This study aimed to investigate the association between leavator ani (LA) elasticity using Shear wave elastography (SWE) and bladder base displacement using transabdominal ultrasound (TAUS)  for evaluation of Pelvic floor muscle (PFM) function. The secondary objective was to assess the elastic properties of LA muscle and provide a standard reference to evaluate PFM function in nulliparous women.

This observational study is in accordance with the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) statement.  All participants were fully informed about experiment procedures and gave their written informed consent before the beginning of the study. Forty-two nulliparous women (mean age 23.5 ± 3.2 years) with no history of pregnancy were included in the study. All participants were educated on how to contract PFM and asked to practice maximum voluntary contraction (MVC) of PFM three times for familiarization. Another 5 minutes were given to the participants to get relaxed. Then, they were asked to maximally contract PFM and maintain MVC for 5 seconds while SWE and TAUS measurements were obtained simultaneously; 30 seconds was given for relaxation between each contraction to prevent muscle fatigue caused by repeated contraction of PFMs.  The elasticity of the LA muscle at rest and during contraction using SWE and the bladder base displacement using TAUS were obtained simultaneously.

The intra-class correlation coefficient for repeat measures of LA elasticity at rest and contraction was 0.906 at rest and 0.687 during contraction.  The LA elasticity measured by SWE was 24.7 ± 4.5 kPa at rest and 62.1 ± 10.4 kPa during contraction. There was a significant increase in mean LA elasticity when the muscle was voluntarily contracted compared to rest (95% CI: 34.3 – 40.4, p < 0.001).  The mean bladder base displacement of the participants was 7.2 ± 2.5 mm, and normalized bladder base displacement by BMI was 0.3 ± 0.1 mm . Analysis of the Pearson’s correlation showed that the bladder base displacement was significantly associated with the elasticity of LA differences between contraction and resting when simultaneously performed (r = 0.486, p = 0.001).

The results of our study assessed non-invasive two different types of PFM function. Our results  found a moderate correlation coefficient between the elastic properties of LA measured using a non-invasive method, SWE, and the bladder base displacement measured with TAUS, a gold standard that indirectly measures the function of PFM. An excellent reproducibility (ICC (2,1) =0.906) at rest and a moderate reproducibility (ICC (2,1) =0.687) during contraction were found. 
Several studies assessed PFM using direct and indirect methods, but it is difficult to compare with previous studies due to the different metric values. The most direct comparison can be processed with the study of Gachon et al., which assessed elastic properties of LA with similar characteristics of population using SWE . They reported a 21.9 - 22.8 kPa shear modulus at rest and 55.1 - 61.4 kPa during a contraction while our results showed an average 24.7 kPa at rest and average of 62.1 kPa during contraction, indicating that the results are similar [1]. As our study only included nulliparous women with no history of pregnancy, the bladder base displacement is similar to the study of Sherburn et al., which had pre-menopausal nulliparous women as their subjects [2] . In addition, our result showed that the ICC for repeat measures of PFM was stronger when LA was resting than during contraction, which supports the result of a previous study [1]. This can be explained by the lower detection accuracy of SWE measurements due to the generation of high-speed shear waves that are too fast to detect when  measuring strongly contracted muscles [3].

This study showed that SWE is a reliable tool to investigate the elastic properties of PFM in nulliparous women. In addition, this paper adds important findings in terms of establishing the association between the bladder base displacement and elasticity of LA differences between contraction and resting for pelvic floor function. Also, the study findings allow a reference baseline data to have an objective method for a clinical measure and establish an appropriate plan of care.

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. Sci Rep 2021, 11, 15532, doi:10.1038/s41598-021-95012-8.
Sherburn, M.; Murphy, C.A.; Carroll, S.; Allen, T.J.; Galea, M.P. Investigation of transabdominal real-time ultrasound to visualise the muscles of the pelvic floor. The Australian journal of physiotherapy 2005, 51, 167-170, doi:10.1016/s0004-9514(05)70023-4.
Brandenburg, J.E.; Eby, S.F.; Song, P.; Zhao, H.; Brault, J.S.; Chen, S.; An, K.N. Ultrasound elastography: the new frontier in direct measurement of muscle stiffness. Archives of physical medicine and rehabilitation 2014, 95, 2207-2219, doi:10.1016/j.apmr.2014.07.007.

Continence 2S2 (2022) 100297
DOI: 10.1016/j.cont.2022.100297