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.