The tone and contractility of the pelvic floor muscles play a fundamental role in the pathophysiology of pelvic floor disorders such as urinary incontinence, pelvic organ prolapse, and sexual dysfunctions (1).
Traditionally, the assessment of pelvic floor muscles has been carried out using digital evaluation employing the Oxford scale. It is a subjective measure with some controversy regarding reproducibility studies.
Perineal tonometry is another method used especially in the field of rehabilitation. Both basal tone and contractility in g/cm2 during a maximal contraction can be evaluated. Perineal tonometry represents a more objective test than the Oxford scale and also allows for biofeedback to patients.
In recent years, with the rise of ultrasound imaging, other methods for evaluating contractility have been described. An indirect sign of muscle strength is measured using bidimensional ultrasound, from the bladder neck to the lower edge of the pubic symphysis. Another method involves measuring the distance between the most anterior part of the pubic symphysis and the most anterior part of the levator ani muscle at the anorectal angle in a mid-sagittal plane (2). The advent of 3-4D ultrasound in urogynaecology has allowed for obtaining an axial view of the levator ani hiatus, thus enabling the calculation of hiatus area at rest, during contraction and during Valsalva manoeuvre, allowing inference of muscle strength (3).
The main objective is to assess whether there is correlation in the measurement of pelvic floor muscle contractility using digital evaluation, perineal tonometry, 2D ultrasound, and 3-4D ultrasound. As secondary objectives, we evaluate potential influencing factors on pelvic floor contractility and define which technique is better tolerated and accepted based on a self-administered questionnaire.

A cross-sectional study was designed in which patients serve as their own control by measuring the same variable using four different techniques. The study was approved by the ethics committee of the centre. Accepting an alpha risk of 0.05 and a beta risk below 0.1 in a one-sided test, 34 subjects are needed to detect a difference equal to or greater than 0.25 units. A follow-up loss rate of 10% has been estimated.  Measurements were carried out by three independent investigators: a gynaecologist for digital evaluation, a physiotherapist for perineal tonometry, and another gynaecologist for 2D and 3D ultrasound assessment. 
Volunteers with pelvic floor dysfunctions from pelvic floor clinics, as well as healthy volunteers, were included. During the first visit, the purpose and procedure of the study was explained, informed consent was obtained, and demographic data were collected. Volunteers were then scheduled for an appointment in a designated consultation room. All measurements were performed with the patient in the lithotomy position and with empty bladder.
First, digital evaluation of pelvic floor muscle contractility was performed using the modified Oxford scale. The gynaecologist inserted two fingers into the vagina, exerting slight pressure on the posterior vaginal wall, and asked the patient to perform a maximum contraction of the pelvic floor muscles. The procedure was performed three times, and the average score of the three attempts was calculated. Subsequently, the physiotherapist performed perineal tonometry of the pelvic floor muscles using a Phoenix tonometer (DPM®). Contractility was measured while maintaining for at least 5 seconds with the speculum open at 5° and 10°. This was repeated three times, and the average of the three maximum contractility values in g/cm2 was calculated. Finally, contractility was assessed using 2D and 3-4D ultrasound by another gynaecologist. A Voluson S10 ultrasound machine with a 4-8 MHz curved 3D/4D transabdominal probe, with a capture angle of 85° (GE Medical Systems, Zipf, Austria), was used. Both 2D and 3D assessments were performed by transperineal approach. The 2D measurement were performed in the mid-sagittal plane, obtaining the plane of minimal dimensions. The distance from the edge of the pubic symphysis to the internal point of the lowest area of the levator ani muscle at the anorectal angle was measured both at rest and on contraction. Muscle strength was calculated as a percentage using the formula: ((Rest Distance - Contraction Distance) / Rest Distance) x 100. The percentage of movement of the bladder neck on maximum contraction was also calculated. Finally, muscle strength was estimated by 3-4D ultrasound. An axial view of the levator hiatus was obtained in the plane of minimal dimensions. Hiatus area at rest and on maximum contraction was measured in rendered volume. The same formula as in the 2D technique was used. Additionally, it was assessed whether there were complete or partial defects of the fascicles of the levator ani muscle, as well as the area of the levator hiatus on maximum Valsalva manoeuvre.
Both measurements and clinical data of the patients were blinded to the three investigators conducting the tests. 
Volunteers were invited to fill out a questionnaire about the degree of comfort, duration, and tolerance of each test (Figure 1). 
Statistical analysis was performed using R software (version 4.3.1). Mean, standard deviation, minimum value, and maximum value were calculated for each variable. To measure test agreement, the percentage value was compared with the Pearson correlation coefficient being calculated for continuous quantitative variables with normal distribution, and the Spearman correlation coefficient for the Oxford scale, which is ordinal qualitative. A statistically significant value was accepted when p < 0.05. We also analyzed the correlation between tests within the subgroups: injury of the levator ani muscle, obesity, and presence of pelvic floor disorder.

The study involved 94 women. Table 1 shows the demographic data of the volunteers. The mean age, BMI, and parity were 45.7 years, 24.9, and 1.7 births respectively. 81 women with at least one childbirth and 13 nulliparous women were included. Among them, 35.1% were asymptomatic, 58.4% presented symptoms of urinary incontinence, and 17% had genital prolapse. 9.6% of them had undergone pelvic floor surgery. 3-4D ultrasound showed that 27 participants had hyperdistention or ballooning of the levator ani hiatus on Valsalva (>25 cm2) and 22 had unilateral avulsion. A high association was found with a Pearson correlation coefficient (r) between 0.47 and 0.94 in all techniques used except for bladder neck movement (Figure 1). Regarding the subgroup analysis, we observed that the correlation between tests remained consistent among the volunteers when stratifying by injury to the levator ani muscle, obesity, and presence of pelvic floor disorder (Figure 2). 
70.2% of women indicated a preference for transperineal ultrasound over the other tests for the assessment of contractility.

The data obtained in our study show that the four methods for evaluating pelvic floor muscle contractility have a high degree of correlation, making them valid techniques for measuring the same variable, except for the percentage of bladder neck movement. This correlation remained consistent when stratifying by the most common factors involved in pelvic floor disorders.
Most of women indicated a preference for ultrasound over other methods.

Transperineal ultrasound is as effective as the Oxford test and perineal tonometry for evaluating the contractility of the pelvic floor muscles. The influential factors assessed in our study did not have an impact on the correlation between tests.

1.	Navarro, B. et.al. (2017). The evaluation of pelvic floor muscle strength in women with pelvic floor dysfunction: A reliability and correlation study. Neurourology and Urodynamics. 2017;9999:1–9.
2.	Thibault-Gagnon, S. (2017). Relationships between 3-Dimensional transperineal ultrasound imaging and digital intravaginal palpation assessments of the pelvic floor muscles in women with and without provoked vestibulodynia. 346 J Sex Med 2018;15:346e360.
3.	Van Delft K.; Thakar R.; Sultan A. H..(2015) Pelvic floor muscle contractility: digital assessment vs transperineal ultrasound. Ultrasound Obstet Gynecol 2015; 45: 217–222.

Continence 12S (2024) 101632
DOI: 10.1016/j.cont.2024.101632