In histologic studies, aging is associated with a 2% annual decrease in striated urethral sphincter muscle fibers in women after age 50 [1] due to fiber dropout within a dense connective tissue field. It is unknown whether these changes can be quantified using MRI. The aims of this study are to test the hypotheses that 1) striated external urethral sphincter thickness decreases with age and 2) changes in striated muscle density can be assessed using magnetic resonance imaging (MRI).

We conducted a sub-analysis of women undergoing pelvic floor MRI as part of a larger study on aging effects on pelvic floor anatomy. Two groups of nulliparous women, young (<40 yrs) and older (≥ 70 yrs), were included. 3D MRIs were performed using a 3-Tesla Ingenia scanner with 2 mm slice thickness. All women had normal pelvic support, no demonstrable stress incontinence on examination and no history of surgery for prolapse or urinary incontinence. Measurements were made using 3D Slicer. Transverse and anteroposterior measures of the inner and outer diameters of the striated external urethral sphincter were made at six equidistant locations (Figure 1 A). Striated urethral thickness was calculated by subtracting the inner diameter from the outer diameter (Figure 1 B). Striated urethral sphincter length was also measured. Tissue density of urethra sphincter muscle layers were quantified using as a contrast noise ratio (CNR) with vaginal gel as the reference (CNR = (intensity (gel) – intensity(muscle))/standard deviation(noise) [2].

10 women were included, 5 young and 5 older. On average, the older group was 50 years older than the young group (Table 1). Striated urethral thickness did not significantly differ between groups for both transverse and anteroposterior directions except for the distal end in the transverse direction which was 89% larger in the older group (Table 1). However, most thickness measures were larger in the older group.  Striated urethral sphincter length was similar between groups (young: 21.1 ± 1.7 mm vs older: 24.2 ± 4.8 mm, p = 0.25). MRI signal CNRs of both the external striated and internal smooth muscle sphincters were significantly greater in the young versus older group (Table 1). Post-hoc power analysis showed 42% power to detect a difference between groups with α = 0.05.

Aging is associated with changes to MRI signal CNR of the striated urethral sphincter independent of effects on muscle thickness.

Our preliminary results suggest that MRI signal intensity decreases with aging, although the thickness of the region occupied by striated muscle does not. These MRI-visible changes are consistent with histologic findings of fiber dropout seen with aging and represent a way to measure this phenomenon in living women.

Perucchini D, DeLancey JOL, Ashton-Miller JA, Galecki A, Schaer GN. Age effects on urethral striated muscle: II. Anatomic location of muscle loss. Am J Obstet Gynecol. 2002;186(3):356–60.
Knight MJ, McCann B, Tsivos D, Couthard E, Kauppinen RA. Quantitative T1 and T2 MRI signal characteristics in the human brain: different patterns of MR contrasts in normal ageing. Magn Reson Mater Physics, Biol Med. 2016;29(6):833–42.