One of the hypotheses for female stress urinary incontinence is that the sphincter deficiency in incontinent women results from decreased sphincter muscle volume, which is a factor of age, hormonal and neurological status, and parity. Our purpose was to evaluate the urethral sphincter in women with urinary incontinence and age-matched continent volunteers, using intraurethral MRI that allowed detailed visualization of the urethral sphincter muscular layers.

We evaluated 10 incontinent women (mean age 59.7 ± 14.5) and two groups of continent volunteers, one group of 8 volunteers was age-matched to the incontinent group (mean age 51.7 ± 5.9), and the second group of 6 volunteers was not age-matched, mean age 43.8 ± 5.8. We imaged patients using 1.5T GE scanner and 14F endourethral coil. Diagnostic sequences included T2-weighted FSE axial, coronal, and sagittal images (TR/TE 3000-6300/60-75 ms, slice/space 2.5-3.0/0.5-2.0 mm, FOV 5-7 cm, 6-8 NEX, 256x256). We measured the urethral length, thickness of the striated and smooth muscle layers, and area of the muscles at the proximal, mid, and distal urethra. We compared the sphincter muscle sizes between the incontinent patients and two groups of volunteers. We used the student's T-test to assess for significant differences between the patient groups.

Intraurethral MRI allowed detailed visualization of the urethral sphincter striated and smooth muscle layers on T2-weighted images. When the sphincter muscle size in incontinent patients was compared to that of age-matched volunteers group, there was no significant difference in the striated or smooth muscle layers. However, when the sphincter muscle size in incontinent patients was compared to that of non age-matched volunteers group, there was a significant difference (p < 0.05) in the size of the striated muscle in the mid anterior urethra and lower posterior urethra, and in total urethral length.

Although, the decrease in urethral sphincter volume has been associated with stress incontinence it is also associated with normal aging. Urethral support system may play a more significant role in maintenance of urinary continence than the size of the sphincter.