Neuromodulation Techniques for the Treatment of Pain

Milan P. Stojanovic

Divinum est sedare dolorem—“It is divine to allay pain.”

—Galen, 129–199

In recent years, complex interventions for pain control have become part of everyday practice in the pain clinic setting. Although more invasive than nerve blocks, these interventions have the advantage of not being neurodestructive. Unlike nerve ablation, these complex interventions may be reversible and therefore more appropriate for use in patients with nonmalignant pain. The clinical efficacy of these approaches has been widely documented in literature. In carefully selected patients, these interventions can reduce pain and suffering, increase functional status, decrease oral medication intake, and ensure early return to normal day-to-day activities. In comparison with more conservative measures for pain control, interventional treatments may appear costly. However, assuming that good outcome is achieved, the overall cost of interventions can be lower than that for conservative measures (as a result of decreased spending on medications and emergency room visits and fewer absences from work, etc.). It is important that the implementation of these interventions be integrated into a multidisciplinary treatment plan. Benefit from these procedures is achieved through careful evaluation of scientific evidence, good clinical judgment, and excellent technical skills.

I. SPINAL CORD STIMULATION FOR CHRONIC PAIN

Electrical stimulation for the treatment of pain was first documented in 600 BC; the process utilized electrical power from the torpedo fish. Renewed interest in pain medicine arose in 1967 when spinal cord stimulation (SCS) was introduced by Shealy et al. Their work was based on the Melzack and Wall “gate control” theory of pain. Initially, SCS implantation involved an open
laminectomy. With advances in technology, SCS became minimally invasive, and is currently performed percutaneously. Further improvements in hardware design and patient selection have increased efficacy, with recent published success rates of 50% to 70%. As an alternative to SCS, peripheral nerve stimulation (PNS) can be performed in patients with localized neuropathic pain.

1. Mechanism of Action

The Melzack and Wall gate control theory of pain was the foundation for initial SCS trials. It was based on the idea that stimulation of A-β fibers closes the dorsal horn “gate” and reduces the nociceptive input from periphery. However, it seems that other mechanisms actually play a more important role. One proposed mechanism involves increased dorsal horn inhibitory action of neurotransmitters such as γ-amino butyric acid (GABA) and adenosine A-1 during SCS. Activation of descending analgesia pathways by serotonin and norepinephrine is another proposed mechanism. In patients with peripheral ischemic pain, the SCS may act by a combination of two mechanisms: suppression of sympathetic activity and suppression of a calcitonin gene–related peptide (CGRP). SCS also relieves angina in ischemic heart disease, probably via redistribution of the coronary blood flow.

2. Indications and Patient Selection

Patients with complex regional pain syndrome (CRPS) and patients with extremity neuropathic pain are the best candidates for SCS. Published reports reveal an excellent long-term success rate for SCS in patients with CRPS, with a reported efficacy of 50% to 91%, and a decrease in analgesic consumption by 50%. A recent study suggests that patients with good response to sympathetic block before SCS are more likely to have a positive response during their SCS trial and long-term pain relief after placement of a permanent SCS device. Phantom limb pain, stump pain, and spinal cord injury pain seem unresponsive to SCS. The likely explanation is that central nervous system (CNS) remapping, which may be critical to the development of these pain syndromes, is not affected by SCS. Diabetic neuropathy may respond well to SCS. However, the risk of infection in these patients is higher than in patients who do not have diabetes. The use of SCS in postherpetic neuralgia is controversial.

Patients with failed back surgery syndrome (FBSS) may respond well to SCS. It has been documented that patients with FBSS respond better to SCS than to a second operation. This applies in particular to low back pain (LBP) with a radiating component to the leg. In these patients, the chances of long-term success with SCS vary from 12% to 88%, with an average efficacy of 59%, as indicated by systematic review of literature. In addition, 25% of patients may return to work, 61% show an improvement in activities of daily living and 40% to 84% decrease consumption of analgesics. Opinion on axial LBP (pain limited only to the low back area) is divided. Some studies show that the dual lead system provides better pain relief for axial LBP than single lead stimulation does, whereas others find the opposite to be true.

