Application of shear-wave elastography to estimate the stiffness of the male striated urethral sphincter during voluntary contractions

Objectives To investigate whether increases in stiffness can be detected in the anatomical region associated with the striated urethral sphincter (SUS) during voluntary activation using shear-wave elastography (SWE); to identify the location and area of the stiffness increase relative to the point of greatest dorsal displacement of the mid urethra (i.e. SUS); and to determine the relationship between muscle stiffness and contraction intensity. Subjects and Methods In all, 10 healthy men participated. A linear ultrasound (US) transducer was placed mid-sagittal on the perineum adjacent to a pair of electromyography electrodes that recorded nonspecific pelvic floor muscle activity. Stiffness in the area expected to contain the SUS was estimated via US SWE at rest and during voluntary pelvic floor muscles contractions to 5%, 10% and 15% maximum. Still image frames were exported for each repetition and analysed with software that detected increases in stiffness above 150% of the resting stiffness. Results Pelvic floor muscle contraction elicited an increase in stiffness above threshold within the region expected to contain the SUS for all participants and contraction intensities. The mean (SD) ventral-dorsal distance between the centre of the stiffness area and region of maximal motion of the midurethra (caused by SUS contraction) was 5.6 (1.8), 6.2 (0.8), and 5.8 (0.7) mm for 5%, 10% and 15% maximal voluntary contraction, respectively. Greater pelvic floor muscle contraction intensity resulted in a concomitant increase in stiffness, which differed between contraction intensities (5% vs 10%, P < 0.001; 5% vs 15%, P < 0.001; 10% vs 15%, P = 0.003). Conclusion Voluntary contraction of the pelvic floor muscles in men is associated with an area of stiffness increase measured with SWE, which concurs with the expected location of the SUS. The increase in stiffness occurred in association with an increase in perineal surface electromyography activity, providing evidence that stiffness amplitude relates to general pelvic floor muscle contraction intensity. Future applications of SWE may include investigations of patient populations in which dysfunction of the SUS is thought to play an important role, or investigation of the effect of rehabilitation programmes that target this muscle.