With the patient in the prone position, the three sacral foramina S2, S3, and S4 are located using bony landmarks (Fig. 17.1). The sacral foramen S2 is typically found just under the projection of the posterior superior iliac spines and approximately one finger-breadth lateral to the median line. The sciatic notches, which correspond to level S3, are identified. S4 is about 2 cm under foramen S3. The foramina of S3 and S4 are also positioned about one finger-breadth across from the median line.
The acute phase test is performed under local anesthesia using a 20-gauge spinal insulated needle [Medtronic™ (Minneapolis, MN) No. 041828-004] and an external neurostimulator (Medtronic model 3625 screener or Medtronic model 3628 dual screener). The needle is inserted perpendicular to the sacrum, with an angle to the skin of 60 to 80 degrees (Fig. 17.2). After the needle is positioned in the chosen foramen, it is connected to the external neu-rostimulator. The stimulation parameters used in the acute phase are a pulse width (PW) of 210 |isec, a frequency of 5 to 25 Hz, and an amplitude that results in an increased contraction of the pelvic floor and a deepening and flattening of the
3065U or model 3057-1) is inserted through the needle, following which the needle is removed. The lead is connected to an external stimulator (screener model 3625) to allow evaluation of the functional responses to the test,both subjectively to continence and objectively using rectoanal manometry. Two to 3 weeks of stimulation is the minimum accepted period for a correct evaluation of the effectiveness of sacral stimulation on constipation. To evaluate the functional results of PNE, patients completed a clinical diary of bowel movements in the 2 weeks preceding, during, and following the PNE.
buttock muscle. This response usually occurs at between 1 and 6 V; simulation of specific sacral nerves typically results in specific movements of the perineum, anal sphincter, and ipsilateral lower extremity. This test ensures correct lead placement. Stimulation of S2 causes some movement of the perineum and the external sphincter along with a lateral rotation of the leg and contraction of the toes and foot. Stimulation of S3 causes a contraction of the pelvic floor and the external sphincter, the "bellows" contraction, and a plantar flexion of the big toe. Stimulation of S4 causes a contraction of the anus with a clamp-like perineal movement without any leg or foot movement. Vesicle, vaginal (or scrotal), and rectal paresthesia may be perceived by the patient during sacral nerve stimulation.
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