelvic organs can offer indirect clues about its integrity[13,14].

Pelvic diaphragm

The second layer consists of the pelvic diaphragm, which is made up of the levatorani and coccygeus muscles. These muscles are visible on MRI. The levatorani is a group of three muscles: The puborectalis, pubococcygeus and iliococcygeus (Figure 2). The levator plate is composed of the posterior fibers of the pubococcygeus, which fuse in the midline to create a median raphe (Figure 2). The puborectalis muscle is visualised on T2-weighted axial images, while the iliococcygeus and pubococcygeus muscles are more clearly depicted in T2-weighted coronal images[13,14].

Figure 2 Normal supporting structures of pelvic floor. A: T2-weighted image in sagittal plane shows the posterior aspect of puborectalis muscle (blue arrow) just behind the anorectal junction (orange star). Levator plate (red arrow) extends from the coccyx (pink arrowhead) to the anorectal junction (orange star). Note the normal vertical orientation of the urethra (yellow star) behind pubic bone. Bladder (B) and rectum (R) are distended; B: Axial T2-weighted image shows the sling of puborectalis muscle (red arrows) attached to the pubis (white arrow) anteriorly and encircling around the anorectal junction (R) posteriorly; C: Axial T2-weighted image cephalad to (B) shows pubococcygeus muscle (red arrows) coursing from the pubis anteriorly to the levator plate (blue arrow) posteriorly; D: Axial T2-weighted image cephalad to (C) shows iliococcygeus muscle (red arrows) extending from the obturator fascia laterally to the coccyx (blue arrow) posteriorly.

Urogenital diaphragm (perineal membrane)

The third and lowest layer consists of the urogenital diaphragm, made of connective tissue and the deep transverse perineal muscle, which attaches to the perineal body posteriorly[15,16].

The anal sphincter comprises of internal anal sphincter which is formed by the longitudinal muscle fibres within the anal wall, and the external anal sphincter, which is the inferior or caudal continuation of the puborectalis muscle.

PATIENT PREPARATION AND IMAGE ACQUISITION

MRD examinations are typically conducted using widely available MRI scanners, employing closed magnets with field strengths of 1.5 Tesla or 3 Tesla. MRD acquisition can be performed with the patient in either a sitting position on an open configuration MRI or a supine position on a closed configuration MRI. Several studies have yielded conflicting results regarding the impact of patient positioning on MRD outcomes. Some studies suggest that MRD performed in the supine position tends to underestimate the degree of organ prolapse, particularly in cases of posterior compartment dysfunction, when compared to sitting MRD, which more accurately simulates the physiological process of defecation. However, numerous other studies have shown no significant difference between sitting and supine MRD in terms of diagnosing clinically relevant pelvic floor dysfunctions. Regardless of patient positioning, ensuring adequate defecatory efforts to facilitate thorough rectal emptying is crucial in minimizing false-negative results.

Patient education regarding the procedure is essential to ensure the acquisition of a good-quality scan. Prior to the examination, patients must be thoroughly instructed on the procedural steps to promote cooperation during the procedure and to enhance the likelihood of rectal evacuation during the drain-out phase. The patient is positioned in the left lateral position on the gantry table, after which approximately 120 cc of ultrasound gel is introduced into the rectum via a flexible tube. Alternatively, some researchers have employed a mixture of mashed potatoes and a gadolinium-based contrast agent for rectal distension. The administration of vaginal gel is optional and depends on the clinical decision, which is made on a case-by-case basis after a consultation with the clinician. To avoid spillage of the gel onto the MRI scanner table, patients are provided with protective materials like towels, diapers, or an enema ring. These items help maintain hygiene and prevent contamination of the MRI equipment. After preparation, the patient is repositioned in the supine position with the knees slightly bent.

A multichannel body coil is positioned around the patient's pelvis. Static images are obtained to evaluate the pelvic floor anatomy, focusing on the muscles, including the puborectalis and iliococcygeus. T2-weighted fast spin-echo (FSE) or fast recovery FSE sequences without fat saturation are obtained in all three orthogonal planes.

Dynamic imaging is then performed using fast T2-weighted sequences in the sagittal plane. Dynamic cine imaging utilizing balanced steady-state free precession sequences is conducted in the mid-sagittal plane, with cine loops captured to visualize the movement of the pelvic floor and the expulsion of the gel. These images are acquired during the rest, squeeze, strain, and defecation phases. During the squeeze phase, the patient is instructed to contract the anal sphincter inward to elevate the pelvic floor (Kegel maneuver), enabling an evaluation of puborectalis muscle function. In the defecation phase, the patient is instructed to expel the gel from the rectum, a process crucial for assessing the coordination of the pelvic floor's supporting structures.

