Objective
We describe a new parameter based on magnetic resonance 3-dimensional (3D) reconstructions proposed to evaluate levator ani muscle (LAM) laxity in women with pelvic organ prolapse (POP).
Study Design
This is an institutional review board–approved, retrospective chart review of 35 women with POP, stages I-IV. The 3D Slicer software package was used to perform 2-dimensional and 3D measurements and the levator ani subtended volume (LASV) was described. Basically, the LASV represents the volume contained by LAM between 2 planes, which coincides with pubococcygeal line and H line. Correlations among measurements, ordinal POP stages, POP Quantification (POPQ) individual measurements, and validated questionnaires were performed.
Results
The LASV differentiated major (III and IV) from minor (I and II) POPQ stages, which positively correlated to POP stages and POPQ individual measurements.
Conclusion
The LASV is a promising parameter to evaluate the LAM laxity.
The levator ani muscles (LAM) have an important function to support the pelvic floor. They interact with the supportive ligaments and stabilize the closure of the levator hiatus and the positions of the pelvic organs. The morphology of the muscle has a critical role in the levator ani function, and evaluations can be performed through magnetic resonance imaging (MRI) and ultrasound. Using MRI, the specific muscle subdivisions can be identified, including the pubovisceral (puboanal, puboperineal, and pubovaginal), puborectal, and iliococcygeal. MRI can also evaluate the integrity of the supportive structures associated with muscle laxity, such as defects, disruptions, pelvic distortions, alterations in levator hiatus dimensions, and increased mobility of pelvic viscera. Three-dimensional (3D) reconstructions add an advantage over traditional examination, neutralizing discrepancies in acquisition angles and improving the interobserver and intraobserver reliability of pelvic floor measurements.
The shape and dimensions of LAM differ among race and parity. Defects and architectural distortions of the LAM are more common among parous than nulliparous women and are considered risk factors for pelvic floor dysfunctions. In fact, these muscle defects are more frequent among women with all described types of pelvic floor disorders, such as pelvic organ prolapse (POP), rectal intussusception, urinary incontinence, and fecal incontinence. Women with POP have decreased LAM volumes (LAMV), and reduced thickness and cross-sectional areas of the anterior portions of these muscles. Women with high-grade prolapses and larger muscle disruptions demonstrate impaired function or weakness.
The levator hiatus can be assessed to indirectly evaluate the function of the LAM. Muscle rest, contraction, Valsalva, and evacuation affect the dimensions of the levator hiatus. The levator hiatus increases with POP Quantification (POPQ) stage progression, aging, impairment of levator ani function, severity of levator ani defects, and after delivery. The 2-dimensional (2D) measurements of the levator hiatus have been correlated with POPQ stages and with validated symptom questionnaires with different success rates.
The aim of this study was to describe the levator ani subtended volume (LASV), a novel parameter based on MRI 3D reconstructions, proposed to evaluate the levator ani laxity in women with POP. Basically, the LASV represents the volume contained by LAM; these limits are defined by levator hiatus, pubococcygeal line (PCL), and H line planes. The secondary objectives are to: (1) compare different POP stages using this parameter and 2D measurements; (2) test the correlation of this parameter with POP ordinal stages, POPQ individual measurements, and validated questionnaires of symptoms; and (3) perform a repeatability analysis through the determination of the LASV interclass correlation coefficient.
Materials and Methods
Charts of patients followed up at the Urogynecology Department of the University of South Florida from August 2008 through August 2010 were reviewed. The study included women between 18-80 years of age with pelvic floor symptoms, who completed validated symptom questionnaires and underwent dynamic pelvic MRI as part of their initial evaluation, following a complementary investigative protocol. Subjects were excluded if they were pregnant or underwent prior pelvic irradiation. Subjects were grouped according to their stage of pelvic support, ranging from POPQ stages I-IV. This study was considered a pilot project and the plan proposed to include 10 subjects in each group. Charts were reviewed in the same order as recruited by the clinic, and the first subjects to meet the inclusion/exclusion criteria were selected for each group. A total of 35 patients were included, 10 patients each in groups 1-3. Only 5 subjects with stage IV prolapse met the inclusion criteria, and were included in group 4. The retrospective chart review was approved by the University of South Florida Institutional Review Board.
Diagnosis, age, ethnicity, medical history, surgical history, obstetrical history, body weight, body mass index, and POPQ measurements, for each patient, were extracted from the medical charts. Pelvic floor, urinary, sexual, and colorectal symptoms were assessed through the following validated questionnaires of symptoms: Pelvic Floor Distress Inventory-short form 20; and Medical, Epidemiological, and Social Aspects of Aging and Pelvic Floor Impact Questionnaire-short form 7. The total scores and subcategorical scores were recorded for analysis.
Imaging protocol
MRI was performed on a 3-Tesla GE system (General Electric Company, GE Healthcare, Buckinghamshire, UK) using an 8-channel torso phased-array coil with the patient in the supine position. Standard imaging for detailed anatomic evaluation of the pelvic floor muscles was performed using T2-weighted fast-recovery-fast-spin-echo sequence acquired in the axial, coronal, and sagittal planes. These standard imaging acquisitions were used for 3D rendering. Each T2 sequence presented the field of view 26 cm and the slice thickness of 3 mm. For T1 imaging, a spoiled gradient sequence in the axial plane was acquired. Prior to imaging, 60 mL of ultrasound gel was placed in the rectum for visualization of the colon.
Dynamic imaging was performed in the supine position. Dynamic imaging is a multiphase, single-slice sequence. This was acquired midsagittally for 23-27 seconds using a T2-weighted single-shot fast-spin-echo sequence. According to the imaging protocol, subjects were coached on how to perform an adequate Valsalva and a pelvic floor contraction. Dynamic imaging was performed through cycles of Valsalva and pelvic muscle contraction.
MRI source (2D) images were transferred to a computer workstation with appropriate graphics capability. Specialized software was used to measure linear parameters on the 2D source images. The images were also segmented into anatomically significant organs, eg, levator ani, bony pelvis, and selected anatomic enclosures, as described below.
Two-dimensional measurements
The 3D Slicer software version 3.6 ( www.slicer.org , a open source software; Brigham and Women’s Hospital, Boston, MA) was used to measure the PCL, H line, and M line on the midsagittal slice, according to the definitions described by Law and Fielding in 2008. A representation of these reference lines is shown in Figure 1 , A . The point in the posterior wall of the rectum, at the level of anorectal junction, used to define both H line and M line is referred to as “Fielding point.” A urogynecology clinical fellow, blinded to the POPQ staging of the patients, chose the Fielding points in the midsagittal slices, and another specialist used this reference point to draw and measure the H lines and M lines on the gray-scale images.
