Anatomical variation and morphological differences in young and older nulliparous urethras

Objectives: Recent Level I evidence reveals that low urethral closure pressure is the single factor most responsible for stress urinary incontinence. Studies of nulliparous women across the age spectrum reveal three-fold variation in closure pressure even among continent nulliparous women of the same age and 15% decline per decade. The structural basis for these facts is unknown.

Objective: Measure variation in urethral morphology and age-related differences between young and older nulliparas.

Methods: A convenience sample of younger (age < 40 years) and older (age ≥ 70 years) healthy continent research subjects was selected from an MRI repository (REP00000220, HUM00194834). MRI had a 2 mm slice thickness. The cross-sectional shape was determined at 6 mm intervals from the external urethral meatus (EUM) to the internal urethral meatus (IUM) in 3D Slicer using the closed curve markups feature of 3D Slicer in the axial plane. Based on prior work (Umek, 2003), the following regions were defined: fibrous “nozzle”, arch muscles (which are the compressor urethrae and the urethrovaginal sphincter), striated sphincter and vesical neck (Figure 1). For each subject, length of each region was calculated; for the striated muscle region (arch muscles and striated sphincter), cross-sectional area and volume were calculated using Python. Comparisons between groups were performed.

Results: The younger group (n = 9) was 24.2±3.5 y.o. (range: 20-32) and the older group (n =11) was 74.4±4.4 y.o. (range: 70-84), with similar BMI. Regional lengths were measured for all subjects, but cross-sectional area and volume were only measured in six younger women and in ten older women, due to the absence of 2 mm axial scans. Variation in nozzle length was 3.9- and 3.5-fold in the younger and older group, respectively; the length of the arch muscles varied 1.9- and 2.3-fold; the length of the striated sphincter varied 1.9- and 4.7-fold and the length of the vesical neck varied 6.4- and 8.6-fold (Figure 2). The cross-sectional area of the muscular urethra region varied 1.5- and 2.9-fold and the volume varied 2.5- and 6.0-fold for younger and older group, respectively (Figure 2). The urethras of younger and older women are not statistically different, however, younger women tend to have striated sphincters that are longer (+22%, p = 0.08), but narrower (-33%, p =0.08), lending to similar muscular urethra volumes (Figure 2).

Conclusions: There is substantial variation (up to 8.6-fold) among nulliparous women in the length, cross sectional area and volume of the urethra even in the absence of potential effects of childbirth. The urethras of younger women tend to be longer and narrower, lending to similar muscular urethra volumes. Comment: Differences in urethral development, their relationship to function, and their role in incontinence deserve further study. (

Funding: NIH-RC2-DK-122379)