Neural pathway of bellows response during SNM treatment revisited: Conclusive evidence for direct efferent motor response.

Background: In sacral neuromodulation (SNM) patients, it is thought the bellows response elicited upon sacral spinal nerve stimulation is reflex-mediated. Therefore the mechanism of action of SNM is considered to be at the spinal or supraspinal level. These ideas need to be challenged.

Objective: To identify the neural pathway of the bellows response upon sacral spinal nerve stimulation.

Design, Setting, And Participants: Single tertiary center, prospective study (December 2017-June 2019) including 29 patients with overactive bladder refractory to first-line treatment.

Intervention: Recording of the pelvic floor muscle response (PFMR) using a camcorder and electromyography (EMG) (intravaginal probe and concentric needles) upon increasing stimulation during lead or implantable pulse generator placement.

Outcome Measurements And Statistical Analysis: The lowest stimulation intensity needed to elicit a visual PFMR and electrical PFMR was determined. Electrical PFMRs were subdivided according to their latency.

Outcome: the association between visual and electrical PFMRs. Statistical analyses were performed using the weighted kappa coefficient.

Results: Three different electrical PFMRs could be identified by surface and needle EMG, corresponding with a direct efferent motor response (R1), oligosynaptic (R2), and polysynaptic (R3) afferent reflex response. Only the R1 electrical PFMR was perfectly associated with the visual PFMR (κ = 0.900).

Conclusions: The visual PFMRs upon sacral spinal nerve stimulation are direct efferent motor responses. A reopening of the discussion on the mechanism of action of SNM is possibly justified.