To ensure optimal results, the defecation or evacuation sequence should be repeated a minimum of three times to allow for adequate gel expulsion. The defecation phase is the most critical component of MRD, as it provides the most accurate insights into the presence and severity of pelvic floor dysfunction.

INTERPRETATION OF MRD

MRD interpretation commences with a brief assessment of the anatomical supporting structures within the pelvis on static images. The evaluation includes assessment of the levator ani muscle and external anal sphincter, noting any changes in signal intensity, atrophic changes, surgical scars, or tears. Special attention is given to the puborectalis muscle, which is frequently observed to be thinned in individuals with stress or mixed urinary incontinence, and thickened in those with pelvic floor dyssynergia. While no standardized reference values exist for the normal thickness of the puborectalis muscle, a commonly cited anecdotal rule suggests that the muscle is deemed thickened when its thickness exceeds three times that of the pubococcygeus muscle, which lends support to the diagnosis of pelvic floor dyssynergia[1].

Subsequent analysis involves the evaluation of defecatory effort using dynamic sagittal cine sequences, which assess the level of defecatory effort (classified as good, moderate, or poor). The amount of instilled rectal gel expelled is quantified as none, one-third, two-thirds, or nearly all[1]. The cine sequence demonstrating maximal strain is selected for interpretation, in comparison to a baseline resting image. Several reference lines are then drawn to assess the functionality of the pelvic floor. The pubococcygeal line (PCL) is first drawn from the inferior aspect of the pubis anteriorly to the last coccygeal articulation posteriorly[1,2,6,17] (Figure 3). The H-line and M-line are subsequently drawn to assess pelvic floor dilation and descent, respectively. The H-line is marked on a mid-sagittal T2-weighted image, extending from the lower border of the pubic symphysis to the posterior aspect of anorectal junction (Figure 3). The M-line is drawn at a right angle to the H-line, starting from its posterior end and reaching the PCL (Figure 3). The H-line quantifies the anterior-posterior width of the levator hiatus, while the M-line represents the descent of the pelvic floor muscles. The normative values for the H-line and M-line are typically ≤ 6 cm and ≤ 2 cm, respectively[1,2].

Figure 3 Sagittal T2-weighted image illustrating the placement of various reference lines. The pubococcygeal line (PCL), denoted by the red line, spans from the inferior aspect of the pubis anteriorly to the last coccygeal articulation posteriorly. The H-line, represented by the green line, extends from the inferior aspect of the pubis anteriorly to the posterior wall of the anorectal junction. The M-line, depicted by the yellow line, is drawn perpendicularly to the PCL, originating from the posterior point of the H-line.

A vertical line extending from the PCL superiorly to the neck of urinary bladder inferiorly provides an estimate of anterior compartment descent. The urethral angle is estimated by drawing a line along the long axis of the urethra and the patient’s body axis, both at rest and under maximal strain. Urethral hypermobility is identified when the urethral angle increases by more than 30 degrees during strain as compared to rest, indicating weakness, laxity, or disruption of the urethral ligaments[2,9]. Additionally, a line is drawn from the PCL superiorly to the anterior cervical lip or superior vaginal cuff inferiorly to measure the degree of descent of the middle compartment[1]. A similar line from the PCL superiorly to the anorectal junction inferiorly is used to calculate the descent of the anorectal junction. Another line is drawn along the most anterior aspect of the rectal wall to its anticipated location to evaluate the presence and severity of an anterior rectocele. The anorectal angle (ARA) is measured to evaluate pelvic floor dyssynergia. The angle is calculated by drawing two lines: One parallel to the long axis of the anal canal and the other along the posterior rectal wall. The intersection of these lines gives the ARA (Figure 4). At rest, the ARA ranges between 108° and 127°[1,2]. During the squeeze maneuver, the ARA decreases by 15%–35% as a result of the contraction of the puborectalis muscle, which pulls the anorectal junction anteriorly and superiorly. During the defecation phase, the puborectalis muscle relaxes, moving the anorectal junction posteriorly and inferiorly, which increases the ARA by 15°–20° from the resting phase, signifying the opening of the anal canal[1,2]. Once all these measurements are obtained, the dynamic cine images are revisited to identify any abnormalities within the cul-de-sac. The rectum is also scrutinized for signs of intussusception, which is often most conspicuous toward the completion of the evacuation process[16].

