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1
Institute of Diagnostic Radiology, University Hospital,
Rämistr. 100, CH-8091 Zurich, Switzerland.
2
Present address: Department of Diagnostic Radiology, University Hospital
Essen, Hufelandstr. 55, D-451222 Essen, Germany.
3
Department of Biostatistics, University of Zurich, CH-8006 Zurich,
Switzerland.
4
Department of Gynecology and Obstetrics, University Hospital, CH-8091 Zurich,
Switzerland.
Received August 18, 2000;
accepted after revision October 2, 2000.
Address correspondence to R. A. Kubik-Huch.
Abstract
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MATERIALS AND METHODS. Thirty continent women were divided into three equal groups (nulliparous, previous cesarean delivery, previous vaginal delivery) and compared with 10 women with stress-incontinence with a history of at least one vaginal delivery. MR imaging of the pelvic floor at rest and on maximal strain was performed, using axial T2-weighted fast spin-echo images followed by sagittal ultrafast T2-weighted single-shot fast spin-echo sequences. Mean population age (age range, 22-45 years; mean ± SD, 36 ± 5.4 years), was similar in the four groups, as was parity in the three parous groups.
RESULTS. Mean distances between the bladder floor and pubococcygeal line at rest did not differ between the four groups. On straining, bladder floor descent was 1.1 ± 0.9, 1.0 ± 1.1, and 1.9 ± 0.9 cm in continent nulliparous, cesarean delivery, and vaginal delivery women, respectively, versus 3.2 ± 1.0 cm in incontinent women (p = 0.0005). Cervical descent was greater in incontinent versus nulliparous women (p = 0.0019). Bladder floor descent was greater in the continent vaginal delivery group than in continent cesarean delivery control patients (p = 0.04). In patients with stress incontinence, symptoms did not correlate with amplitude of descent. The right levator muscle was thinner overall than the left, regardless of frequency direction (p = 0.001).
CONCLUSION. Ultrafast MR imaging using the T2-weighted single-shot fast spin-echo sequence allows dynamic evaluation of the pelvic compartments at maximal strain with no need for contrast medium. Pelvic floor laxity and supporting fascia abnormalities were most common in patients with stress incontinence followed by continent women with a history of vaginal delivery. The results are therefore compatible with the hypothesis of vaginal delivery as a contributory factor to stress incontinence in older parous women.
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The purpose of our dynamic MR imaging study, using an ultrafast T2-weighted single-shot fast spin-echo technique, was to test the etiologic hypothesis of trauma by comparing pelvic floor laxity in continent nulliparous women and continent parous women subgrouped by mode of delivery (cesarean delivery and vaginal) with women with stress incontinence as a positive control group.
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Imaging Technique
All MR imaging studies were performed on a 1.5-T system (CTi; General
Electric Medical Systems, Milwaukee, WI) in the supine position using a pelvic
phased array surface coil. After an initial coronal localizer sequence, axial
T2-weighted fast spin-echo MR imaging (TR/TE, 5000/105; field of view, 18 cm;
slice thickness, 5 mm; gap, 0 mm; number of excitations, 4; matrix, 256
x 256; echo train length, 12; kHz, 21; acquisition time, 6 min) was
performed to assess the anatomy and any abnormality of the pelvic floor (i.e.,
levator ani, vaginal attachments, and uterus). A right-to-left frequency
direction was chosen in all patients. In five patients, the same sequence was
repeated with identical parameters except that an anteroposterior frequency
direction was used.
This sequence was followed by dynamic MR imaging to assess pelvic descent. Sagittal ultrafast T2-weighted single-shot fast spin-echo sequences (TR, indefinite; TE, 93 msec; field of view, 26 cm; slice thickness, 5 mm; gap, 0 mm; number of excitations, 0.5; matrix, 256 x 192; echo train length, 12; kHz, 21; acquisition time, <1 sec/slice) were performed (three adjacent slices in the pelvic midline) at rest and intriplicate during Valsalva's maneuver at maximal pelvic floor strain. The MR image showing maximal maneuver effect was used for analysis.
Image Analysis
Consensual image analysis by the same two radiologists who were unaware of
the clinical history was based on multiple pelvic measures using the MR
console. Axial images were evaluated at the level of pubic symphysis (at
approximately the proximal urethra) for vaginal morphology (normal H-shape,
asymmetric, or flat). Vaginal angles were measured as shown in
Figure 1 at the level of pubic
symphysis, and differences between the right and left sides were calculated.
