Pelvic Measurements & Compartments

Interpretation of MR defecography requires knowledge of the relevant anatomic compartments and their contents, as illustrated in Figure 1. 

• Anterior compartment: Bladder, Urethra
• Middle compartment: Vagina, Uterus
• Posterior compartment: Rectum

Fig 1: Summary of major pelvic compartments in a 68-year-old female undergoing MR defecography.

Anatomic relationships within the compartments can be described further with the pubococcygeal line, H-line and M-line as described in Figure 2 and 3. 

Pubococcygeal line

• Inferior pubic symphysis to the most inferior coccygeal joint
• Plane of levator muscle/pelvic floor

H-Line

• Symphysis to posterior anorectal junction
• AP Diameter of levator hiatus
• < 6 cm normal

M-Line

• Perpendicular to PCL & intersects posterior H-line
• Reference for descent of hiatus
• < 2 cm considered normal

Fig 2: Pubococcygeal line, H-Line, M-Line & tricompartmental reference points in a 68-year-old female undergoing MR defecography.

Fig 3: Applications of the H-line and M-line.

Anorectal Angle (Figure 4)

• Angle between longitudinal axis of the rectum & longitudinal axis of the anal canal.
• During normal defecation, relaxation of puborectalis muscle causes the anorectal angle to widen
• Non-widening or shortening of the angle during defecation may be pathologic

Fig 4: Normal widening of the anorectal angle in a 79-year-old female.

MR Defecography Technique & Comparison with Fluoroscopic Defecography

MR defecography is a dynamic exam, with images acquired during multiple phases of straining and defecation (Figure 5).

Fig 5: Summary of MR defecography technique with representative images in a 28-year-old female undergoing MR defecography.

MR defecography provides greater level of anatomic detail and direct visualization of structures but, unlikely fluoroscopy, is performed in a non-anatomic position (Figure 6).

Fig 6: Contrasting MR defecography and conventional fluoroscopic defecography.

Common Entities Encountered in Defecography

Endopelvic Fascial Defects (Figure 7)

Individual components fascia often not well visualized with standard MR defecography technique:

• Defects often inferred indirectly due to secondary findings.
• Endovaginal coil may improve visualization.

Signs of Endopelvic Defects:

• Chevron Sign | Level 1 Fascial Laxity: Posterior drooping of posterolateral walls of the vagina.
• Saddlebag Bladder Sign | Level 2 Fascial Laxity: Posterior drooping of posterolateral walls of urinary bladder.
• Drooping moustache sign | Level 3 Fascial Laxity: Herniation of fat into the retropubic/pre-vesical space.

Fig 7: Signs of endopelvic fascial defects on MR defecography.

Urethral Hypermobility (Figure 8)

• One of the structural abnormalities underlying stress urinary incontinence (along with intrinsic sphincter deficiency).
• Laxity of the urogenital diaphragm or denervation (level 3 fascia).
• Managed by urogynecology or female urology
  • Mid-urethral sling procedure commonly performed for correction

Fig 8: Urethral hypermobility in a 20-year old female.

Cystocele (Anterior Vaginal Wall Prolapse, Figure 9)

• Term “Anterior vaginal wall prolapse” preferred by surgeons as it correlates with the physical exam findings of anterior vaginal wall bulge.
• Bladder neck descends below PCL
  • ± urethral hypermobility
  • Pitfall: bladder neck beaking ≠ cystocele or hypermobility but may indicate intrinsic sphincter deficiency
• Managed by Urogynecology or Female Urology service
  • Anterior colporrhaphy
  • Apical suspension appears important to reduce recurrence

Fig 9: Cystocele (anterior vaginal wall prolapse) in a 59-year-old female.

Apical Compartment Prolapse (Figure 10)

• Descent of the vaginal apex into the lower vagina to hymen, or beyond the introitus
  • Laxity of level 1 endopelvic fascia
  • Typically after hysterectom, patient sees or feels a bulge
• Characterized by descent of vaginal apex below PCL
  • Cervix at the level of pubic symphysis
  • ± enterocele
• Managed by Urogynecology or Female Urology service
  • Conservative management: pelvic floor strengthening exercises & pessaries
  • Surgical: Suspension of uterus or vaginal vault to uterosacral ligament, sacrospinous ligament or anterior longitudinal ligament

Fig 10: Apical compartment prolapse in a 56-year-old female.

Rectocele (Posterior Vaginal Wall Prolapse, Figure 11 & 12)

• “Posterior vaginal wall prolapse” preferred term, as rectum bulges anteriorly into posterior vaginal wall
  • Laxity in level II & level III endopelvic fascia
  • Difficulty with defecation, outlet obstruction
• Extension of the rectum beyond the expected location of the anterior rectal wall
  • <2 cm considered mild
  • 2-4 cm medium
  • >4 cm large
• Managed by Urogynecology or Female Urology service
  • Conservative management: pelvic floor strengthening exercises & pessaries
  • Surgical: Suspension of uterus or vaginal vault to uterosacral ligament, sacrospinous ligament or anterior longitudinal ligament

Fig 11: Rectocele (posterior vaginal wall prolapse) in a 26-year-old female.