Figure 4  Anorectal angle is the angle formed at anorectal junction by the intersection of two lines, one drawn along the central axis of anal canal and another drawn along the posterior border of lower rectum.

At rest, the anal canal remains closed, with the puborectalis muscle creating an impression on the posterior wall of anorectal junction. Effective defecation requires the coordinated interplay between intra-abdominal pressure, the sphincter complex, and the pelvic floor musculature. During defecation there is rise in intra-abdominal pressure, which is followed by a rise in rectal pressure. This culminates in the relaxation of the anal sphincters and pelvic floor which facilitates the opening of the anal canal. At MRD, the rectum should evacuate the majority of the contrast medium within 60 seconds. However, some individuals with normal defecatory function may experience difficulty expelling the rectal gel when positioned supine[1,2,17].

Pelvic floor dysfunction, refers to a range of abnormalities due to weakened or deficient muscles or tissues of pelvic floor. It may encompass both pelvic floor relaxation and pelvic organ prolapse. As a result, one or more than one compartments may descend during defecation. This condition leads to a descent of the pelvic floor itself, along with an expansion of the levator hiatus. These alterations are assessed by using H and M lines, with the grading system detailed in Table 1. Patients with pelvic organ prolapse, demonstrate abnormal descent of the pelvic organs—such as the bladder, cervix, or bowel—through the hiatus. To quantify the extent of pelvic organ prolapse, PCL is used as a reference line (Table 2).

Table 1 Pelvic floor measurements using H-line and M-line¹.

Grade	H-line (measure of hiatal enlargement)	M-line (measure of pelvic floor descent)
Normal	< 6 cm	< 2 cm
Mild	6-8 cm	2-4 cm
Moderate	8-10 cm	4-6 cm
Severe	> 10 cm	> 6 cm

¹H-line and M-line are measured in mid-sagittal cine gradient echo sequence at maximum strain during defecation.

Table 2 Grading of pelvic organ prolapse using pubococcygeal line as reference line.

Grade	Descent below pubococcygeal line1
Mild	1-3 cm
Moderate	3-6 cm
Severe	> 6 cm

¹Descent is measured from the pubococcygeal line to the inferior bladder base (cystocele), anterior cervical lip (cervical descent) and anorectal junction (anorectal junction descent).

ANTERIOR COMPARTMENT DISORDERS

Disorders affecting the anterior compartment, such as cystocele and urethral hypermobility, primarily involve the urinary system and do not have a direct impact on defecation[1,2]. Although these two conditions can sometimes occur together, they are distinct clinical entities, and each should be evaluated and documented individually. Under normal conditions, the urethra is positioned vertically and lies posterior to the pubic symphysis[1]. When the ligaments supporting the urethra become weakened or disrupted, it leads to urethral hypermobility, characterized by a posterior tilt of the urethra during strain[18,19]. This phenomenon is most clearly observed on sagittal MRI images, where the urethra demonstrates a posteriorly inclined orientation at rest and a downward shift with strain. The hypermobility of urethra is often linked with the downward displacement of both the urethra, which may move below the pubic symphysis, and the bladder, which can descend past the PCL, a key reference for the pelvic floor level (Figure 5). Recognizing urethral hypermobility is crucial, as it typically requires surgical intervention, often through a sling procedure[20,21]. A cystocele occurs when the bladder experiences insufficient support from the anterior abdominal wall or due to defects in the pubocervical fascia, leading to abnormal bladder descent. Clinically, this is observed as bulging of the anterior vaginal wall, with or without full-blown vaginal prolapse[22]. On MRI, the bladder normally rests above the PCL at rest, and may descend to or just below the PCL during strain. However, in cases of cystocele, the bladder moves in a curved, posterior, and downward direction, often dropping significantly below the PCL, resulting in a mass effect that distorts the anterior vaginal wall (Figure 5).

Figure 5 Urethral hypermobility and cystocele. A: Sagittal true fast imaging with steady-state free precession (TRUFI) at rest, which shows minimal posterior angulation of the urethra (yellow line), approximately 5° relative to the vertical axis of the pubis; B: In the sagittal TRUFI image obtained during the defecation phase, the urethral axis rotates posteriorly to 162° and descends below the pubic symphysis, indicating urethral hypermobility. This is accompanied by a descent of the bladder neck below the pubococcygeal line, consistent with cystocele.

