1. Mechanism of action
2. Indications and patient selection 3. Stimulator trials
4. Choice of hardware 5. Implantation techniques
6. Troubleshooting and complications
II. Chronic intrathecal therapy for cancer and nonmalignant pain 1. Patient selection
2. Screening
3. Hardware selection
4. Medication selection and dosage 5. Complications and side effects III. Discography
1. Brief overview of disc anatomy and pathophysiology 2. Discogenic low back pain: diagnostic studies 3. Technical aspects of lumbar discography 4. Discogenic low back pain: treatment options IV. Intradiscal electrothermal therapy
V. Vertebroplasty Selected Readings
In recent years, complex interventions for pain control have become part of everyday practice in pain clinics. Although interventions are more invasive than nerve blocks, many of them are not neurodestructive. Unlike nerve ablation, they may be reversible and therefore more appropriate for use in patients with nonmalignant pain. Their clinical efficacy has been widely documented. In carefully selected patients, these interventions can reduce pain and suffering, increase functional status, decrease oral medication intake, and facilitate an early return to work. In comparison with the more conservative measures for pain control, interventional treatments may appear costly, but when a good outcome is achieved, their overall cost can actually be lower (e.g., decreased cost of medications, fewer emergency room visits, less absence from work).
The implementation of these interventions should be integrated into a multidisciplinary team treatment plan. Patient benefit from these procedures can be achieved only by careful evaluation of scientific evidence, good clinical judgment, and excellent technical skills.
I. SPINAL CORD STIMULATION FOR CHRONIC PAIN
Electrical stimulation for treatment of pain was first documented in 600 B.C., utilizing electrical power from the torpedo fish. However, electrical treatment did not find a place in pain medicine until 1967, when spinal cord stimulation (SCS) was introduced by Shealy and associates. Their work was based on the “gate control” theory of pain proposed by Melzack and Wall and published just 2 years earlier. Initially, the SCS implantation involved open laminectomy, performed only by neurosurgeons. With recent advances in technology, the SCS has become a minimally invasive treatment and it is currently performed by physicians from various specialties. Further improvements in hardware design and patient selection criteria have enhanced the efficacy of SCS, and success rates of 50% to 70% have been recently reported. Besides SCS, peripheral nerve stimulation (PNS) can be performed in selected patients with localized neuropathic pain. Today, SCS presents a valuable tool for treatment of many chronic pain conditions.
1. Mechanism of action
The Melzak and Wall gate control theory of pain was a foundation for the first SCS trials. It was based on the idea that stimulation of A-beta fibers closes the dorsal horn “gate” and reduces the nociceptive input from the periphery. However, it seems that other mechanisms play a more significant role in the mechanism of SCS action.
One proposed mechanism involves increased dorsal horn inhibitory action of neurotransmitters, such as g-aminobutyric acid (GABA) and adenosine A-1, during SCS. The potential activation of descending analgesia pathways by serotonin and norepinephrine is another explanation for SCS action. 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)–mediated mechanism. The probable mechanism for pain relief in ischemic heart disease is redistribution of the coronary blood flow from regions with normal perfusion to regions with impaired perfusion. Also, SCS may suppress the excitatory effects of myocardial ischemia on intrinsic cardiac neurons.
2. Indications and patient selection
Patients with complex regional pain syndrome (CRPS) or with neuropathic pain with upper and lower extremity involvement are the best candidates. Excellent long-term success rates (50% to 91% efficacy and a decrease in analgesic consumption by 50%) have been reported for SCS used in patients with CRPS.
However, the same does not apply to phantom limb pain, stump pain, or spinal cord injury pain. The most 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, but the infection risks in these patients are higher than in the nondiabetic population. 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
reoperation. This applies in particular to low back pain (LBP) with a radiating component to the leg. In these patients, the chance of long-term success with SCS varies from 12% to 88%, with an average efficacy of 59% as indicated by a systematic review of the literature. In addition, 25% of patients may return to work, 61% show an improvement in activities of daily living, and 40% to 84% decrease their consumption of analgesics. Opinions on axial LBP (pain limited only to the low back area) are divided. Some studies show that the dual-lead system provides better pain relief for axial LBP than single-lead stimulation, but others find the opposite.
Severe peripheral vascular disease is also an indication for SCS. Patients with advanced peripheral vascular disease who are not surgical candidates respond well to SCS, with reported efficacy rates 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. These patients have demonstrated a reduction in anginal pain, decreased use of short-acting nitrates, and increased exercise capacity. SCS does not completely abolish anginal pain, but it raises the anginal threshold. Fear of a potential increase in myocardial damage does not seem to be justified.
