How best to measure the levator hiatus: evidence for the non-Euclidean nature of the 'plane of minimal dimensions'.

OBJECTIVE: To clarify whether the 'plane of minimal dimensions' of the levator hiatus on three-dimensional (3D) ultrasound accurately represents the minimal anatomical transverse hiatal dimension during a Valsalva maneuver. METHODS: In this retrospective study of 3D transperineal ultrasound and magnetic resonance (MR) imaging, datasets from 19 female participants were used to measure the transverse diameter of the levator hiatus using the plane of minimal dimensions on maximum Valsalva maneuver. The term 'apparent minimal transverse diameter' (aMTD) was used to define the transverse diameter measured using axial ultrasound and comparable axial or coronal MR images. Coronal MR images, using the plane of the vagina as a reference, were also obtained on maximum Valsalva. The transverse diameter measured between the caudal margin of the pubococcygeus/puborectalis on the MR coronal image is denoted by the term 'true minimal transverse diameter' (tMTD). Statistical comparisons between the aMTD and tMTD were made using Student's t-test. RESULTS: No significant difference was demonstrated between the aMTD as measured by ultrasonography and MRI. However, there were significant differences found between the aMTD measured by both ultrasound and MRI and the tMTD measured on coronal MR images (both P < 0.01), with mean ( ± SD) values of 4.36 ± 0.85, 4.13 ± 1.09 and 3.23 ± 0.49 cm, respectively. CONCLUSION: This study highlights the complexity and 3D nature of the levator hiatus and pelvic floor muscles. Investigators have previously assumed that the plane of minimal dimensions of the hiatus can be measured in a flat plane, however, the 3D nature of the hiatus means that the true levator hiatus occupies a warped (non-Euclidean) plane. Hiatal measurements on Valsalva may be subject to systematic error if performed in a single section, i.e. using a flat (Euclidean) plane.

Pelvic floor function in nulliparous women using three-dimensional ultrasound and magnetic resonance imaging.

OBJECTIVE: To compare biometric measures of pelvic floor function obtained using three-dimensional (3D) ultrasound and magnetic resonance imaging (MRI) in a group of nulliparous asymptomatic young women. METHODS: Twenty-seven asymptomatic nulliparous volunteers were assessed prospectively, using translabial 3D ultrasound and multiplanar 3D MRI. Levator hiatal dimensions were measured in the axial plane in both modalities. All participants were imaged supine, after voiding with data acquired at rest, on maximum Valsalva and maximum pelvic floor contraction. Interobserver variability was determined for both methods. Normally distributed continuous ultrasound data were compared with equivalent MRI parameters, and intraclass correlation coefficients were used to estimate correlation between the two methods. Bland-Altman analysis was also used to estimate agreement between methods. RESULTS: Interobserver repeatability was fair to excellent for all parameters measured with both methods. Moderate-to-substantial agreement between methods was shown for all tested parameters (intraclass correlation coefficients 0.587-0.783). There was a systematic but nonsignificant difference between methods, in that measurements on Valsalva tended to be larger for MRI, and the poorest agreement (intraclass correlation coefficient 0.587) was found for hiatal area on Valsalva. CONCLUSION: Agreement between the two methods was moderate to substantial for all parameters except for hiatal area on Valsalva. Magnetic resonance imaging yielded higher area measurements on Valsalva, which may indicate difficulties in identifying the plane of minimal dimensions due to poorer temporal resolution compared with ultrasound imaging.

Pelvic floor function in elite nulliparous athletes.

OBJECTIVE: There is preliminary evidence linking long-term participation in high-impact exercise with poor performance in labor and increased incidence of stress urinary incontinence, which may be due to altered pelvic floor function. Recent work has shown that HIFIT (high-impact, frequent intense training) athletes have an increased cross-sectional area of the levator ani muscle group as visualized using magnetic resonance imaging (MRI). The aim of this study was to further characterize pelvic floor muscle function and pelvic organ descent in a nulliparous athletic population and compare it with non-athletic controls matched for age and body mass index, using three-dimensional/four-dimensional (3D/4D) pelvic floor ultrasound imaging. METHODS: In this prospective comparative study translabial ultrasound imaging was used to assess pelvic floor anatomy and function in 46 nulliparous female volunteers (aged 19-39 years), 24 HIFIT and 22 controls. Two-dimensional (2D) and 3D translabial ultrasonography was performed on all subjects, after voiding and in the supine position. Descent of the pelvic organs was assessed on maximum Valsalva maneuver, whilst volume datasets were acquired at rest, during pelvic floor muscle contraction and during a Valsalva maneuver. Participants performed each maneuver at least three times and the most effective was used for evaluation. RESULTS: HIFIT athletes showed a higher mean diameter of the pubovisceral muscle (0.96 cm vs. 0.70 cm, P < 0.01), greater bladder neck descent (22.7 mm vs. 15.1 mm, P = 0.03) and a larger hiatal area on Valsalva maneuver (21.53 vs. 14.91 cm(2), P = 0.013) compared with the control group. There were no significant differences in hiatal area at rest or on maximal voluntary contraction between the two groups. CONCLUSION: HIFIT athletes show significant differences in several of the measured parameters for both function and anatomy of the pelvic floor. Further research into the impact of this altered function on childbirth and continence mechanisms is needed.