MIDDLE COMPARTMENT DISORDERS

Uterine or vaginal prolapse can occur due to damage to the pubocervical fascia, cardinal and uterosacral ligaments, and other supportive tissues. Like other pelvic floor disorders, the likelihood of genital prolapse increases with factors such as previous pregnancies, aging, obesity, and activities that involve excessive abdominal pressure[4]. Normally, the cervix should be located above the PCL, and the lower vagina should maintain a vertical alignment[1]. On MRD imaging, it is considered abnormal for any part of the uterus, cervix, or vaginal apex to descend below the PCL (Figure 6). Uterine prolapse is often observed alongside cystocele. In cases of posterior compartment disorders, uterine descent can contribute to rectal outlet obstruction, and abnormal vaginal descent can lead to the formation of cul-de-sac hernias due to increased space in the cul-de-sac region[4,6].

Figure 6 Cervical descent. A: Sagittal true fast imaging with steady-state free precession image at rest, the cervix is positioned well above the pubococcygeal line (PCL); B: During defecation, the anterior lip of the cervix descends 2.1 cm below the PCL, indicating cervical descent.

Cul-de-sac hernias, also called pouch of Douglas hernias, develop as a result of herniation of abdominal contents into the rectovaginal space. It occurs due to the weakening or disruption of the supporting structures of vaginal wall. These hernias are more frequently encountered in patients who have had a hysterectomy, as this surgical procedure leads to damage to the endopelvic fascia. These hernias are categorized according to their contents, such as enterocele, peritoneocele, and sigmoidocele (Figures 7 and 8). It has been estimated that 37% of patients with pelvic floor disorders also have cul-de-sac hernias, however this statistic may be underestimated due to the difficulty in diagnosing these hernias during a routine clinical examination[23,24]. MRD is useful for assessing whether these hernias contribute to obstructed defecation, which can then guide decisions regarding the need for surgery. On MRD, cul-de-sac hernias are seen as a widening and deepening of the rectovaginal space and its filling with bowel and/or peritoneum. These hernias may not be apparent in the initial defecatory MRD images, but become more evident in post-evacuation images, after the rectum and bladder have emptied. When reporting cul-de-sac hernias, it is important to include information about the type of hernia, its contents, and whether it obstructs rectal evacuation[25,26].

Figure 7 Peritoneocele. A: Sagittal true fast imaging with steady-state free precession image at rest shows the normal position of cul-de-sac (white arrow) above pubococcygeal line (PCL); B: During the defecation phase there is caudal descent of the peritoneum (peritoneocele) (white arrow) below the PCL. Note descent of anorectal junction (white star).

Figure 8 Enterocele. A: Sagittal true fast imaging with steady-state free precession image at rest shows a small bowel loop (red star) well above pubococcygeal line (PCL); B: During the defecation phase there is caudal descent of this small bowel loop (red star) along with the peritoneum (green star) below the PCL. Note descent of anorectal junction (yellow star).

POSTERIOR COMPARTMENT DISORDERS

The posterior compartment is comprised of the rectum and anal canal, which are anatomically connected at the anorectal junction. It is crucial to run the defecation phase over a sufficient duration in order to fully capture the range of posterior compartment disorders[16]. MRD reveals a variety of abnormalities within the posterior compartment in patients with ODS, including anorectal junction descent, rectocele, intussusception, pelvic dyssynergia and solitary rectal ulcer syndrome[1,2].

Anorectal junction descent

In the resting state, the anorectal junction should ideally lie above the PCL (Figure 9). Anorectal junction descent is generally considered excessive when the junction descends more than 2 cm below the PCL[27,28]. Various studies have documented anorectal junction descent as a common finding in anorectal dysfunction, with rates of 70% in Saraya et al[22], 60% in Rentsch et al[28], and 65% in El-Nashar et al[29].

Figure 9 Anterior rectocele and anorectal junction descent. A: Sagittal true fast imaging with steady-state free precession (TRUFI) image at rest shows anorectal junction located above pubococcygeal line (PCL); B: During the squeeze phase there is reduction in anorectal angle with anorectal junction located above PCL; C: During the strain phase, anorectal junction reaches PCL; D: During the defecation phase there is caudal descent of the anorectal junction (yellow line) below the PCL. There is the formation of an anterior rectocele (green line), which is seen as a bulge of the anterior rectal wall beyond the expected anterior wall of rectum (blue line).

