Study design, materials and methods
Thirty 8-week-old female Sprague-Dawley (SD) rats weighing 200-220g were randomly divided into three groups: Control(n = 10), NB(n = 10), and NB+SNM (n = 10). In the Control group, the T9-T10 vertebral lamina was removed to expose the spinal cord without further injury. In the NB group, the right T9-T10 spinal cord was hemisected. In the NB+SNM group, after hemisection of the right T9-T10 spinal cord, a specialized stimulation electrode was implanted through the S2 foramen(Figure 1). On the second postoperative day, the NB+SNM group received daily electrical stimulation for 1 hour, which continued for 3 weeks. Bladder function was assessed using ultrasound, cystography, and urodynamic measurements. Bladder tissue samples were collected for HE and Masson staining.
Results
Bladder ultrasound revealed significantly higher bladder volume in the NB group compared to the Control and NB+SNM groups[(4.02±0.16)ml vs(0.37±0.09)ml, (1.00±0.16)ml, P<0.0001], which was confirmed by cystography (Figure 2A). The bladder weight in the NB group were also significantly greater than that in the Control and NB+SNM groups[(0.66±0.26)g vs(0.11±0.01)g, (0.28±0.07)g, P<0.01]. Urodynamic analysis showed that the voiding contraction interval in the NB+SNM group was shorter than that in the Control group but longer than that in the NB group[(2.47±1.27)min vs(5.92±0.46)min, (1.01±0.45)min, P<0.001]. During bladder filling, the NB group exhibited multiple non-voiding contraction waves(4.33±3.20) with a maximum pressure of (18.61±4.46)cmH2O, whereas no significant non-voiding contractions were observed in the NB+SNM group 0(0,1.25), with a maximum pressure of 0.00(0.00,12.89)cmH2O (Figure 2B).These differences were statistically significant (P<0.05). Compared to the Control and NB+SNM groups, the NB group displayed disorganized bladder mucosal epithelial cells, loose and disrupted lamina propria, tissue edema, and increased fibrosis (Figure 3).
Interpretation of results
In this study, the NB group showed increased bladder capacity in ultrasound compared to the Control and NB+SNM groups. The number of non-voiding contraction waves during the filling phase was significantly higher in the NB group than in the NB+SNM and Control groups, and the voiding contraction interval was shorter than in the NB+SNM and Control groups. However, both the NB and NB+SNM groups exhibited significantly higher maximum bladder capacity and bladder compliance than the Control group. The NB group also had a lower leak point pressure compared to the Control and NB+SNM groups, which may be attributed to the relatively short experimental duration, preventing bladder contracture and severe fibrosis in the NB group. The NB group had significantly greater bladder weight than the Control and NB+SNM groups, along with abnormal gross morphology and imaging findings. HE staining revealed disorganized muscle bundles, loss of muscle fibers, and residual cytoplasmic vacuolization in the NB group. Masson staining indicated subserosal fibrosis in the NB group, whereas these pathological changes were alleviated in the NB+SNM group. These findings suggest that the NB group exhibited overactive bladder during the storage phase, along with structural remodeling and fibrosis, all of which were suppressed by SNM.