The perception of pain involves signals transmitted by the nerves throughout the body to the brain. Because this is an electrochemical process, it can be influenced by electricity. Typically, people think of electricity as causing pain; for example, when a person receives an electric shock from a wall outlet. In other cases, however, electricity can disrupt and reduce pain. Beginning in the 1960s, researchers found that delivering mild electrical pulses to the nervous system of a person in pain could replace some of that pain with a mild tingling sensation. This has led to the development of a spinal cord stimulator and other neurostimulation devices to manage pain.
How does a spinal cord stimulator work?
In the late 1960s, researchers discovered a technique for relieving pain using electrical pulses. Though intense electrical shocks can be painful, they found that less powerful pulses delivered to the nerves in a painful area decreased the perception of pain.
This finding was explained by the Gate Control Theory of Pain.
According to this theory, when there is a large amount of nerve stimulation coming from a part of the body, the brain “closes the gate,” which results in a substantial reduction in how much of that stimulation is perceived. We use this principle unconsciously when we are hurt and touch or hold the area around the injury. The sensation of our touch can relieve the pain a small amount.
Non-painful electrical pulses are very effective at relieving pain this way. This technique is called neuromodulation or neurostimulation. It is especially useful for pain arising from nerve damage or inflammation.
Current applications of neurostimulation
This practice of neurostimulation has since been applied through several types of medical devices.
TENS units (Transcutaneous Electrical Nerve Stimulation) deliver neurostimulation through patches placed on the skin. This stimulation affects the nerves in the area under the patch.
Another type of neurostimulation device is a spinal cord stimulator. These devices are implanted along the spine and include leads positioned against nerve roots in the spine. The positions of the leads are selected so that when they deliver stimulation, it covers that patient’s painful areas.
Arizona Pain doctors talk to one patient who was suffering from agonizing chronic pain. He explains how a spinal cord stimulator helped him find pain relief.
Finally, there are peripheral nerve stimulators. Much like a spinal cord stimulator, these devices are implanted. However, they target nerves outside of the spine or head (for example, in the shoulder or knee). There are even some neurostimulators that are implanted directly in the brain (deep brain stimulators), though these have primarily been used for non-pain related conditions, such as epilepsy.
Current neurostimulation devices
The discovery that electrical stimulation of nerves, or neurostimulation, could produce pain relief led to the development of devices such as spinal cord stimulators, which involve implanting a system of leads along the spinal cord attached to a small battery and pulse generator. These devices delivered electrical pulses to the spinal cord.
Because the pain signals for the body pass through the spinal cord, the stimulation produced by these devices is able to alleviate pain in many different body regions. Spinal cord stimulators are particularly effective for treating various types of nerve pain as well as pain that radiates from the spine into the arms, buttocks or legs. A number of new developments in neurostimulation devices have the potential to improve the pain relief that patients experience.
Current neurostimulation devices deliver a constant stimulation when they are active. This stimulation replaces pain with a mild tingling sensation called paresthesia. Recent studies have experimented with stimulation delivered in a series of five brief bursts, each lasting one thousandth of a second followed by one thousandth of a second with no stimulation. These five burst series are repeated around 40 times per second.
These bursts of stimulation seem to disrupt pain signals as well as, or even better than, constant stimulation. Further, the bursts of stimulation generally do not produce any paresthesia. This is an advantage for some patients, because the side effect of paresthesia itself is sometimes considered unpleasant.
Current spinal cord stimulation devices typically have a frequency of ten to a few hundred cycles per second. Some recent devices have been tested which deliver stimulation at a very high frequency, such as 10,000 cycles per second.
Like burst stimulation, high-frequency stimulation does not produce paresthesia. Often this type of stimulation does not produce the very fast relief of a traditional neuromodulation treatment, but within hours many patients get a level of pain relief greater than most neurostimulation devices. This can even last for some time after the stimulation is discontinued.
Dorsal root ganglion stimulation (or DRG stimulation)
Most current neurostimulation devices target the spinal cord itself. This has the advantage of being able to cover large regions of the body. Certain body regions tend to be hard to get consistent and complete relief in. For example, treating pain in the feet may require more stimulation in the legs than would be desired.
Some of the latest research involves placing leads along a particular bundle of nerves right where they branch off from the spinal cord, called a dorsal root ganglion. Each of these bundles of nerves handles pain signals from a particular region of the body. By stimulating a specifically chosen dorsal root ganglion, it may be possible to focus the stimulation more precisely on the body region that needs it, including difficult to cover areas such as the hands and feet.
Our own Arizona Pain doctors discuss the possibilities of DRG stimulation, especially for patients with complex regional pain syndrome, or CRPS.
New developments in neurostimulation
In addition to the different types of neurostimulation devices being produced, there are other advancements in how these can be used. Some of these advance the pain management available through neurostimulation, while others make it easier for daily use.
One of the trends in the development of neurostimulators is for devices to get smaller over time. One example of this trend is the Bioness Stimrouter that is currently undergoing testing in a clinical trial we are taking part in.
This is a peripheral nerve stimulator in which only a small six inch lead is implanted under the skin. The battery and pulse transmitter are not implanted, but are placed on the skin with an adhesive patch when the device is in use. Future neurostimulation devices may be even smaller. One neurostimulation device tested by researchers at Arizona State University is so small that it could be injected into the body via syringe.
One of the challenges in neurostimulation is that some patients’ pain is located in areas that are difficult to cover with a traditional spinal cord stimulator. For example, some types of low back pain can be hard to cover. Some new neurostimulation devices attempt to address this problem.
For example, some doctors have found that having both spinal cord stimulation and stimulation of an area of peripheral nerves (peripheral nerve field stimulation) can cover this type of pain effectively. The St. Jude SENSE study is testing whether such a device (combined spinal cord and peripheral field stimulation) covers this sort of pain better than a spinal cord stimulator alone. We are taking part in this new trial.
Other devices target a different part of the nerves, such as the dorsal root ganglion, rather than the dorsal column typically targeted by spinal cord stimulators. This may also help with pain in areas that are harder to cover.
Though the stimulation delivered by neurostimulation devices is calibrated so that it is not painful, at certain times or for certain people, this stimulation may seem unpleasant. As discussed, new devices are being developed that can potentially avoid this problem by delivering stimulation at higher frequencies or in rapid bursts.
These types of stimulation are still capable of disrupting pain, but the stimulation itself is not even felt by the patient.
Current neurostimulation devices, such as a spinal cord stimulator or DRG stimulator, are able to deliver effective pain relief to many patients. They are often a good treatment option for many people with nerve pain or other types of spine-related pain that hasn’t responded to conservative treatments or surgery.
These devices are in many ways safer and produce fewer side effects than many pain medications and types of surgery. Future technologies, such as those described above, have the potential to provide even greater relief and other benefits to patients. Some of the current devices are even capable of being upgraded to new forms of neurostimulation as they are tested and approved.
If you are interested in learning more about the trials mentioned above, you can speak with your pain management doctors or contact me directly at TedS@arizonapain.com.
Ted Swing has 13 years of research experience in psychology and pain medicine and four years of teaching experience, has published in top psychology and medical journals, and has presented his research at major conferences. He received his Ph.D. in Social Psychology from Iowa State University and has been the Research Director at Arizona Pain since May 2012.
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