Rectocele

Rectocele refers to the protrusion or outward bulging of the rectal wall during defecation, arising as a consequence of weakening the pelvic floor’s supportive structures, particularly the recto-vaginal fascia. Rectoceles may be directed anteriorly or posteriorly, with anterior rectoceles being significantly more prevalent than posterior ones. Clinically, anterior rectoceles may present as a bulge or fullness against the posterior vaginal wall. However, clinical examination proves to be suboptimal for detecting anterior rectoceles, with sensitivity ranging from 30% to 80%[27]. Furthermore, it can be challenging to differentiate between anterior rectoceles and cul-de-sac hernias during clinical evaluation. Rectoceles can manifest with vaginal symptoms, such as bulging or dyspareunia, or rectal symptoms including constipation, a sensation of incomplete evacuation, or defecatory dysfunction. Anterior rectoceles are measured by the anteroposterior dimension, from the most anteriorly protruding part of the rectal wall to the anticipated position of the rectal wall (which can be inferred by the location of the anterior aspect of the anal canal (Figure 9). Rectoceles are typically classified as mild (< 2 cm), moderate (2-4 cm), and large (> 4 cm)[1,2]. Posterior rectoceles are exceedingly rare and are often associated with levator plate damage. Lateral rectoceles are also uncommon, typically occurring in patients with damage to the rectovaginal septum and iliococcygeus fascia[6].

Rectal intussusception

Rectal intussusception refers to telescoping or invagination of the rectal wall into the lumen of the distal rectum or anal canal during the process of evacuation[1,2]. Depending on the extent and length of the intussusception, it can be classified into three categories: Intrarectal (where the tip of the intussusception remains confined within the rectum), intra-anal (where the tip extends into the anal canal), and extra-anal (where the tip protrudes externally beyond the anal orifice). The latter is also referred to as rectal prolapse. Rectal intussusception can further be categorized based on the thickness of the intussuscepted wall into partial-thickness (mucosal) intussusception, which involves only the mucosal layer, and full-thickness intussusception, which encompasses all layers of the rectal wall[30-32].

MRD reveals mucosal intussusceptions as thin, hypointense curvilinear folds on the rectal wall during defecation. In contrast, full-thickness intussusceptions appear as the entire rectal wall folding upon itself (Figure 10).

Figure 10  Rectal intussusception. A: In the first patient, the anorectal junction (orange star) is above the pubococcygeal line (PCL) at rest; B: During the defecation phase, the anorectal junction descends caudally below the PCL (orange line). There is associated formation of three mucosal intussusceptions along the posterior wall of the rectum (yellow arrows); C: In the second patient, the anorectal junction is above the PCL at rest (orange star); D: During defecation, the anorectal junction descends below the PCL (orange star), with an associated full-thickness rectal intussusception (yellow arrows). Note the presence of an enterocele (green star) and peritoneocele (yellow star).

The pathophysiology of rectal intussusception is often attributed to chronic straining during defecation, which results in damage to the supporting fascial structures[4]. Patients typically present with a range of symptoms, including a sensation of incomplete evacuation, excessive straining, the need for digital maneuvering, frequent attempts at evacuation, and mucous discharge[12,14].

MRD, with a reported sensitivity of 70%, is capable of detecting rectal intussusception, classifying it as partial or full-thickness, and assessing its vertical extension (whether intra-anal or extra-anal)[1]. These distinctions are crucial when determining the appropriate management strategy.

Management decisions primarily depend on the thickness of the intussusception, in addition to the clinical severity of symptoms. For instance, partial-thickness intussusceptions may be amenable to conservative treatment, such as a high-fiber diet and the use of laxatives, or through transanal resection of redundant mucosa. Conversely, full-thickness intussusceptions typically require surgical intervention, such as rectopexy. While intrarectal and intra-anal intussusceptions may respond to conservative measures, patients with extra-anal intussusception frequently necessitate surgical correction[33,34].

Solitary rectal ulcer syndrome

Solitary rectal ulcer syndrome (SRUS) is characterized by chronic, localized inflammation and thickening of the distal rectum[33]. On endoscopy, it may present as mucosal hyperemia, a solitary ulcer, or multiple ulcers. Patients commonly exhibit symptoms consistent with ODS, including constipation, excessive straining, rectal bleeding[33]. MRD often reveals nonspecific imaging findings, such as rectal intussusception or localised wall thickening[1]. Treatment for SRUS depends on the severity of symptoms and may include conservative measures, such as dietary modifications, or, in more severe cases, surgical intervention, such as rectopexy[2].

