The approach to the DGN was practised in a male and female cadaver. Needle electrodes were inserted at several positions close to the pubic bone in the direction of the DGN. The DGN is a branch of the pudendal nerve. It appears from underneath the pubic bone after it leaves Alcock's canal (Figure 1). The DGN continues bilaterally on the dorsal aspect of the penis. The electrode was directed in between the base of the penis and the pubic bone along the suspected course of the DGN. The position of the electrode in relation to the pubic bone and the DGN was determined by dissections of the perineum.
A study in four male and four female healthy volunteers was conducted to determine the feasibility of needle electrode insertion for stimulation. Volunteers were in the supine position. The skin was disinfected and sterile covered. Local anaesthetics were applied to the skin at the electrode suprapubic insertion site. A surface electrode was attached to the lower abdomen to serve as anode. A needle electrode (041828, Medtronic, Minneapolis, MN, USA) was connected as cathode. Both a midline insertion and lateral-to-midline insertion of the electrode were tried (Figure 1).
Sensation to stimulation and the GAR, monitored by anal palpation or anal pressure measurements using a pressure balloon catheter, were used to position the electrode near the DGN. Electrical stimulation (Medelec Synergy, Oxford Instruments Medical, UK) with pulse train stimulation (biphasic, rectangular, 20 Hz, pulse width 200 μs, 0-25 mA) was used in all eight volunteers, and burst stimulation (1 Hz, five pulses per burst, interpulse interval 4 ms, biphasic, rectangular, pulse width 200 μs, 0-10 mA) in four out of eight volunteers in additional sessions. Burst stimulation was used in these four patients, as GAR activation is more effective with burst stimulation compared with pulse train stimulation.
Two complete and six incomplete SCI patients with IDCs at a bladder volume below 400 ml were included to verify the feasibility of electrode insertion and effect of conditional stimulation on IDCs. IDC was defined as an involuntary detrusor contraction not related to desired and voluntary voiding. Patients stopped anticholinergic treatment 5 days before the study date.
A comparable study setting and method for electrode insertion as in healthy volunteers were used (Figure 1). Detrusor and abdominal pressures were measured using water-filled catheters. An additional pressure balloon catheter was positioned anally to monitor anal contractions.
The bladder was emptied before needle electrode insertion. The skin of the insertion site was disinfected, sterile covered and locally anaesthetized. The electrode was inserted craniolaterally to the os pubis and directed inferiomedially over the pubic bone towards the base of the penis or clitoris, according to the lateral-to-midline approach in healthy volunteers (Figure 1).
The nearer the tip of the electrode to the DGN, the lower the threshold amplitude to elicit the GAR with burst stimulation. In addition, patients were asked to describe the site where they sensed stimulation. Two surface electrodes were used in patient 3 on the dorsal site of the penis, as the GAR could not be evoked using the needle electrode. In the last four patients, surface electrodes on the dorsum of the penis or the clitoris were used to test the presence of the GAR, and the resulting anal pressure responses before needle electrode insertion.
The needle electrode was repositioned until the minimum amplitude to elicit the GAR was comparable to the results in the study with healthy volunteers and sensation to stimulation was described in the glans penis or clitoris, and until repositioning of the electrode did not further improve these results. The inserted electrode was kept in this final position manually. The lowest pulse train stimulation that resulted in sensation to stimulation (sensation threshold) and maximum tolerable amplitude were determined in this final position by stepwise amplitude changes (0.05-2 mA). The sensation threshold was checked in between fillings with conditional stimulation, and the electrode was repositioned accordingly if there was a change in sensation to stimulation.
After needle insertion, the bladder was filled with water at room temperature at a filling rate of 20 ml min. Pulse train stimulation at maximum tolerable amplitude was started manually at an IDC when bladder pressure increased with 10 cmHO. Stimulation was switched off after an IDC was suppressed. IDC was defined as suppressed when detrusor and vesical pressure returned back to or decreased to near baseline pressure owing to conditional stimulation. After the bladder had been emptied partially, filling was started again with conditional stimulation at a lower amplitude. These stimulation sessions were repeated 2-5 times in each patient. A control cystometry was carried out before needle insertion and after the cystometries with conditional stimulation. Cystometry was stopped at maximum cystometric capacity or at a bladder volume of 500 ml or at a sustained IDC.
Studies on both healthy volunteers and SCI patients were approved by the local ethical committee. We certify that all applicable institutional and governmental regulations regarding the ethical use of human volunteers were followed during the course of this research.