Increased duration of simulated childbirth injuries results in increased time to recovery

Abstract

Stress urinary incontinence (SUI) development is strongly correlated with vaginal childbirth, particularly increased duration of the second stage of labor. However, the mechanisms of pelvic floor injury leading to SUI are largely unknown. The aim of this study was to determine the effects of increased duration of vaginal distension (VD) on voiding cystometry, leak point pressure testing, and histology. Sixty-nine virgin female rats underwent VD with an inflated balloon for either 1 or 4 h, while 33 age-matched rats were sham-VD controls. Conscious cystometry, leak point pressure testing, and histopathology were determined 4 days, 10 days, and 6 wk after VD. The increase in abdominal pressure to leakage (LPP) during leak point pressure testing was significantly decreased in both distension groups 4 days after distension, indicative of short-term decreased urethral resistance. Ten days after VD, LPP was significantly decreased in the 4-h but not the 1-h distension group, indicating that a longer recovery time is needed after longer distension duration. Six weeks after VD, LPP was not significantly different from sham-VD values, indicating a return toward normal urethral resistance. In contrast, 6 wk after VD of either duration, the distended rats had not undergone the same increase in voided volume as the sham-VD group, suggesting that some effects of VD do not resolve within 6 wk. Both VD groups demonstrated histopathological evidence of acute injuries and tissue remodeling. In conclusion, this experiment suggests pressure-induced hypoxia as a possible mechanism of injury in vaginal delivery.

Fig. 1

Cystometry results 4 days, 10 days, and 6 wk after a sham, 1 h, or 4 h vaginal distension. A: voided volume. B: bladder baseline pressure. C: increase in bladder pressure for voiding. Each box plot represents the median and 10th, 25th, 75th, and 90th percentiles of data from 11 or 12 individual animals. Paired numbers indicate a significant difference between 2 groups (P < 0.05).

Fig. 2

Leak point pressure testing results 4 days, 10 days, and 6 wk after a sham, 1-h, or 4-h vaginal distension. A: bladder baseline pressure. B: increase in abdominal pressure to leak (LPP). Each box plot represents the median and 10th, 25th, 75th, and 90th percentile of data from 11 or 12 individual animals. Paired numbers indicate a significant difference between 2 groups (P < 0.05).

Fig. 3

Light micrographs of urethra 4 days after 1-h vaginal distension (A) and sham vaginal distension (B). Stain = Masson’s trichrome. Magnification = ×4. EUS, external urethral sphincter; L, urethral lumen; SM, smooth muscle.

Fig. 4

Light micrographs of urethra, bladder, and vagina 10 days after a sham (A, D, G), short-duration (1 h; B, E, H), or long-duration (4 h; C, F, I) vaginal distension. Stain = Masson’s trichrome. Magnification = ×4(A, B, C, G, H, I) or ×10 (D, E, F). BC, bladder cavity; EC, extravasated cells; Sb, submucosa; SV, small vessel; V, vaginal epithelium.

Fig. 5

Light micrographs of urethra 6 wk after a sham (A, D), short-duration (1 h; B, E), or long-duration (4 h; C, F) vaginal distension. Stain = methylene blue azure II. Magnification = ×4 (A-C), ×20 (D-F). St, striated muscle; U, urethral submucosa; arrow, blood vessel.