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CHAPTER 96  Spinal Cord Stimulation

powered by an external transmitter and antenna worn by the patient. The development of lithium primary cells made the in­ ternally powered generator (IPG) feasible in the 1980s; it was marketed as “totally implanted,” although it still required an external remote control device. Early IPGs required surgical replacement when the battery was exhausted, but newer gen­ erators, introduced in 2004, have rechargeable batteries. Most recently, advances in microwave power and microcircuit tech­ nology have led to development of a wireless implant which can be inserted through a needle and which, as it is passive, need not be modified to accommodate new waveforms deliv­ ered by the external transmitter. Since the 1980s, all generators have allowed noninvasive post­ operative reassignment of each electrode contact (to anode, cath­ ode, or off), and generators now support up to 32 independent contacts. The newest “multichannel” generators are capable of si­ multaneously or sequentially delivering pulses with different am­ plitudes and widths to different combinations of contacts, which of course consumes more energy compared with the simpler gen­ erators and thus motivates use of the rechargeable devices. Factors that dictate the choice of stimulator/power generator include the patient’s ability to control the device, the amount of power required, and patient convenience. The number of contact combinations possible with either a sur­ gical plate/paddle or percutaneous catheter electrode has grown markedly as the number of contacts has increased. In fact, more programming options exist than can be tested (e.g., whereas a four-contact electrode offers 50 functional bipolar combi­ nations of anodes and cathodes, an eight-contact electrode in­ creases the choices to 6050). Thus, all three SCS manufacturers employ computerized equipment to find the best options for an individual patient and save these settings for subsequent use. Not only is initial programming important, skillful reprogram­ ming can compensate for changes in impedance (e.g., due to postural changes or fibrosis), electrode migration, and changes in pain location or intensity. Although each manufacturer’s adjustment system is unique, each asks the patient to describe the area of pain and pares­ thesia and (for conventional SCS) to rate pain overlap and intensity at various settings. Figure 96.8 shows a prototype developed in the 1990s to automate this process and interact directly with patients. Patient Management The patient should have a postoperative surgical check and SCS adjustment and on postoperative day 7 to 14 return for su­ ture or staple removal and any needed additional adjustment. PROGRAMMING A SPINAL CORD STIMULATION SYSTEM

From that point on, monthly visits should gradually taper to yearly visits. Patients should understand that SCS is expected to reduce, not eliminate, pain. It is, therefore, important for clinicians to remember that other pain treatments remain available, espe­ cially those designed to treat nociceptive pain, for which SCS might not be so effective. On/off time has a direct effect on battery longevity, but there are no studies of the impact of an imposed duty cycle on pain relief. In some patients, pain relief persists for a week or more after the device is turned off; others must operate the stimula­ tor continuously. It is possible but rare for an SCS patient to achieve complete resolution of pain and request removal of the system. Stimulation can induce subtle loss of normal sensation in SCS patients, but this loss is not sufficient to cause undesirable side effects, such as Charcot joints. 4,190 A change in paresthesia intensity corresponding to a change in posture is a normal, gen­ erally benign, side effect of SCS related to normal movement of the spinal cord with respect to the electrode(s). 191 Automated methods have been developed to mitigate this. 11,12 These side effects might, however, make certain activities haz­ ardous; thus, we instruct patients to turn off SCS before driving a car or climbing a ladder, etc. The incidence of SCS side effects increases as stimulation amplitude and recruitment increase. 192 As is standard practice, except in emergency cases that re­ quire immediate attention, a physician has the discretion to ac­ cept or reject any new patient who was implanted elsewhere. The device manufacturers provide all SCS patients with identi­ fication cards with the necessary contact information to facili­ tate appropriate emergency treatment. SPINAL CORD STIMULATION PATIENT PRECAUTIONS SCS systems might affect or be affected by electromagnetic fields, and this necessitates certain precautions. When MRI was first introduced, it was generally contraindicated in SCS patients; some systems are now “MRI contingent.” Manufac­ turers generally recommend that implants be disabled before a patient enters an electromagnetic field produced by antitheft devices, a metal detector, or any other security scanning device. A patient with an IPG must also refrain from scuba diving more than (typically) 10 m deep or entering hyperbaric chambers with an absolute pressure above (typically) 2.0 atmospheres. Everyday precautions include avoiding placing excessive me­ chanical stress on the system. Special steps must be taken before an SCS patient undergoes certain routine medical tests (such as cardiac monitoring) or radiation therapy with the pulse generator in the active field. Radiofrequency ablation, electrocautery, and lithotripsy also require caution. Ultrasound over the device and diathermy in any location are contraindicated in SCS patients.

Usage Threshold

Stimulation Overlap Rating

Stimulation Drawing

More

Increase the power until you feel stimulation over the largest area that STILL FEELS COMFORTABLE. When finished, press Done.

Please rate how much of your pain is being covered or overlapped by stimulation by marking the line below.

R

L

L

R

When finished, press Done.

Please outline the areas where you feel stimulation To start over, press Erase, when finished, press Done.

1 2 3 4 5 6 7 8 9

No overlap

Complete overlap

Less

STIM OFF

STIM OFF

STIM OFF

Done

Done

Erase

Done

FIGURE 96.8  This computerized patient-interactive system allows the patient to (1) adjust amplitude to a specified level, (2) draw the area of paresthesia to be compared with a pain drawing, and (3) rate pain/paresthesia overlap.