A Deep Analysis of the Pelvic Floor Motor Response in Sacral Neuromodulation Linking It to Outcome

This study aimed to characterize the pelvic floor muscles (PFM) motor response provoked during sacral neuromodulation (SNM) programming, determining its utility in improving therapy delivery.

This prospective study (January 2018-September 2021) included patients with overactive bladder (OAB) or nonobstructive urinary retention (NOUR) who underwent unilateral SNM. An external pulse generator was connected for three weeks. Success was defined as ≥50% improvement. Sensory threshold (ST), motor threshold (MT), and their ratio (MT-ST) were analyzed. PFM electromyography was recorded using a multiple array probe. A linear regression model with ST, MT, MT-ST, and mean natural log transformation (peak-to peak-amplitude) vs outcome (percentage improvement) was performed. Differences in electrical PFM motor response amplitude between different electrical stimulation levels (ST ± 0.5 mA) and different parts (four sides, three depths) of the pelvic floor were modeled using linear mixed model analysis (LMM).

The study population comprised 64 women (overall success 80%). Responders presented with significantly lower MT and MT-ST (unpaired t-test: p = 0.0271 and p = 0.0158, respectively). MT and MT-ST proved a significant relationship with percentage improvement (linear regression [lin. Regr.] p = 0.0304, R² = 0.0745 and lin. Regr. p = 0.0107, R² = 0.1020, respectively). PFM amplitude showed a significant relationship with percentage improvement for all stimulation amplitudes (ST ± 0.5 mA) (lin. Regr. p < 0.0001, R² = 0.2560), and subsensory stimulation intensities (lin. Regr. p = 0.0008, R² = 0.1673). Responders presented with a different evolution in increase in overall peak-to-peak amplitude over increased stimulation intensities (LMM: p = 0.0160), presenting with a significantly higher slope. This was significantly different depending on the percentage improvement for all the different sides and depths, with contralateral superior being the only exception (LMM: p = 0.0071, range: 0.0663-<0.0001).

A clear correlation was found between therapy efficacy and PFM motor response elicited by unilateral sacral spinal nerve stimulation, linking lead placement and SNM outcome. Responders presented with lower MTs, lower MT-STs, higher compound muscle action potentials, and higher continuous activation of their PFM at subsensory stimulation levels than did nonresponders.