Programming concerns with spinal cord stimulation

Scenario

A 50-year-old woman with failed back surgery syndrome (FBSS) has been identified as a spinal cord stimulation (SCS) candidate by her pain management physician and is undergoing a one week trial. She is met shortly before the trial procedure to confirm the pain areas that the physician has evaluated to be treatable with SCS, provide an overview of the procedure, set expectations, and answer questions. She says that she has pain in the low back and both legs including the feet, equal in intensity on both sides of the body. She rates her pain as an 8 out of 10 in the low back and 6 in the legs. The patient is informed that during the procedure, she will be awakened for “intraoperative testing” to ensure that stimulation is covering all of her pain areas or a reasonably large amount of the areas. She will feel a tingling sensation (paresthesia), and she needs to identify where she feels it. The goal is for her to feel the tingling sensation in all of her pain areas if possible. It is also explained to her that sometimes longer programming sessions may be needed to obtain complete coverage and that this may not be feasible in the operating room (OR) but a large proportion of the pain areas can be captured most of the time.

• Variation : Intraoperative testing is not performed

• The physician may choose not to do intraoperative testing and to place the leads over a specific vertebral region. In this case, the patient is informed that she may be asleep for the entire procedure or may not be asked for much feedback during the procedure.

She is informed that SCS does not usually provide 100% pain relief, although sometimes it does, and on average, patients experience 60%–80% pain relief. The patient is informed that after the procedure, she will be provided with a few initial programs to take home as well as details on what will happen during the trial week.

Within the OR, she is given quickly reversible sedation and local anesthetic. In this case, the physician chooses to use two 16-electrode percutaneous leads with 1-mm electrode spacing and a 67-mm electrode span to allow for assessing appropriate levels for coverage of the back and most of the lower extremities during testing in the trial. The 67-mm electrode span typically covers almost three full vertebral levels. The leads are placed paramedial and cover vertebral levels T8 to T10. Before intraoperative testing, impedances of the electrodes are checked, and they are all within the normal range. High-impedance electrodes may not be useable because of system constraints.

• Variation : Two electrodes in the middle of the right lead (electrodes 8 and 9 counting from the distal end of the lead) have high impedance

• The physician wipes the proximal end of the lead that inserts into the cable that connects to the external trial stimulator to clear any material that may be causing the high impedance. Impedances are now normal.

• Variation : Impedances are still high

• The physician moves the right lead in the rostral direction by two electrodes. The impedances move to electrodes 10 and 11, indicating an anatomical issue such as an air bubble. This usually clears after the procedure. During intraoperative testing, these electrodes are not used for programming.

• Variation : Impedances are still high

• The impedances stay high on electrodes 8 and 9, indicating a mechanical issue. The cable that connects the lead to the external trial stimulator is replaced, and the impedances are normal.

• Variation : Impedances are still high

• A different external trial stimulator is used, and the impedances are normal.

• Variation : Impedances are still high

• The lead itself is then replaced, and the impedances are found to be normal.

• During intraoperative testing, a pulse width of 200 μs and a frequency of 40 Hz are used initially. Because the physiological midline does not always coincide with the anatomical midline, stimulating with each lead separately is helpful for identifying the physiological midline and ensuring that stimulation provides bilateral coverage. Stimulation is delivered from the left lead in the middle of T9, and the patient is asked where she feels it. She says she feels it in her left thigh. Stimulation is delivered from the right lead in the middle of T9, and she feels it in her right thigh, confirming that the physiological midline coincides with the anatomical midline and the patient has bilateral coverage.

• Variation : Physiological midline is offset from anatomical midline

• The patient feels abdominal stimulation when the left lead is stimulated and left-sided stimulation when the right lead is stimulated, indicating that the physiological midline is toward the right of the anatomical midline. The physician is informed and attempts to reposition the leads by moving the left lead to the right of the right lead, thus trying to more closely bracket the midline. The leads now provide bilateral stimulation.

  Amplitude is slowly increased until the intensity is strong but not uncomfortable for the patient. She is asked several times during this process where she feels the stimulation in her body and whether the amplitude is high enough or too high. Stimulation often can be moved to the midline by using equal amounts of current on each lead, which stimulates the space between the leads, and the patient reports feeling stimulation equally in both thighs. As the patient is assessed further, she is asked where she feels stimulation, trying to gain coverage all the way into her lower legs and feet, if possible, as well as the low back. Attempts to capture the low back usually require moving the stimulation in the rostral direction. At the caudal aspect of T8, she feels stimulation in her low back. The patient is asked if the stimulation is covering all of her low back pain, and she confirms that it does. Stimulation is then moved in the caudal direction. Again, the patient is frequently asked where she feels stimulation. At the rostral aspect of T10, she feels stimulation in both legs but not in the feet. The pulse width is increased to 250 μs to capture more dorsal column fibers, and she now reports feeling stimulation in the feet. The patient confirms that stimulation is covering all of her leg pain.

  • Variation : Coverage of low back is only at the very top of the lead

  • The patient has low back pain coverage when electrodes 1 and 2 are being used. Because lead migration is possible, especially during a trial when the leads are not usually anchored in place, it is more ideal if the physician moves the lead in the rostral direction by approximately two electrodes.

  • Variation : The patient has focal pain

  • If the patient has focal pain (e.g., only in the left knee), which often happens in complex regional pain syndrome (CRPS), the physician may elect to try to place the lead laterally on the side of pain to target the dorsal root fibers at the vertebral levels corresponding to the pain area (e.g., L4) and leverage their dermatomal selectivity. However, it should be noted that focal pain can still be selectively covered using dorsal column stimulation as long as the appropriate fibers can be selectively targeted.