Dyssynergia

Pelvic floor dyssynergia, also referred to as spastic pelvic floor or anismus, is a functional cause of constipation characterized by the uncoordinated interaction between the pelvic floor and abdominal muscles involved in defecation[34]. Dyssynergia accounts for approximately 7% of cases of ODS[35]. It arises from inadequate relaxation of the anal canal, resulting in increased resistance to evacuation. Patients with dyssynergia typically report excessive straining, incomplete evacuation, and the need for digital manipulation during defecation[36].

Dyssynergia is considered a functional cause of ODS and may occur due to the paradoxical contraction of the puborectalis muscle (paradoxical puborectalis syndrome) or failure of the anal sphincter to relax properly[1]. The Rome IV criteria provide the diagnostic framework for dyssynergic defecation, which includes an increased contraction of the pelvic floor muscles (as measured by anal electromyography or manometry) in the presence of adequate rectal propulsive forces[2]. For a diagnosis to be made, these criteria must be met for a minimum duration of three months[2].

MRD assists in making a confident diagnosis and also helps in assessing any coexisting abnormalities, such as intussusception, rectocele or cul-de-sac hernias[37]. On dynamic MRD dyssynergia is evidenced by the failure of the ARA to open despite repeated attempts at defecation, due to the abnormal contraction of the puborectalis (Figure 11).

Figure 11  Dyssynergic defecation. A: Sagittal true fast imaging with steady-state free precession image at rest shows an obtuse anorectal angle (ARA); B: During the squeeze maneuver the ARA reduces; C: During the strain phase, there is further reduction in the ARA with the prominent pubococcygeal muscle (yellow arrow) indenting the posterior wall of the anorectal junction; D: During defecation, the anorectal junction shows further indentation (yellow arrow) by the puborectalis muscle, which further reduces the ARA.

MRD is capable of detecting incomplete and prolonged evacuation, defined as the failure to completely expel the rectal gel during the examination or to eliminate two-thirds of the inserted gel within 60 seconds, caused by the anal canal's failure to open (Figure 12). This 60-second time frame is derived from studies conducted on conventional defecography[2].

Figure 12  Dyssynergic defecation. A: Sagittal true fast imaging with steady-state free precession image at rest demonstrates an anorectal angle (ARA) of greater than 90°; B: During the squeeze maneuver there is reduction in ARA; C: During the strain phase ARA reduces further; D: During the defecation phase, there is further paradoxical reduction in the ARA, with failure of the anal canal to open up.

A study by Reiner et al[20], conducted on patients (n = 48) with clinically suspected dyssynergic defecation revealed that impaired evacuation of rectal jelly exhibited an exceptionally high sensitivity of 100% for diagnosing dyssynergia. In contrast, paradoxical contraction of the puborectalis muscle demonstrated a sensitivity of 83%. The failure of the ARA to open displayed a lower sensitivity of 50%, but a high specificity of 97% for diagnosing dyssynergic defecation. The combination of failure to open the ARA and paradoxical puborectalis contraction further increased the diagnostic sensitivity to 94%.

Thanaracthanon et al[21] reported that various findings on MRD aid in diagnosing dyssynergic defecation, including the descent of the anorectal junction, alterations in the ARA, prominence of the puborectalis muscle, and incomplete or failed evacuation. Notably, changes in the ARA and the prominent indentation of the anorectal junction by the puborectalis muscle demonstrated the highest specificity, with values of 95.5% and 100%, respectively. Additionally, it was determined that the optimal diagnostic thresholds for dyssynergic defecation include an ARA at defecation of less than 122°, a change in ARA of less than 1.5°, and a descent of the anorectal junction greater than 3.25 cm below PCL[37].

The primary treatment approach for dyssynergia defecation includes dietary modifications and biofeedback therapy[37]. Although myectomy and botulinum toxin injections have been investigated as potential treatments for dyssynergia, they have not demonstrated superior outcomes compared to conservative management.

