Abstract
Intra-operative electrode placement for sacral neuromodulation (SNM) relies on visual observation of motor contractions alone, lacking complete information on neural activation from stimulation. This study aimed to determine whether electrophysiological responses can be recorded directly from the S3 sacral nerve during therapeutic SNM in patients with fecal incontinence, and to characterize such responses in order to better understand the mechanism of action (MOA) and whether stimulation is subject to changes in posture. Eleven patients undergoing SNM were prospectively recruited. A bespoke stimulating and recording system was connected (both intraoperatively and postoperatively) to externalized SNM leads, and electrophysiological responses to monopolar current sweeps on each electrode were recorded and analyzed. The nature and thresholds of muscle contractions (intraoperatively) and patient-reported stimulation perception were recorded. We identified both neural responses (evoked compound action potentials) as well as myoelectric responses (far-field potentials from muscle activation). We identified large myelinated fibers (conduction velocity: 36–60 m/s) in 5/11 patients, correlating with patient-reported stimulation perception, and smaller myelinated fibers (conduction velocity <15 m/s) in 4/11 patients (not associated with any sensation). Myoelectric responses (observed in 7/11 patients) were attributed to pelvic floor and/or anal sphincter contraction. Responses varied with changes in posture. We present the first direct electrophysiological responses recorded from the S3 nerve during ongoing SNM in humans, showing both neural and myoelectric responses. These recordings highlight heterogeneity of neural and myoelectric responses (relevant to understanding MOA of SNM) and confirm that electrode lead position can change with posture.
Highlights
Sacral Neuromodulation (SNM, referred to as sacral nerve stimulation), was first established as a therapy for treatment of refractory fecal incontinence (FI) in 1995 (Matzel et al, 1995)
When the response was only observable on one channel due to low signal-tonoise or signal-to-artifact ratios, the conduction velocity could not be assessed. This is the first study presenting evoked electrophysiological responses recorded from the human sacral nerve during active stimulation
We recorded neural responses of varying conduction velocities as well as myoelectric responses. These electrophysiological responses showed a large degree of inter-patient variability indicative of fundamental differences in the nature of the fibers activated in each patient
Summary
Sacral Neuromodulation (SNM, referred to as sacral nerve stimulation), was first established as a therapy for treatment of refractory fecal incontinence (FI) in 1995 (Matzel et al, 1995). A recent retrospective study showed that efficacy is maintained in 45% of patients at 10 years post-implantation (Desprez et al, 2020). Refinement of surgical techniques and intraoperative testing for a motor response both aim to minimize the distance between the electrodes and the sacral nerve. While this ensures that the stimulus reaches some fibers of the sacral nerve, testing for a motor response alone gives a very limited account of the fibers being activated by stimulation. A recent study comparing SNM trial success in patients with overactive bladder, urinary retention and FI found no continuous association between the motor thresholds and trial success (Adelstein et al, 2019). Lead migration post-implantation is a common occurrence which requires reprogramming and can lead to loss of efficacy (Ezra et al, 2020)
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