New indications and techniques for PNS have emerged recently. Some patients with occipital neuralgia seem to respond well to PNS. In those cases, the SCS lead is placed subcutaneously around the C1-2 spinous process. In patients with pelvic pain (e.g., interstitial cystitis, pain of unknown origin), sacral placement of two to four SCS leads may provide adequate analgesia. Sacral placement can also be helpful in patients with impaired bladder control. Some cases of lumbar radiculopathy may
respond better to SCS leads placed directly through neural foramina (retrograde lead placement).
Infection, drug abuse, and severe psychiatric disease are major contraindications for SCS implantation. Before SCS implantation, a psychological evaluation of patient is recommended.
3. Stimulator trials
Before proceeding with permanent SCS implantation, a stimulation trial is warranted. The trial allows patients to evaluate the SCS analgesic activity in their everyday surroundings. The criteria for a successful trial include at least a 50% pain reduction, a decrease in analgesic intake, and a significant functional improvement. The SCS trial is a minimally invasive procedure (similar to placing an epidural catheter), and it can positively predict a long-term outcome in 50% to 70% of cases. There is no consensus on the length of an SCS trial. Minimal trial time should be 24 hours, although many centers perform 3- to 5-day trials. The trial begins in the hospital with proper SCS adjustment, after which the patient is discharged for several days of home trial. In cases of equivocal results, the trial time can be extended. There are two technical approaches for an SCS trial. In the first approach, the SCS lead is placed percutaneously. This has the advantage of minimal invasiveness. At trial completion, the lead is removed, and a new lead and internal pulse generator (IPG) are placed (on a separate occasion). The other approach is to tunnel in and anchor the trial lead via a surgical incision. This approach simplifies the final procedure and ensures that stimulation coverage remains the same during both the trial period and the permanent implantation. The major disadvantage of the second approach is the need for a second visit to the operating room for lead removal in the case of an unsuccessful trial.
A percutaneous trial followed by lead placement via a laminotomy is another, less frequently utilized approach for SCS. In this case, a lead with wider electrodes is placed via laminotomy during permanent implantation. Wider electrodes might provide better coverage in certain patients, and they are less prone to migration than standard SCS leads.
4. Choice of hardware
The permanent SCS hardware consists of the SCS lead, an extension cable, a power source, and a pulse generator.
The number of electrodes in the lead varies 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 case of lead migration. The leads with minimal space between electrodes (such as the Medtronic Quad compact lead) are better suited for localized pain (such as foot pain) or 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) the completely implantable pulse generator containing a battery, and (b) an IPG supplied by external power through the radiofrequency antenna applied to the skin. The implanted pulse generator is more convenient to use and can be easily adjusted by the patient using a small telemetry device. Patients can turn the stimulator on and off, and they can control the stimulation amplitude, frequency, and pulse width. A separate external programmer allows more complex IPG reprogramming by the physician. In case of inadequate stimulation, the physician can change the polarity and the number of functioning electrodes to provide better stimulation coverage. The batteries have to be changed every 3 to 6 years, which requires a brief visit to the operating room. The battery life depends on the time the stimulator is used and the stimulation amplitude. The externally powered IPG, therefore, has an advantage over the implanted one in patients requiring higher amplitudes of stimulation, which deplete implanted batteries in a short time.
5. Implantation techniques
For lumbar lead placement, the patient is placed in the prone position, and for cervical placement both prone and lateral decubitus positions are used. The patient is prepared and draped in usual fashion. Both trial and permanent implantations are performed under local anesthesia with light intravenous (IV) sedation. The most common entry sites are the T12-L1 and L1–L2 spinal interspaces for the lumbar area and C7-T1 for the cervical area. These interspaces are first identified with fluoroscopic guidance, making sure to obtain a true anteroposterior (AP) view. The true AP view is achieved by C-arm rotation until the spinous process is placed on the midline in relation to the spinal pedicles.
For the percutaneous SCS trial, the Tuohy needle entry site is at the level of the spinous process below the desired interspace. It is important to achieve a shallow entry angle or to use the alternate Piles needle. The needle tip should stay close to midline during insertion. As the needle is advanced, lateral fluoroscopic view can be obtained to assess needle depth. Once adequate depth is achieved, the loss-of-resistance technique is used to identify the epidural space. At this point, the SCS lead is inserted into the epidural space under continuous fluoroscopic guidance. The curved stylet, or curved lead tip, allows lead steering. The lead tip during insertion and at final position should lie at the lateral border of the spinous process on the ipsilateral side of the pain.
