Implant Technologies for Severe Pain: Why, When, and the Outcomes
For more than 40 years, clinicians have been using and developing implantable technologies for the control of severe pain. For the purpose of this article, the technologies are categorized into three groups:
- Spinal cord stimulators deliver electric current to the dorsal columns of the spinal cord in an effort to block or alter neural pain signals.
- Peripheral nerve stimulation is a technique that was not widely used because of concerns for potential trauma to the peripheral nerves. However, in recent years, there is renewed interest in stimulation within peripheral nerve fields, with electrodes kept at a safe distance from the major peripheral nerves.
- Spinal pump technology allows for the continuous infusion of medication directly into the cerebrospinal fluid (CSF). This has enabled clinicians to deliver more potent doses of analgesics with lower systemic side effects.
In most circumstances, these implantable technologies are reserved for situations of severe pain and when less invasive techniques are limited by side effects or have proven ineffective.
This article provides the reader with a current and comprehensive review of these three implantable technologies, focusing on their indications and efficacy.
Spinal Cord Stimulators
Spinal cord stimulation (SCS) has been used for the treatment of refractory pain, especially for failed back surgery syndrome (FBSS) and for complex regional pain syndrome (CRPS). It has undergone significant advancements in technology and placement techniques.1
The SCS device consists of a radiofrequency-controlled neurostimulator or generator—about the size of a stopwatch, which is surgically implanted in the abdomen or buttocks. It delivers mild electrical signals to the epidural space near the spinal cord through one or more thin wires called leads.
SCS is based on generating an electrical field over the spinal cord that blocks or diminishes the perception of neuropathic pain, not nociceptive pain.1 The neurostimulator produces mild electrical impulses (a tingling sensation) that reaches the brain before the pain signal arrives.2
The mechanisms by which electrical stimulation of the dorsal columns and afferent fibers attenuate or modulate a patient’s sensation of pain are not completely understood. However, their efficacy in practice has been established with decades of literature describing SCS techniques.3
SCS consists of placing lead(s) in the epidural space along the posterior aspect of the dorsal columns. Leads can be placed essentially at any point along the spinal cord. The placement of SCS leads depends on the location of the patient’s pain (Table 1).
Individual electrode arrays may be placed over each hemicord for the independent manipulation of left and right sides separately.3
To achieve optimal pain relief effects, stimulation paresthesias should cover the area of pain. The electric field is propagated by an implanted, programmable generator containing a battery pack, an antenna, and a computer module for external programming.1
Currently, there are two different SCS systems routinely used. One involves percutaneously placed electrode leads. The other involves laminectomies for placement of the electrodes.4
The first system uses percutaneous insertion of electrodes into the epidural space and either transcutaneous connection to an external generator (allowing a trial period of stimulation) or subcutaneous connection to the implanted receiver or an implanted pulse generator (IPG).4 When trial stimulation is used, if the test stimulation alleviates the pain, then the electric pulse generator is internalized in a second procedure.
The second system involves the implantation of paddle-type leads into the epidural space after laminectomy. As with percutaneously placed electrodes, the electrode leads may be connected to an external generator (allowing a trial period of stimulation), or they may be connected subcutaneously to an IPG—identical to the programmable generator used for the percutaneous electrodes.4
With either system, the patient has the option to set the intensity, frequency, and the pulse width with the transmitter. The battery-powered unit can be transcutaneously programmed and customized to meet the needs of the patient by allowing the alteration of stimulation parameters, including electrode selection via a computerized telemetry system.
The patient can adjust the strength and location of stimulation using a handheld programmer. Furthermore, the patient can adjust the levels of stimulation at various times of the day or for various activities.
Indications/Uses for SCS
A recent consensus document published by the British Pain Society recommended the following as good indications (patients likely to respond) to SCS implantation5:
- Neuropathic pain in the leg or arm following lumbar or cervical spine surgery (FBSS)
- Neuropathic pain secondary to peripheral nerve damage
- Pain associated with peripheral vascular disease
- Refractory angina
- Brachial plexopathy
There has also been effective use of SCS for radiculopathies, peripheral neuropathy, peripheral vascular disease, chronic unstable angina, tumors, brachial plexus injuries, spinal cord injury, phantom limb pain, ischemic limb pain, multiple sclerosis, and arachnoiditis.3,4 The FDA’s approved indications for SCS are outlined in Table 2.
SCS implantation is most effective in managing patients with neuropathic pain. In patients with mixed nociceptive and neuropathic pain, such as FBSS, those with predominant radicular pain should be considered candidates. Typically, patients with a past or current history of substance abuse are excluded.6
SCS implantation is a relatively safe procedure. However, less invasive alternatives should be attempted before a patient is recommended for SCS. A formal psychologic evaluation is typically recommended before implantation.3 We have found these evaluations to be very helpful for ensuring that the patient understands the procedure and the long-term implications of having an implanted device. It is also important to assess how the patient will respond to treatment failure: Will the patient be overwhelmed by feelings of hopelessness? Furthermore, significant psychopathology, if present, should be treated prior to proceeding with implantation.
Contraindications and Cautions
Kries and Fishman provide a useful summary of contraindications and cautions when selecting a patient for SCS implantation (Table 3).7