The urethra during pelvic floor contraction: observations on three-dimensional ultrasound

To investigate with three-dimensional ultrasound how voluntary pelvic floor contractions influence the morphology of the female urethra's components.

Twenty female patients with benign gynecologic disorders (mean age: 29 years; range: 19-40) had transrectal sonography using a 7.5-MHz mechanical sector endoprobe with three-dimensional features during both pelvic floor muscle relaxation and pelvic floor muscle contraction. The multiplanar display of the scanned volumes allowed detailed morphologic assessment of the urethra and the measurement of distances and volumes of the urethral components. Statistical end points were maximum sagittal and transverse urethral diameter, maximum sphincter length and thickness, maximum smooth muscle thickness, and the volumes of the sphincter, the smooth muscle, and the entire urethra.

All 20 rectal scans were feasible. Two patients had to be excluded from analysis because of poor image quality, leaving 18 patients for evaluation. When compared with pelvic floor relaxation, the following measures were smaller during pelvic floor contraction: sagittal urethral diameter (10.4 versus 11.5 mm; P =.004), transverse urethral diameter (14.1 versus 15.0 mm; P =.009), urethral sphincter thickness (2.4 versus 2.7 mm; P =.012), urethral sphincter volume (0.5 versus 0.6 mL; P =.003), and total urethral volumes (1.4 versus 1.5 mL; P =.007). Sphincter length and smooth muscle thickness, as well as smooth muscle volume, did not change significantly during pelvic floor contraction.

On three-dimensional ultrasound, the morphologic changes of the female urethra during pelvic floor contraction suggest external compression of the urethra rather than contraction of the sphincter muscle.