Once adequate lead position is obtained, the trial stimulation is performed. It is important that stimulation paresthesias provide 70% to 80% overlap with the patient's pain location. Adequate patient feedback during this stage is important. Maximal effort should be used to provide adequate pain coverage, since this optimizes the trial. Frequent lead repositioning might be needed during this stage. Once adequate coverage is achieved, the needle is removed under continuous fluoroscopy, ensuring no change in lead position. The lead is then taped to the skin.
Permanent stimulator placement technique is similar to the trial. Although the trial is usually done in the pain clinic setting, permanent SCS placement is performed in the operating room. Under local anesthesia and IV sedation, a skin incision is made along the cervical or lumbar insertion site. Tissue dissection is performed until lumbar fascia is encountered. At that point, the Tuohy needle and the stimulator lead are inserted as done in the SCS trial. Once adequate coverage is obtained, the Tuohy needle is removed under continuous fluoroscopic guidance and the SCS lead is anchored with sutures to the fascia and supraspinous ligament. The pocket for the IPG is made in the gluteal or abdominal area. The SCS lead is then connected to the IPG through an extension cable tunneled through the skin. The skin and subcutaneous tissues are closed in layers.
Patients should avoid any extreme activity for the first 6 to 8 weeks following permanent SCS implantation to prevent lead migration and allow for epidural scar tissue formation.
Lead positioning
The SCS topographic coverage depends on the spinal level at which the SCS lead tip is positioned. The following landmarks are for orientation only; the variance can be very high in individual patients. Careful intraoperative mapping is needed for optimal coverage (“sweet spot placement”).
Upper extremity: SCS tip at a level between C2 and C5. The shoulder area can be difficult to cover (Fig. 1).
Figure 1. Spinal cord stimulation (SCS) lead at the C2-3 level as seen on lateral fluoroscopic view (A) and anteroposterior (AP) view (B). Note the alignment of the SCS lead with a lateral margin of the odontoid process in the AP view.
Foot: SCS lead tip at a level between T11 and L1 (Fig. 2).
Figure 2. SCS lead placed at the thoracic spinal level as seen in lateral (A) and AP fluoroscopic view (B).
Lower extremity: SCS lead tip at the T9–10 level
Low back: SCS lead tip at a level between T8 and T10; two parallel leads can be used. Chest: SCS lead tip at the T1–2 level.
Occipital neuralgia: SCS lead placed around C1–2 subcutaneously.
Pelvic pain: multiple SCS leads placed retrogradely within the sacrum or through foramina at S2 to S4. 6. Troubleshooting and complications
SCS Not Functioning or Inadequate Coverage
1. Obtain AP and lateral fluoroscopic images of SCS lead tip to rule out lead migration. 2. Image the IPG and all connections, and search for disconnection or breakage. 3. Using programmer, check the batteries.
4. Change amplitude and pulse width.
5. Reverse electrode polarity, and change electrodes activated if there is no response to prior measures.
6. If adequate pain coverage cannot be obtained, measure impedance of each electrode in relation to the IPG. Exactly the same impedance on two electrodes raises the possibility of a short circuit between the two electrodes. Some mechanical failures might require surgical revision and replacement of affected SCS components.
Progressive decrease in stimulation threshold
Consider intrathecal migration of the SCS lead. If it stays unnoticed, it can lead to serious complications such as spinal cord injury. Intrathecal migration is most
common in patients with significant spinal canal stenosis. If this condition is suspected, a magnetic resonance imaging (MRI) scan of the targeted spinal level should be obtained before anticipated SCS placement.
SCS and pacemakers
The SCS can cause interference and inhibition of a cardiac pacemaker if they are used simultaneously. However, both devices can be used in the same patient if these guidelines are followed: (a) both devices should be programmed in bipolar mode, (b) the SCS frequency should be set at 20Hz, and (c) each SCS programming should be performed using continuous electrocardiographic (ECG) monitoring. A cardiology consult should be obtained in these patients, and the recommendations of the pacemaker's manufacturer should be closely followed.
Other complications
The most common other complications of SCS are hardware failure, lead migration, infection, skin irritation at the IPG site, and failure to provide pain relief. Bleeding at the IPG site (subcutaneous hematoma) is usually self-limiting and gradually reabsorbs in a few weeks. If infection occurs at the IPG insertion site, make sure to
aspirate the site before initiating antibiotic coverage and removing the hardware.