|ECR 2019 / C-1978|
|MRI of pelvic floor dysfunction: a practical review.|
Findings and procedure details
Interpretation and Grading of Pelvic Floor Disorders with MR Imaging: The HMO System
To help standardize interpretation and grading of PFD with MR imaging, the HMO (H line, M line, organ prolapse) system was developed, which is applied to a midsagittal rapid half-Fourier T2-weighted image obtained during maximal patient strain. On the midsagittal image obtained during maximal strain, three points of reference are first defined: A, the inferior margin of the symphysis pubis; B, the convex posterior margin of the puborectalis muscle sling (the posterior levator plate); and C, the junction between the first and second coccygeal segments. Two anatomic fixed references in the HMO system are (a) the pubococcygeal line (PCL), which is drawn between points A and C, and (b) point B. The degree of pelvic floor relaxation is then graded as a measure of two components: hiatal widening (enlargement) and hiatal descent.
The puborectal hiatus line (H line) allows grading of the maximal widening of the pelvic sling in the anteroposterior dimension during straining and is the linear distance between points A and B. (Fig.9).
HMO Grading of Pelvic Organ Prolapse
Pelvic organ prolapse is any protrusion of a given organ (bladder, urethra, vagina, uterus, small bowel, or rectum) through the puborectal hiatus or the H line.In case of the rectum, prolapse may also be defined as bulging of the anterior rectal wall anterior to the transverse perineus muscle (anterior rectocele). This constitutes the final O component of the HMO classification system and is measured as the shortest distance between the most caudal aspect of a given organ during the Valsalva maneuver and the H line. The HMO system allows consistent definition, differentiation, and grading of pelvic organ prolapse and pelvic floor relaxation (Table. 1,2).
Interpretation of Dynamic MRI:
(A)Urinary Incontinence and Anterior Compartment Prolapse:
- Small urethral muscle volume or a short urethra:
The mean normal thickness of the urethral sphincter was reported as 4.3 mm +/- 0.9 (total striated and smooth muscle thickness anteriorly at the midurethra level), and the length of the urethra has been reported as 38 mm +/- 3(Fig. 10).
- Defects in the Urethral Sphincter:
A simple urethral diverticulum is round or oval and usually located lateral or posterior to the urethra, a U-shaped diverticulum extends partially around the urethra, and a circumferential diverticulum extends completely around the urethra, often having a “saddlebag” appearance. Urethral diverticula are classically most conspicuous on T2-weighted images, since the fluid-containing cystic cavity is hyperintense relative to the surrounding soft tissues(Fig. 11).
- Distortion of the urethral support ligaments:
Normal ligaments are seen as continuous T2 hypointense bands of tissue stretched tight between the points of attachment. Disruption of the urethral ligaments may be complete or partial. In complete disruption, there is discontinuity of the ligament, complete attenuation of a portion of the ligament, or loss of its attachment. In partial disruption, fluttering of the lax ligament or focal thinning or attenuation can be seen(Fig. 12).
- Level III fascial defect:
It is recognizable by the drooping mustache sign, which is caused by the fat in the prevesical space against the bilateral sagging of the detached lower third of the anterior vaginal wall from the arcus tendineus fascia pelvis(Fig. 13).
- Cystocele and Urethral Hypermobility:
At MR imaging,
a cystocele is diagnosed when the bladder base descends more than 1 cm below the PCL.When the bladder base descends,
it tents to bulge into the anterior vaginal wall; consequently,
eversion of the vaginal mucosa can be observed in severe cases(Fig.
14 and 15).
- Asymmetric Pubococcygeus Muscle:
Loss of the symmetric appearance of the pubococcygeus muscle and lateral deviation and thinning or complete attenuation have been shown in patients with urinary incontinence(Fig. 16).
- Increased Vesicourethral Angle:
The vesicourethral angle is best evaluated at MR imaging on sagittal images as the angle between the axis of the urethra and the posterior bladder base.
A posterior vesicourethral angle below 115° is considered normal; however,
this angle is variable in both the continent and incontinent populations and is not a reliable marker in pelvic floor assessment.
- Enlargement of the Retropubic Space:
The retropubic space is defined by the distance between the posterior aspect of the symphysis pubis and the anterior urethral wall. It has been shown that the retropubic space may enlarge in incontinent patients. This may be explained as being a result of the damage to the posterior urethral support mechanism, leading to posterior displacement of the urethra.
- Abnormal Shape of the Vagina:
The vagina with maintained paravaginal attachments assumes an H-shaped configuration, as seen on axial images. Alteration of the morphologic features of the vagina may be indicative of paravaginal tears. These tears lead to urinary incontinence by weakening the urethral support mechanism provided by the vagina to the middle and distal portions of the urethra embedded in the anterior wall of the vagina. With the loss of paravaginal attachments, the vagina has a flattened appearance (Fig.16).
(B) Middle compartment prolapse:
- Uterine and Vaginal Vault Prolapse:
As previously discussed for cystocele,
in cases of uterine prolapse the H and M lines are elongated. The vagina loses its normal vertical-oblique orientation and is directed in a more horizontal axis(Fig.
- Level I and II endopelvic fascial defect:
In the axial plane, a paravaginal defect in the fascia is visualized as sagging of the posterior urinary bladder wall, caused by the detachment of the vaginal supporting fascia from the lateral pelvic wall, known as the saddlebags sign. A central defect is indicated by sagging of the central part of the urinary bladder posterior wall(Fig. 18).
- Iliococcygeus muscle In the coronal plane:
The iliococcygeus muscle is assessed for loss of the normal symmetrical appearance of its muscle slings or defect and/or disruption of its attachment to the obturator internus muscle(Fig. 19).
(C) Posterior Compartment Abnormalities:
- Intussusception and Rectal Prolapse:
Rectal prolapse is an invagination of the rectal wall and may be classified as internal or external. Internal rectal prolapse may be classified as intrarectal and intraanal internal prolapse.The distance of parietal inversion from the anal verge should be assessed when evaluating invagination and further classified as intrarectal (distal, middle, or proximal with respect to rectal length(Fig.20).
- Descending Perineal Syndrome:
On dynamic images, perineal descent can be quantified by measuring the descent of the anorectal junction from the PCL, considered abnormal if exceeding 2 cm. Since the width of the pelvic hiatus is greater in descending perineal syndrome, the H and M lines will be longer as well(Fig. 21).
- Fecal incontinence:
The Anorectal angle(ARA) as measured with dynamic MR was substantially more obtuse in patients with fecal incontinence than in patients with constipation or in control subjects. Also moderate or severe descent of the rectum was noted in patients with fecal incontinence. In addition, results show that abnormal descent of the posterior pelvic floor compartment is often combined with abnormal descent of the middle or anterior pelvic floor compartment.
- Spastic pelvic floor syndrome- Anismus:
MR imaging clearly shows lack of descent of the pelvic floor during defecation and paradoxical contraction of the puborectalis muscle with failure of the ARA to open, thus resulting in prolonged or incomplete evacuation. The puborectalis muscle is frequently hypertrophic and makes a prominent impression on the posterior rectal wall during voiding (Fig. 22).
Thematically related posters
ECR 2019 / C-0914
A pictorial review of the congenital uterine anomalies according to the ESHRE/ESGE classification