Intraurethral sonography and the test-retest reliability of urethral sphincter measurements in women

PURPOSE: The purpose of this prospective study was to determine the test-retest reliability of urethral sphincter morphologic measurements obtained with intraurethral sonography. METHODS: The cross-sectional urethral sphincter anatomy of 29 asymptomatic nulliparous women was studied in a blinded fashion. Each patient returned for a repeat examination on a different day. At the point of maximal rhabdosphincter thickness, the urethral diameter and circumference and the longitudinal smooth muscle and rhabdosphincter thickness, diameter, circumference, and area were measured using the ultrasound scanner's integrated software. For each measured variable, the reliability between patients was assessed with a paired t test. Intraclass correlation coefficients were calculated to assess the reliability of each intraurethral sonographic measurement obtained from the same patient. RESULTS: On test-retest analysis, the differences for each measured variable between patients were not statistically significant (p > 0.05). Of the measurements obtained from the same patient, however, longitudinal smooth muscle thickness (rho = 0.44; p = 0.006), diameter (rho = 0.49, p = 0.003), circumference (rho = 0.49, p = 0.003), and area (rho = 0.43; p = 0.009) were significantly correlated. CONCLUSIONS: The urethral longitudinal smooth muscle layer is the only structure that can be measured reliably using sonography for diagnostic use. Sonographic measurements of the rhabdosphincter may not be reliable because the outer portion of that structure lies outside the depth of penetration of a 12.5-MHz transducer.