COMPARISON OF MRD WITH CONVENTIONAL X-RAY DEFECOGRAPHY

According to the American College of Radiology, only two imaging modalities are deemed appropriate for the evaluation of pelvic floor dysfunction: MRD and Fluoroscopic Cystocolpoproctography (CCP), both of which have been assigned an appropriateness rating of 7 to 9—scores that indicate a high level of clinical utility[38]. As such, dynamic CCP serves as a viable alternative imaging modality for assessing pelvic floor disorders. Dynamic CCP entails fluoroscopic imaging during the act of defecation, performed with the patient seated in a physiologically upright position on a specialized fluoroscopic commode. Image acquisition occurs at multiple functional phases: Rest, kegel maneuver (pelvic floor muscle contraction), straining, and defecation. CCP is capable of identifying clinically occult prolapse that may otherwise go undetected on physical assessment. A principal advantage of dynamic CCP lies in its capacity to provide functional evaluation in the upright, seated posture, closely mirroring normal physiological conditions.

A study conducted by Poncelet et al[39] compared the diagnostic performance of CCP and MRD in the evaluation of posterior compartment disorders. The investigators assessed the sensitivity of both dynamic CCP and MRD against a composite reference standard. The results of the combination of CCP and MRD were used as the standard of reference.

According to the study, X-ray defecography demonstrated a sensitivity of 90.9% for detecting peritoneocele, 71.4% for rectocele, 81.1% for rectal prolapse, and 63.6% for anismus. MRD, in comparison, showed a sensitivity of 86.4% for peritoneocele, 78.6% for rectocele, 62.2% for rectal prolapse, and 63.6% for anismus.

These findings revealed no statistically significant differences in sensitivity between the two imaging modalities across the spectrum of posterior compartment abnormalities. Consequently, the authors concluded that either X-ray defecography or MRD may be employed in clinical practice, with the choice largely dependent upon local availability and the expertise of the interpreting radiologist. In complex or ambiguous cases, the combined use of both modalities may be used to enhance diagnostic precision and guide management.

However, several limitations are inherent to fluoroscopic CCP. Notably, it lacks soft-tissue contrast resolution and is incapable of directly visualizing pelvic floor musculature, fascial structures, or post-surgical anatomical changes. Furthermore, the procedure requires the administration of contrast media into the bladder and vagina, in addition to oral contrast ingestion prior to the examination—elements that may render the study cumbersome and uncomfortable for patients. Finally, dynamic CCP involves exposure to ionizing radiation, with an estimated dose ranging from 1 to 10 mSv[38].

MRD offers several distinct advantages over X-ray defecography. MRD is free of ionizing radiation, rendering MRD a safer option for patients. In addition, MRD enables detailed visualization of pelvic organ morphology and facilitates a more precise evaluation of key pelvic floor musculature and ligaments. Perhaps most critically, it permits a comprehensive, simultaneous assessment of all three pelvic compartments within a single examination—an essential consideration in the formulation of an effective therapeutic strategy as patients usually have a dual or tricompartmental insufficiency.

MRD offers direct visualization of pelvic floor dysfunction through cine loop imaging. This dynamic representation enhances diagnostic clarity and serves as a valuable tool in multidisciplinary team discussions, thereby facilitating more informed and collaborative treatment planning. Furthermore, MRD is increasingly accessible and allows for repetition or acquisition of additional sequences in cases of suboptimal or inconclusive studies, thereby enhancing diagnostic reliability.

However, MRD has certain limitations. Primarily, it is typically conducted with the patient in a supine position, which does not replicate the physiological posture assumed during natural defecation. This discrepancy may restrict the accurate assessment of pelvic floor descent and prolapse, as the biomechanics of defecation can vary significantly when performed supine. Moreover, inadequate straining effort by the patient during the examination may result in the underdiagnosis or complete omission of rectal intussusception.

CONCLUSION

In conclusion, pelvic floor disorders represent a significant and multifaceted clinical challenge, manifesting through a broad spectrum of symptoms. ODS is an important cause of chronic constipation associated with a range of pelvic floor abnormalities. MRD serves as a versatile diagnostic tool in the management of ODS by offering detailed visualization of the pelvic floor abnormalities underpinning ODS. Its dynamic imaging capabilities enable the identification of multicompartmental pelvic disorders, thereby playing a crucial role in the accurate diagnosis and formulation of an appropriate management plan tailored to the patient’s needs. Further research is needed to deepen our understanding of the complex pathophysiology underlying pelvic floor dysfunction. Such efforts are essential to advance diagnostic accuracy and optimize clinical management.