A comparison of the effect of age on levator ani and obturator internus muscle cross-sectional areas and volumes in nulliparous women

Abstract

Aims: Functional tests have demonstrated minimal loss of vaginal closure force with age. So we tested the null hypotheses that age neither affects the maximum cross-sectional area (CSA) nor the volume of the levator muscle. Corresponding hypotheses were also tested in the adjacent obturator internus muscle, which served as a control for the effect of age on appendicular muscle in these women.

Methods: Magnetic resonance images of 15 healthy younger (aged 21-25 years) and 12 healthy older nulliparous women (aged >63 years) were selected to avoid the confounding effect of childbirth. Models were created from tracing outlines of the levator ani muscle in the coronal plane, and obturator internus in the axial plane using 3D Slicer v. 3.4. Muscle volumes were calculated using Slicer, while CSA was measured using Imageware™ at nine locations. The hypotheses were tested using repeated measures analysis of variance with P < 0.05 being considered significant.

Results: The effect of age did not reach statistical significance for the decrease in levator ani muscle maximum CSA or the decrease in volume (4.3%, P = 0.62 and 10.9%, 0.12, respectively). However, age did significantly adversely decrease obturator internus muscle maximum CSA and volume (24.5% and 28.2%, P < 0.001, respectively). Significant local age-related changes were observed dorsally in both muscles.

Conclusions: Unlike the adjacent appendicular muscle, obturator internus, the levator ani muscle in healthy nullipara does not show evidence of significant age-related atrophy.

Figure 1. a (above), sample traced outlines of levator ani (yellow) in coronal MR images from a young subject, and b (below), obturator internus outlined (yellow) in axial MR images

Figure 2

(Top) Ventrocaudal view looking down into a model of the pelvis showing the left levator ani (purple). The red line in the direction of the fibers is divided into eight segments by nine (yellow) equidistant lines. (Bottom) Strategy for assessing average levator ani line-of-action. The background colored lines are reproduced from Shobieri (11). Black dots indicate the upper, lower and median points along the mid-portion of the levator ani. A best fit line was constructed (black) to indicate the predominant fiber orientation. The nine perpendicular cross sections (red lines) were then constructed across the muscle to yield nine estimates of anatomic CSA (red dot “cloud”). PS, V, PP, PA, A, PR, PV, IC, C, CC, ATLA, and ATFP are usual abbreviations for pelvic floor structures (11).

Figure 2

(Top) Ventrocaudal view looking down into a model of the pelvis showing the left levator ani (purple). The red line in the direction of the fibers is divided into eight segments by nine (yellow) equidistant lines. (Bottom) Strategy for assessing average levator ani line-of-action. The background colored lines are reproduced from Shobieri (11). Black dots indicate the upper, lower and median points along the mid-portion of the levator ani. A best fit line was constructed (black) to indicate the predominant fiber orientation. The nine perpendicular cross sections (red lines) were then constructed across the muscle to yield nine estimates of anatomic CSA (red dot “cloud”). PS, V, PP, PA, A, PR, PV, IC, C, CC, ATLA, and ATFP are usual abbreviations for pelvic floor structures (11).

Figure 3

The mean (SE, vertical bars) anatomical CSAs from young (n=15) and old (n=12) obturator internus. Location 1 is nearest the pubic symphysis and location 9 is closest to the ischial spine. In this and the following figure, * p<0.05.

Figure 4

The mean (SE, vertical bars) anatomical CSAs of the levator muscle from young (n=15) and old (n=10) donors. Location 1 is close to the origin at the pubic symphysis and location 9 is close to the iliococcygeal/coccygeus insertion. [Note: the maximal values in the graph at location 7 are less than those reported for the mean maximal CSA because in some cases the maximal value occurred at a different location (usually 6 or 8).