Levator thickness (i.e., transverse diameter) was measured bilaterally.
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On sagittal dynamic MR images, the pubococcygeal line drawn between the inferior aspect of the symphysis and the distal joint of the coccyx was used as the reference line. Bladder volume was calculated using the formula for an ellipse: length x width x height x 0.5. The vertical distance from the pubococcygeal line to the most inferior portion of the bladder floor and cervix was measured at rest and at maximal strain. Laxity was defined as an absolute value greater than 2.5 cm below the pubococcygeal line at maximal strain [1]. The mobility of the bladder floor and cervix (i.e., the difference between rest and maximal strain) was calculated for each patient. MR imaging parameters were then analyzed as a function of symptoms and mode of delivery.
Statistical Analysis
Continuous variables are presented as mean ± standard deviation
(SD). Differences between groups were analyzed using the Kruskal-Wallis test
followed by post hoc comparison using the Mann-Whitney test with Bonferroni
adjustment. Nominal variables were analyzed using the chisquare test.
Relations between symptom amplitude and pelvic floor descent were analyzed
using Spearman's rank correlation coefficients. A significance level of
p less than 0.05 was used in all tests.
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Qualitative analysis showed lesions of vaginal suspension structures in five patients with stress incontinence (Fig. 2) and none in nulliparous women. Quantitative analysis, however, was unable to show a significant intergroup difference in vaginal angle (right, p = 0.52; left, p = 0.22) or vaginal shape in terms of the absolute right-minus-left difference in vaginal angle (p = 0.19).
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Levator thickness did not differ between groups (p = 0.11), but, overall, the left side was thicker than the right (p = 0.001), regardless of frequency direction (Fig. 3A,3B). Additional findings on the T2-weighted axial fast spin-echo MR images included fluid in the pouch of Douglas (n = 17), functional ovarian cysts (n = 14) indicative of the secretory phase of the menstrual cycle, retroverted uterus (n = 1), and nabothian cysts (n = 3).
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At rest, on the sagittal MR images, the mean distance between the bladder floor and pubococcygeal line did not differ between the groups: 2.8 ± 0.7 cm in nulliparous women, 2.7 ± 0.6 cm in the cesarean delivery group, 2.7 ± 0.6 cm in the vaginal delivery group, and 2.4 ± 0.6 cm in the incontinent group (Fig. 4) (p = 0.46). On straining, however, bladder descent was less marked in the continent groups: 1.1 ± 0.9 cm (nulliparous women; Fig. 5), 1.0 ± 1.1 cm (cesarean delivery), and 1.9 ± 0.9 cm (vaginal delivery; Fig. 6) than in women with stress incontinence (3.2 ± 1.0 cm [Fig. 7]) (p = 0.0005).
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Similarly, at rest the mean distance between the cervix and pubococcygeal line did not differ between the groups: 3.2 ± 1.3 cm (nulliparous women), 3.4 ± 0.8 cm (cesarean delivery), 3.4 ± 1.5 cm (vaginal delivery), and 3.4 ± 0.9 cm (stress incontinence) (p = 0.76). On straining, however, cervical descent was greater in the stress-incontinent group than in the nulliparous group: 2.2 ± 0.5 cm versus 0.8 ± 0.9 cm, respectively, (p = 0.0019). Values in the cesarean delivery and vaginal delivery groups were 1.4 ± 1.2 cm and 2.2 ± 0.8 cm, respectively.
Pelvic floor laxity in the vaginal delivery group (Fig. 6) did not significantly differ from that in nulliparous women (p = 0.11) but was greater than that in the cesarean delivery group (p = 0.04).
Results are summarized in Table 1.
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Assessment of the posterior compartment (i.e., exact delineation of the anorectal junction) was somewhat limited by the absence of rectal contrast medium in patients in whom the rectum was collapsed. Qualitative assessment showed descent (> 2.5 cm below the pubococcygeal line) in five patients with stress incontinence (Fig. 7) versus none in the nulliparous group.
