Hypothesis / aims of study
Clinically available neuromodulation therapies in urology either use the sacral nerve roots or the posterial tibial nerve as target for stimulation. Another promising stimulation target is the dorsal genital nerve (DGN). Studies have shown that DGN stimulation can suppress detrusor contraction  and could be used to prevent incontinence when stimulation is started soon after the onset of a detrusor contraction (on demand stimulation). The main hurdle for a clinically available DGN stimulation system is to develop a suitable stimulation electrode that provides a stable long-term electrode-nerve interface. An implanted electrodes is invasive and without good options for fixation the electrode will most likely migrate. Surface patches may work in males but are not suitable for females. A new electrode type, a percutaneous electrode may solve this problem. The percutaneous electrode is anchored to the skin using a fistula like a piercing. In this study, we investigated the suitability the percutaneous electrode and compared this electrode with a standard surface electrode.
Study design, materials and methods
For this study we used healthy volunteers with a nipple piercing, belly button piercing and/or genital piercing. The study was approved by the local ethical committee. Experiments were conducted while the subject was lying on his/her back in a bed. The existing piercing was removed and a surface electrode (3.2 cm diameter, round, Pals, Axelgaard, Denmark) was placed over the area where piercing had been. A return electrode (5x5 cm, square, Pals, Axelgaard, Denmark) was placed about 5 cm away. Both electrodes were connected to stimulator 1 (DS5, Digitimer, UK) for impedance measurement or to stimulator 2 (DS7A, Digitimer, UK) to measure perception threshold and maximum tolerable stimulation amplitude.
Impedance was measured using a sinewave current of 1 mA at 3 different frequencies (0.1 kHz, 1 kHz and 10 kHz). At each frequency the corresponding voltage was measured using an oscilloscope and the impedance was calculated by dividing the peak voltage by 1 mA.
To determine sensation threshold, 200 µs monophasic square current pulses at a frequency of 20 Hz were used. The return electrode was anode while the other was cathode. The amplitude was slowly increased until the subject reported sensation. This was taken as perception threshold. The amplitude was further increased until the subject asked to stop stimulation. This amplitude was taken as maximum tolerance amplitude.
After the measurements for the surface electrode were finished, the surface electrode over the piercing area was removed and a custom made stainless steel rod was inserted into the fistula. Care was taken that the ends of the rod were not in contact with skin. Both metal rod and return electrode were again connected to the stimulator and the measurements for impedance, perception threshold and maximum tolerable level were conducted in the same way as for the surface electrode.
A total of 10 healthy subjects with relevant piercings were included. There were 3 subjects with a genital piercing, 3 with a nipple piercing and 4 with a belly piercing. All experiments were conducted without any complications.
The impedances are shown in Table 1. It shows that for the nipple, surface electrodes had a higher impedance than piercing electrodes. For the other 2 locations, the impedance of surface electrode was somewhat lower than the piercing electrode impedance.
Perception and max tolerance thresholds (Table 2) show that, compared with the surface electrodes, the piercing electrodes has the lowest threshold for perception.
Interpretation of results
It was expected that impedance of the surface electrode was much lower than the impedance of the percutaneous electrode. This because (1) the contact area between tissue and electrode is much larger for the surface electrode and (2) the surface electrode contains gel to reduce impedance. It turned out that the difference in impedance between percutaneous and surface electrode is not very large. The surface electrode impedance at both nipple and belly was higher compared with genital. The reason may be that the surface at both nipple and belly button is not flat. It is therefore likely that not the whole area of the surface electrode was in contact with the skin, which increases the impedance.
The thresholds for perception and tolerance were lowest for the percutaneous electrode while still sufficiently high to allow DGN stimulation if the electrode is at the right location.