Aims: To determine the correlation of InterStim amplitudes required to evoke motor and sensory responses in subjects with overactive bladder (OAB) and to determine if subjects reprogrammed to achieve motor response have improvement in voiding diary parameters.
Methods: Descriptive pilot study of patients with an existing sacral nerve stimulation (SNS) device for OAB. Subjects completed voiding diaries and reported subjective improvement at current settings. Subject's implantable pulse generator was interrogated while surface electromyography (EMG) was performed using a rectal sponge electrode. Electromyography was evaluated for baseline motor response demonstrated by compound muscle action potential (cMAP) at current settings. Stimulation amplitude was reduced to zero then incrementally increased. Amplitudes evoking subject sensation and motor response were recorded and used for correlation calculation. Subjects without baseline motor response were reprogrammed to achieve cMAP, and voiding diaries were completed after reprogramming.
Results: Thirty-one subjects were recruited. Twelve (39%) had motor response (cMAP) at baseline settings. Subjects with motor response were significantly more likely to report sensation of stimulation versus those without (55% vs 17%; P = 0.04). Amplitudes evoking motor response were found to be significantly correlated to those for sensory response (r = 0.90; P < 0.0005). Sixteen of 19 subjects without baseline motor response were successfully reprogrammed to achieve cMAP. Improvements in nocturia, incontinence, and urgency incontinence episodes (14.4%, 19.8%, 18.2%; nonsignificant) were seen in the reprogrammed group.
Conclusion: Sacral nerve stimulation motor and sensory amplitudes were highly correlated in our cohort, and subjects with motor response were significantly more likely to feel sensation of stimulation. This supports the theory that both motor efferent and sensory afferent portions of the sacral nerve may contribute to SNS mechanism.