Currently, only one system for sacral nerve stimulation is commercially available (InterStim, Medtronic, Minneapolis, MS). The surgical procedures are identical irrespective of the indication in question. First, a peripheral nerve evaluation test (PNE test) is undertaken to establish whether the patient will benefit from sacral nerve stimulation. Under general anaesthesia, sedation or local anaesthesia one or several sacral foramina are cannulated, usually foramen 3 or foramen 4. When the cannula is inserted through the sacral foramen, the tip will be located immediately adjacent to the corresponding sacral nerve root (fig. 2).
Figure 2: Schematic drawing of cannulation of the third sacral foramen on the left side. The tip of the cannula (arrow) will ideally be placed against the corresponding sacral nerve when the cannula is inserted at an angle of 60° to the skin over the sacrum.
The cannula is supplied with pulsating current from a stimulator. If the operation takes place under general anaesthesia or sedation, the stimulation may lead to contraction of the anal sphincter (fig. 3). If it takes place under local anaesthesia, the stimulation may also cause a sensory response corresponding to the rectum, the bladder or the vagina. If an appropriate response is obtained, an electrode is inserted in the same place using percutaneous technique.
Figure 3: A cannula has been placed in the third sacral foramen on the left side. The cannula is attached to an electrical cable to test the motoric response of the anal sphincter. Photo: Stig Norderval. The patient has consented to publication of the picture.
The electrode is linked to an external stimulator that the patient carries during the test period. The duration of this period depends on the patient’s condition. During the test period, the patient keeps a daily record of the frequency of the symptoms. If the test is carried out using a temporary electrode, this can be easily removed by the patient him/herself once the test period is over. If a permanent electrode has been used, the extension wire emerging through the skin at the flank can be cut at the skin level.
After completion of the test, the frequency of the symptoms prior to and during the test are compared. There is a broad agreement that a PNE test should lead to a reduction in symptoms of 50 % or more before implantation of a permanent stimulator is offered.
Two different stimulators are available – one with a service life of up to ten years and another, considerably smaller model, with a service life of 3 – 5 years. The stimulator is implanted deeply subcutaneously in the left or right buttock, according to the patient’s wishes (fig. 4). If the patient was tested using a temporary electrode, a permanent electrode is inserted during the implantation of the stimulator. If a permanent electrode was used for the test, the stimulator will be attached to this electrode. Patients are supplied with a remote control device that enables them to turn the stimulator on and off, change the voltage and switch between up to four different stimulation programmes.
Figure 4: A permanent stimulator attached to an electrode which has been tunnelled subcutaneously from the third sacral foramen on the right side (arrow) to an established subcutaneous pocket on the right side (stippled arrow). The stimulator is ready to be implanted in the subcutaneous pocket. Photo: Stig Norderval. The patient has consented to publication of the picture.