The patients with stress incontinence who had undergone pelvic floor training (n = 6) showed less cervical descent than their untrained within-group controls (p = 0.02); however, no such correlation was observed with either cervical or bladder floor descent in the continent vaginal delivery group, nor between episiotomy (n = 3) and bladder floor descent (p = 0.73)
The women with stress incontinence rated their symptoms as 4.7 ± 3.6 (range, 1-10). Symptom ratings did not correlate with the amplitude of descent of either bladder floor (p = 0.35) or cervix (p = 0.99). Symptoms were not correlated with pelvic floor training.
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These findings are of interest because obstetric trauma is believed to be a main cause of incontinence in older women. Although the trauma was initially thought to be neurologic [3], structural pelvic floor damage is now thought to be a major contributor, fueling current debate about the risks and benefits of elective cesarean delivery. Some women may prefer an elective cesarean to avoid the disabling complications of incontinence [4].
In our study population, independent of parity and mode of delivery, the wall of the funnel-shaped levator muscle was thinner on the right side than on the left (p = 0.001). Right—left difference has previously been described, but was attributed to the presence of either trauma or chemical-shift artifacts [5, 6]. Although the latter may have contributed to the difference to some extent, the fact that it persisted after changing the frequency direction proved that it was anatomically based. It is important to realize that asymmetry can also be a physiologic finding in nulliparous women and, thus, is not necessarily a result of birth trauma and episiotomy.
Various radiologic techniques have been developed to meet the need for accurate visualization and quantitative assessment in the diagnosis of pelvic floor disorders [7,8,9]. Transvaginal and transrectal sonography are well suited for the static and dynamic examination of the anterior compartment [8] but are dependent on operator skill. Vaginography explores the middle compartment and reveals vaginal fistulas [9]. Bead-chain cystourethrography, colpocystorectography, and defecography have become key techniques for investigating all compartments, but their drawbacks include high exposure to ionizing radiation and absence of information on surrounding soft tissue [10]. The advantages of MR imaging are nonexposure to ionizing radiation and high soft-tissue contrast for assessing pelvic floor morphology [11].
Dynamic MR imaging of the pelvic floor was first reported by Yang et al. [1] in 1991 using T1-weighted gradient-echo sequences. Because of the relatively long acquisition times, dynamic imaging was only feasible in cooperative patients. The development of stronger, faster gradients and ultrafast T2-weighted pulse sequences with acquisition times under 1 sec now permits dynamic evaluation of the pelvic compartments at maximal strain. By enhancing tissue differentiation, the heavily T2-weighted images also obviate the administration of contrast medium [12,13,14]. A possible objection to our methodology is that the images were acquired in a physiologically inappropriate supine position. Open-configuration MR imaging systems permit imaging in the sitting position, simulating the conditions most often associated with urinary leakage by combining the effect of gravity with the increased intraabdominal pressure caused by the Valsalva's maneuver and the damaged pelvic floor. However, the dynamic ultrafast single-shot fast spin-echo sequence is not currently available with the open-magnet scanner. Furthermore, in a study on the influence of gravity on a 0.5-T open-configuration magnet system, Fielding et al. [2] showed that, in both the supine and upright positions, all pelvic floor structures remained stable except for the posterior urethrovesical angle. Gufler et al. [12] corroborated these findings by showing that examination in the supine position yields correct diagnoses at maximal pelvic strain.
Another potential objection is that we did not quantify straining. In an attempt at standardization, all patients practiced the Valsalva's maneuver several times under supervision before undergoing MR imaging. Furthermore, only the image showing maximal depressant effect was used for subsequent analysis. We decided against using a device to measure intraabdominal pressure, in case the device itself (i.e., the rectal balloon) influenced the straining effect. A further limitation was that we did not fill the rectum with contrast medium. Unlike Gufler et al. [12], we thus found it difficult to correctly assess the posterior compartments in selected patients.
In conclusion, dynamic ultrafast MR imaging using the single-shot fast spin-echo sequence allows dynamic evaluation of the pelvic compartments at maximal strain. Heavily T2-weighted images give excellent tissue differentiation with no need for contrast medium. In our study, pelvic floor laxity and supporting fascia abnormalities were most common in patients with stress incontinence, all of whom had a history of at least one vaginal delivery, followed by continent women with a history of vaginal delivery. The results are therefore compatible with the hypothesis of vaginal delivery as a contributory factor to stress incontinence in older parous women.
Acknowledgments
We thank Anni Meier and Karl Treiber for technical assistance, and Josef
Wisser and Renate Huch for critical review of our study design.
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