PFM training has been shown to improve stress urinary incontinence and pelvic organ prolapse [17, 18] and is recommended by the International Continence Society as Grade A [19]. However, about 30% of women report failure to contract the PFM correctly [20]. Incorrect PFM contraction is not expected to have a training effect. Therefore, it is recommended that proper PFM training should always include an objective assessment of correct contraction.
In the present study, the reliability and validity of PFM strength assessment using the MizCure perineometer were examined in healthy women. Using transvaginal devices, it is known that the measurement of PFM strength depends on the size and placement of the probe, the subject’s cooperation, and the examiner’s experience and skills [21, 22]. If a small perineometer probe is used, the placement of the intravaginal probe causes reliability problems, because the probe may be located not completely adjacent to the pressure zone [23]. Thus, in the current study, whether the MizCure can properly measure PFM strength was verified using the widely reported Peritron.
The within-session intra-rater reliability results of the MizCure and Peritron vaginal pressures were ICC (1, 1) 0.90 to 0.96 for both examiners 1 and 2 (Table 1). The intra-rater reliabilities of between-session testing using the MizCure are shown in Table 2. ICC (1, 1) was 0.72 for the supine measurement and 0.79 for the standing measurement for examiner 1 and 0.63 for the supine position measurement and 0.80 for the standing position measurement for examiner 2. In the present study, intra-rater reliability for the MizCure measurement was slightly lower between sessions than within sessions. Furthermore, the results for inter-rater reliability of the two perineometers were as follows: Peritron and MizCure had good inter-rater reliabilities in Test 1 (1st day), with ICC (2, 1) of 0.87 to 0.96 for both supine and standing positions measurements. In Test 2 (2nd day), MizCure measured in the supine position had an ICC (2, 1) = 0.69, slightly lower than in Test 1.
In the previous study on the intra-rater reliability of the Peritron, the ICC (1, 1) was over 0.9 for the supine and standing position measurements [3]. Therefore, the results of the present study suggest that the within-session intra-rater reliability of the MizCure is as good as that of the Peritron. Rahmani et al. and Ferreira et al. reported the inter-rater reliability of vaginal pressure measurements using the Peritron. Rahmani et al. reported that the between-session ICC (1, 1) was 0.88 [9]. Ferreira et al. reported that the mean vaginal squeeze pressure had good inter-rater reliability, and the difference between the examiners was not significant [10]. In the current study, the between-session intra-rater reliability of the MizCure was slightly lower than that reported by Rahmani et al [9]. This is primarily because of the size and placement of the probes. In the current study, the insertion and placement of the probe were performed by the subject. The MizCure has a smaller probe than the Peritron, and it is difficult to standardize the position of the probe between subjects. The previous study showed that probe placement can affect measurement results [21]. The fact that intravaginal squeeze pressure changes along the vagina [24] means that the profile of vaginal pressure is not fully understood. This suggests that probe placement may have affected reliability. The MizCure probe used in the current study was 79 mm in length. The vaginal high-pressure zone at rest and during PFM contraction is 2 to 4 cm from the vaginal introitus [25]. The vaginal high-pressure zone was shown to be related to the PFM contraction [24]. Therefore, the MizCure probe is considered to cover the high-pressure zone of the vagina. The other factor that may have affected the results for the reliability of vaginal pressure measurements may be the skill of the examiner. With respect to this factor, the two examiners were skilled physical therapists with urologic education. The examiner checked the correct PFM contraction with 2D ultrasound prior to measurement, and always checked to prevent compensatory movement due to PFM contraction (e.g., pelvic tilt or excessive abdominal muscle contractions). Therefore, the effect of the two examiners on the measured value was thought to be small.
ICC reliability criteria are defined as substantial for 0.61 to 0.80 and almost perfect for 0.81 to 1.0 [26]. From this perspective, the present results indicate that the intra-rater reliability of the MizCure within and between days was substantial to almost perfect. The inter-rater reliability was similar. These results suggest that the use of the MizCure perineometer to evaluate PFM strength can minimize the effects of the examiner and provide good intra-rater reliability.
The validity of the MizCure was evaluated using Pearson’s product-moment correlation coefficient and Spearman’s rank correlation coefficient. The correlation between MizCure and Peritron measurements has not been previously reported. The MizCure measurements showed a significant correlation with the Peritron measurements: r = 0.68 for the supine measurements and rs = 0.82 for the standing measurements. For the correlation coefficient, 0.4 to 0.69 is considered moderate and 0.7 to 0.89 strong [27]. Therefore, the correlation coefficients for the MizCure were moderate to high. The results suggest that the MizCure is a suitable tool for measuring PFM strength. There have been reports in the past examining the validity of the Peritron and other perineometers [28]. Barbosa et al. reported the following differences among the other devices: acquisition of higher or lower measurement values depends not only on the diameter of the probes used, but also on other variables, such as individual vaginal diameter, probes of different materials, and differences in the sensitivity of each piece of equipment to vaginal pressure. Therefore, a detailed examination of the characteristics of the MizCure perineometer and the individual’s vagina will be necessary in the future.
The advantage of the MizCure perineometer is that it is easy to purchase, portable, and simple to use. These results suggest that the MizCure is a tool that can quantitatively reflect PFM function, since the reliability and validity of the measured vaginal pressure values are good. This may help select the measurement position for evaluation and treatment purposes and help the treatment plan.
However, as a limitation of this study, the results of the present study are limited to healthy women without PFM dysfunction, such as stress urinary incontinence or pelvic organ prolapse.