Alterations in the function of pelvic floor muscles (PFM) were shown to be involved in the pathophysiology of various pelvic floor disorders, including incontinence, chronic pelvic pain, and pelvic organ prolapse. Yet, the assessment of the PFM remains an ongoing challenge. Although various methods are currently available, none of them allow for the objective quantification of localized muscle stiffness. Shear wave elastography (SWE) has the potential to fill this gap and may overcome the limitations of the current PFM assessment methods. SWE is a new, non-invasive technique, with a transducer applied externally on the perineum, that enables the assessment of the stiffness of a specific area, providing a real-time spatial representation of the stiffness of the different muscles composing the pelvic floor. The emerging data on SWE in the assessment of PFM have been limited to a few studies investigating only the puborectalis muscle [1,2]. Therefore, no studies thus far have evaluated the reliability of SWE assessment in other PFM. This study aimed to investigate the intra-rater and inter-rater reliability of SWE to measure the stiffness of the different PFM at rest and during maximal voluntary contraction (MVC) in asymptomatic women. To our knowledge, this is the first study using SWE to assess the elastic properties of different PFM and reporting on its reliability.

Thirty healthy women (mean age 25.89±5 years) without pelvic floor disorders (e.g., pelvic pain, incontinence, prolapse) participated in this reliability study. They all attended a single assessment session comprising B-mode ultrasound and SWE evaluation with the use of linear transducers (SLH20-6 and SL10-2, Aixplorer; SuperSonic Imagine). As depicted in Figure 1, for each PFM structure (e.g., bulbospongiosus, ischiocavernosus, perineal body, puborectalis, transverse perineal), the transducer was placed on the perineum and aligned parallel with the muscle fascicles/connective tissues. Examinations were performed at rest and during MVC by two independent raters. Rater 1 performed two consecutive trials with an interval of a few minutes between the measurements (intra-rater reliability assessment). Rater 2 executed all measurements once (for inter-rater reliability assessment). The order of the raters was quasi-random and the raters were blinded to each other’s results. Data were analyzed offline using customized Matlab scripts (R2021a; MathWorks, Inc.). Stiffness (shear modulus) was evaluated by manually defining a region of interest that corresponded to the PFM structures (see example in Figure 2). The value of the shear modulus from 3 consecutive frames with stable SWE maps was averaged. Reliability was quantified using intra-class correlation coefficient (ICC) estimates. ICC values were interpreted as follows: poor reliability (<0.40), fair reliability (0.40-0.59), good reliability (0.60 -0.79), and excellent reliability (≥0.80) [3].

Intra-rater reliability of the bulbospongiosus muscle was fair to good at rest and during contraction (ICC 0.51-0.61). However, inter-rater reliability was poor for both muscle states (ICC 0.22-0.23). For the ischiocavernosus, reliability obtained for intra-rater measurements was excellent (ICC 0.82-0.89), but poor when inter-rater reliability was considered (ICC 0.25-0.38). Perineal body evaluation revealed good to excellent intra-rater reliability (ICC 0.69-0.89) and fair to good inter-rater reliability (ICC 0.48-0.69). Reliability of puborectalis evaluation was excellent for both muscle states for intra-rater measurements (ICC 0.82-0.92, ), and good to excellent when inter-rater reliability was considered (ICC 0.62-0.84). For the transverse perineal muscle, the reliability was good to excellent for both intra-rater (ICC 0.79-0.95) and inter-rater reliability (ICC 0.72-0.89).

Moderate to excellent reliability was found for the majority of the PFM structures, including the puborectalis muscle, transverse perineal muscle, and perineal body. Poor inter-rater reliability for the assessment of the bulbospongiosus and ischiocavernosus muscles may potentially be explained by the small size of these structures jeopardizing the reliable placement of the ultrasound transducer, and unstable contractions impeding data analysis during MVC measurements.

This study showed that SWE is a reliable real-time diagnostic tool that can be used to assess PFM stiffness in women. By enabling the evaluation of localized areas of stiffness, SWE may be a stepping-stone to enhancing our understanding of the role of PFM in the pathophysiology of pelvic floor disorders and improving the management of these conditions.

Morin M, Salomoni SE, Stafford RE, Hall LM, Hodges PW. Validation of shear wave elastography as a noninvasive measure of pelvic floor muscle stiffness. Neurourology and Urodynamics. 2022;41(7):1620-1628.
Gachon B, Fritel X, Pierre F, Nordez A. Transperineal ultrasound shear-wave elastography is a reliable tool for assessment of the elastic properties of the levator ani muscle in women. Scientific Reports. 2021;11(1):15532.
Cicchetti DV. Guidelines, criteria, and rules of thumb for evaluating normed and standardized assessment instruments in psychology. Psychological Assessment. 1994;6:284-290.

Continence 7S1 (2023) 100994
DOI: 10.1016/j.cont.2023.100994