Alterations of cortical electrical activity in patients with sacral neuromodulator

Sacral neuromodulation represents chronic stimulation of the sacral (S3) nerve. So far, the mode of action and neuro-anatomical basis is unclear. Sacral reflex mechanisms as well as pontine or cortical centers of modulation have been postulated. Our aim was to evaluate possible alterations in electroencephalogram (EEG) activity as an indicator of a supraspinally mediated mechanism of sacral neuromodulation.

We analyzed serial EEGs (apparatus: Kölner Vitaport System) using electrodes placed at Fz, Cz, Cz' and Pz in 10 patients. Subsequently, the sacral (S3) nerve was stimulated by means of an impulse generator (Medtronic, Interstim 3023) using an on-off paradigm with a 1.5s "on" interval followed by a 10s stimulation break. Raw data were analyzed using both Matlab 4.0 software and a specially developed averaging routine.

All patients demonstrated a cortical potential complex following sacral root stimulation with an early electronegative component at 50 ms with a mean amplitude of 23 microV followed by a late potential component with a mean latency of 253 ms and a mean amplitude of 5 microV, both with a maximum at Cz, corresponding to the post-central gyrus. This finding occurred irrespective of patient's reports of actually feeling the neuromodulator being switched on and off.

In neuromodulation responders, both short and long latency cortical potentials can be reproduced with a maximum at the sensory cortical area. Although these potentials are similar to cognitively mediated "event-related potentials", they are clearly distinct from any subjective sensory or even painful response since several patients of this series have not been able to feel any neuromodulator action. Therefore, this pilot study indicates a supraspinally mediated site of modulation, most probably in sensory cortex areas.