A previous study demonstrated that in patients with pain in the pelvic floor muscle (PFM), manual stretching of PFM significantly improved pain and symptoms1). The evaluation of PFM property is generally palpation of the vagina. However, this is a subjective assessment. Therefore, to evaluate the PFM property objectively, appropriate modality should be needed. Recently, several studies have investigated the hardness of PFM using ultrasound elastography in healthy subjects2). In the skeletal muscles of the extremities, hardness has been found to increase in the presence of pain. IC/ bladder pain syndrome is characterized by refractory discomfort referable to the lower urinary tract and associated with urinary urgency, frequency, and pain. However, the relation between PFM hardness in patients with interstitial cystitis (IC) and PFM pain is unknown. From the above, it is expected that in patients with IC, PFM hardness may higher than in healthy adults. The aim of this study is to investigate the differences in muscle hardness of PFM between healthy female adults and patients with IC.

The subjects were 18 women patients with IC (IC group; median: age 74.0 years, BMI 21.4 kg/m2) and 17 healthy women (control group; median: age 26.0 years, BMI 19.6 kg/m2). The measurements were performed in the supine position with the knees flexed. At rest and during PFM contraction, the hardness of the striated urethral sphincter was assessed using real-time tissue elastography (RTE) (ARIETTA 70, HITACHI Aloka Medical, Japan) with a linear transducer (frequency range: 7-3 MHz) placed on the perineum in the mid-sagittal plane (Figure 1a). The transducer was oriented perpendicular to the perineal skin surface and was pressed by free-hand manipulation with constant repeated pressure. The measurements were performed three times. The target region of interest (ROI) (ROI A) was set at 10 mm ventral to the mid-urethra, and 15 mm cranially from the pubic bone (striated urethral sphincter). The reference ROI (ROI B) was set at 5 to 10 mm caudal adipose tissue from ROI A (Figure 1b). The relative hardness of the striated urethral sphincter compared to that of the reference was indicated by a strain ratio, the ROI B divided by the ROI A (B/ A). Between-group (healthy women and patients with IC) comparisons of descriptive variables were made using Mann-Whitney U tests (age, at rest and PFM contraction strain ratio). The Wilcoxon test was used to compare strain ratio at rest and during PFM contraction within the groups. Spearman's rank correlation coefficient was used for the correlation between age and strain ratio (At rest and PFM contraction). Statistical significance was set at 5%.

• The striated urethral sphincter strain ratio at rest was significantly higher in IC group than in control group (p=0.038). The striated urethral sphincter strain ratio during PFM contraction was not significantly different between the two groups.
• The striated urethral sphincter strain ratio in control group was significantly higher during PFM contraction than at rest (p=0.003). The striated urethral sphincter strain ratio in IC group was not significantly changed between the resting and PFM contraction (Figure 2a)
• Age was significantly higher in the IC group than in the control group (p <0.001). There was no significant correlation between age and striated urethral sphincter strain ratio (at rest, during PFM contraction) (Figure 2b).

To our knowledge, this is the first study to investigate the differences in muscle hardness of PFM between healthy women and patients with IC. In this study, we have shown that striated urethral sphincter strain ratio at rest was significantly higher in IC group than in control group. The previous study has reported that increased muscle hardness is a factor in the pain of PFM in the IC3). Muscle hardness is generally found to be significantly higher in patients with low back or neck pain than in asymptomatic patients. Therefore, in patients with IC, pain in the bladder or urethra may lead to increased PFM tone, resulting in increased muscle hardness of the striated urethral sphincter. 
The mechanism in which the hardness of PFM increases is considered as follows; Pain may increase muscle hardness by stimulating α-motor neurons and stimulating muscle contraction. Pressure on blood vessels due to hypertonia results in decreased local blood flow and ischemia. These may cause a vicious circle that sustains muscle contraction.
In the current study, age was significantly higher in IC group compared to the control group. Although, there is no correlation between striated urethral sphincter hardness and age in both control and IC group. From this, we consider that age is unlikely to affect striated urethral sphincter hardness. However, further clinical studies will be necessary to examine the muscle hardness of the striated urethral sphincter in healthy elderly women.

We concluded that striated urethral sphincter hardness is significantly higher in patients with IC than in healthy women.

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