Fig 12: Rectocele (posterior vaginal wall prolapse) in a 26-year-old female.

Spastic Pelvic Floor Syndrome / Anismus (Figure 13)

• Cause of functional outlet obstruction during defecation
  • Paradoxical contraction or inability to relax anal sphincter and/or puborectalis
  • Term "dyssynergic defecation" felt to more appropriately described pathophysiology
• Lack of appropriate widening of anorectal angle with straining
  • Prolonged interval between opening of anal canal and start of defecation on cine sequences (>60s)
  • ± puborectalis muscle hypertrophy
• Treatment primarily conservative and includes pelvic floor physical therapy
  • Botox injections to levator ani or anal sphincter muscles may also be helpful

Fig 13: Spastic pelvic floor syndrome (animus) in a 49-year-old female.

Peritoneocele, Enterocele, & Sigmoidocele (Figure 14 & 15)

• Herniation of peritoneal fat from the cul-de-sac into rectovaginal space +/- small bowel (enterocele) or sigmoid colon (sigmoidocele)
  • MRD important in detection of peritoneocele that cannot be reliably detected with clinical exam
• Peritoneocele should prompt search for enterocele or sigmoidocele
  • Re-evaluate peritoneocele on all defecation attempts to look for bowel in peritoneocele
• Isolated defects repaired via transvaginal or trans-anal approach
  • Obliteration of the herniated portion of the cul-de-sac
  • Uterus and/or vagina resuspended
• Sigmoidocele may be require concomitant sigmoidopexy or sigmoidectomy

Fig 14: Peritoneocele & enterocele in a 49-year-old female.

Fig 15: Large peritoneocele & sigmoidocele in a 58-year-old female.

Rectal Prolapse and Intussusception (Figure 16)

• Stratified by level of invagination of the rectal wall into the rectal lumen with respect to the anus
  • Intra-rectal
  • Intra-anal
  • Extra-anal/Complete
• ”Bowel-within-bowel" appearance during evacuation
  • Pitfall: degree of intussusception may be underestimated if rectal vault is not completely emptied during evaluation
• Managed by colorectal surgery
  • Rectopexy ± rectosigmoidectomy

Fig 16: Rectal prolapse in a 46-year-old female & rectal intussusception in a 32-year-old female.

Standardized Techniques, Terminology & Reporting

• Consensus definition and interpretation template provided by Pelvic Floor Disorders Consortium (PFDC). Highlights include (Figure 17):
  • MRD can be performed either upright or sitting
  • Rectal contrast is essential for appropriate MRD exam
  • Vaginal contrast medium is not routinely required
• A sample exam and reporting template are provided in Figure 18.

Fig 17: Highlights from the consensus definition and interpretation templates for MR defecography.

Fig 18: Sample reporting template of an MR defecography exam on a 79-year-old female.

Surgical Perspectives on MR Defecography

Role of Defecography

• Physical exam considered reliable for eval of anterior & middle compartments
  • Anterior & posterior vaginal wall prolapse, apical prolapse are palpable on exam.
  • Q-tip test  for urethral hypermobility.
• MRD provides most value in evaluation of the posterior compartment.
  • May reveal pathology not detected by physical exam & requiring concurrent intervention from a non-gyn surgeon (e.g. colorectal).
• Most common reason to evaluate with MRD:
  • (1) Obstructive defecation not explain by physical exam.
  • (2) Defecatory dysfunction not explained with physical exam.

Choice of Primary Surgical Service & Procedure (Figure 19-21)

• Involved compartments dictate choice of primary surgical service (Figure 19 and 20).
  • Anterior & middle compartment disorders → urogynecology or female urology
  • Posterior compartment disorders → colorectal surgery
• Surgical treatment strategies divided into reconstructive and obliterative approaches (Fig 21)
  • Obliterative procedures close off a portion of the vaginal canal to correct prolapse in patients who no longer require vaginal function (no desire for future sexual activity).

Fig 19: Choice of primary surgical service in cases of pelvic floor dysfunction.

Fig 20: Choice of primary surgical procedure in cases of pelvic floor dysfunction.

Fig 21: Flowchart describing options for primary surgical procedure in cases of pelvic floor prolapse.

Pre-operative considerations (Figure 22 & 23)

• Pelvic Organ Prolapse Quantification (POP-Q) system was devised in to standardize classification among clinicians
  • Identifies key locations during rest & straining which quantify anterior, apical & posterior vaginal prolapse

Fig 22: Pre-operative considerations and highlights of the pelvic organ prolapse quantification (POP-Q) tool.

Fig 23: Pre-operative considerations and examples of the pelvic organ prolapse quantification (POP-Q) tool.