Imaging the urethral sphincter with three-dimensional ultrasound

Abstract

Objective: To image the striated urethral sphincter (rhabdosphincter) using three-dimensional ultrasound and to compare its size in women with genuine stress incontinence and continent controls. Methods: Women with no previous continence or prolapse surgery had transvaginal sonography using a 7.5-MHz mechanical sector endoprobe with real-time and three-dimensional facilities. Three perpendicular planes were displayed simultaneously on the screen. Manipulation of the stored images allowed detailed morphologic assessment of the urethra at different levels using several parallel cross-sectional planes along its length. The length, thickness, and volume of the rhabdosphincter were measured. Results: Forty-six women with genuine stress incontinence (27–73 years, mean 48) and 48 continent controls (23–76 years, mean 49) were studied. In the transverse plane, the inner part of the urethra, which consists of urothelium and smooth muscle, appeared hyperechogenic compared with the outer hypoechogenic ring of striated muscle. The two groups studied were not different in age and parity. Women with genuine stress incontinence, compared with continent controls, had a significantly shorter (mean ± standard deviation 16.9 ± 1.9 mm compared with 19.2 ± 3.6 mm; P = .001), thinner (2.1 ± 0.5 mm compared with 2.5 ± 0.4 mm; P < .001), and smaller volume (0.8 ± 0.2 mL compared with 1.2 ± 0.2 mm; P < .001) of the striated urethral sphincter. There was a close correlation between the urethral sphincter volume and the degree of incontinence assessed on videocystourethrography ( r = −.65, P < .001). Conclusion: Three-dimensional ultrasound allowed examination of the female urethra in planes that could not be visualized by conventional sonography. The rhabdosphincter had a smaller volume in women with genuine stress incontinence than in continent women.