Severe peripheral vascular disease is another indication for SCS. Patients with advanced peripheral vascular disease, who are not surgical candidates, respond well to SCS, with reported efficacy ranging from 60% to 100%. Besides providing pain relief, SCS promotes ulcer healing and potentially contributes to limb salvage.

Ischemic heart disease refractory to pharmacologic and surgical treatments may respond well to SCS, with reported efficacy rates of 60% to 80% several years after implantation. Patients with ischemic heart disease treated with SCS have demonstrated a reduction in anginal pain, decreased use of short-acting nitrates, and increased exercise capacity. SCS does not completely eliminate anginal pain but raises the anginal threshold. Fear of potential increase in myocardial damage does not seem to be justified.

New indications and techniques for PNS are emerging. Some patients with occipital neuralgia seem to respond well to PNS. In those cases, the stimulator lead is placed subcutaneously around the C1-C2 spinous process. In patients with pelvic pain (e.g., interstitial cystitis, pain of unknown origin), sacral placement of 2–4 SCS leads may provide adequate analgesia. Sacral placement also can be helpful in patients with impaired bladder control. Some cases of lumbar radiculopathy respond better to SCS lead placement directly through neural foramina (retrograde lead placement).

Infection, drug abuse, and severe psychiatric disease present major contraindications for SCS implantation. Before SCS implantation, a psychological evaluation of the patient is recommended.

3. Stimulator Trials

Before proceeding with permanent SCS implantation, a stimulation trial is warranted. The trial allows the patients to evaluate the SCS analgesic activity in their everyday surroundings. The criteria for a successful trial include at least a 50% reduction in pain, a decrease in analgesic intake, and significant functional improvement. The SCS trial is a minimally invasive procedure (similar to placing an epidural catheter) and can positively predict a long-term outcome in 50% to 70% of cases.

There is no consensus on the length of an SCS trial. The minimum trial time is 24 hours, although many centers perform 3- to 5-day trials. The beginning of a trial in the hospital setting allows for proper SCS adjustment, after which the patient is discharged home for several days of “home” trial. In cases of equivocal results, the trial time can be extended.

There are three technical approaches for SCS trial. In the first approach, the SCS lead is placed percutaneosly. After successful trial, the lead is removed and a new lead and an implantable pulse generator (IPG) are placed (on a separate occasion). Alternatively, the trial lead is tunneled and anchored via a surgical incision, which simplifies the final procedure and ensures that stimulation coverage remains the same during both the trial period and permanent implantation. The disadvantage of the second approach is the need for a second operative procedure for lead removal in case of an unsuccessful trial.

Percutaneous trial followed by lead placement via laminotomy is another, less frequently utilized, approach. In this case, wider
electrodes are used in the permanent implantation, which may provide better coverage in certain patients, and are less prone to migration when compared to standard SCS leads.

4. Choice of Hardware

The hardware consists of the SCS lead, an extension cable, a power source, and a pulse generator. Lead design varies in the number of electrodes from four (Medtronic and ANS) to eight (ANS). The distance between the electrodes and the length of the leads also can differ. It is not clear whether an increased number of electrodes provides better coverage, but it might be beneficial in the event of lead migration. Leads with minimal space between electrodes (such as Medtronic Quad compact lead) are better suited for localized pain (such as foot pain) or for cases of isolated axial LBP. Many leads contain a removable stylet, which eases lead steering during implantation.

There are two types of pulse generators: (a) a completely implantable pulse generator (IPG) containing a battery, and (b) IPG powered externally through a radio frequency antenna applied to the skin. The IPG is more convenient and can be easily adjusted by the patient by using a small telemetry device. Patients can turn the stimulator on and off and can control the stimulation amplitude, frequency, and pulse width. A separate external programmer allows for more complex IPG reprogramming by the physician. If the stimulation is inadequate, the physician can change the polarity and the number of functioning electrodes to provide better coverage. Batteries have to be changed every 3 to 6 years, which requires a brief operative procedure. The battery life depends on the time the stimulator is used and the stimulation amplitude. The externally powered IPG is preferred in patients requiring higher amplitudes of stimulation.

5. Implantation Techniques

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