Back in 1965, Melczak and Wall published their Gate Control Theory, which led to dramatic changes in the medical community’s understanding of pain and, eventually, to the development of new types of treatment for pain. Per the Gate Control Theory, the brain sometimes regulates incoming pain signals from the body, making them seem less intense. Specifically, when there is a large transmission of signals from a neural pathway, the brain automatically decreases the perceived intensity of those signals (i.e., closing the “gate”). This explains why, if you have an injury to your hand, for example, it may help to hold or rub your arm away from the site of the injury. The increase in total nerve signals (rubbing the arm plus the injury) leads the brain to perceive the injury itself as less painful. People often engage in this sort of behavior after an injury without even realizing it. But, perhaps the most important consequence of the Gate Control Theory was the development of neurostimulation technology.
Neurostimulation involves the delivery of mild electrical pulses to the body to treat pain and other medical conditions. Traditional applications of neurostimulation include transcutaneous electrical nerve stimulation (TENS) units, which deliver stimulation to through the skin to the nerves in a particular region of the body, as well as spinal cord stimulators.
Spinal cord stimulators are implanted devices that consist of a battery connected to one or more leads, which deliver electrical pulses to the dorsal column of the spinal cord. When these traditional devices are operating, the patient feels a tingling sensation known as paresthesia in the targeted body part. For example, if a spinal cord stimulator is positioned to stimulate the part of the spinal cord that gathers pain signals from the legs, the person would feel paresthesia in their legs when the device is on.
In many cases, paresthesia is not a problem. Patients often like it; paresthesia is generally preferable to the pain it is replacing. However, in some cases it can create challenges.
Typically, a spinal cord stimulator is calibrated to deliver a low to moderate level of paresthesia. However, if this calibration is done while the patient is sitting, when the patient changes position (e.g., lying down) the device leads may get closer to the spinal cord, leading to more intense — potentially painful — paresthesia. Patients often turn the device settings down to a lower, less effective level of stimulation to prevent these painful increases in stimulation. This prevents shocks, but at the expense of more effective relief.
Further, certain pain conditions such as complex regional pain syndrome (CRPS) involve sensitization of the nerves, potentially causing even mild paresthesia to increase the pain in the affected body part. For these reasons, paresthesia is increasingly being viewed by pain doctors not as a necessary guide to delivering neurostimulation, but instead as a potentially unwanted byproduct of neurostimulation.
New neurostimulation technologies
Fortunately, several new neurostimulation technologies are being developed to continue to offer the substantial pain relief of spinal cord stimulators but without the potential drawbacks of paresthesia. Many of these devices are still being tested in clinical trials. We are taking part in many of those trials here at Arizona Pain. Other types are already commercially available for appropriate patients.
Traditional stimulators typically deliver stimulation at lower frequencies, such as 100 or 200 times per second. Some newer devices deliver stimulation at high frequencies, such as 10,000 times per second. The high frequency of stimulation leads the patient to not perceive paresthesia, but pain signals are still blocked. We are currently taking part in a study of a high-frequency stimulator designed by Neuros Medical for lower limb amputation pain. We are also participating in a study of a high-frequency neurostimulator created by Nevro Corp for treating non-spinal post-surgical nerve pain.
By delivering many pulses of stimulation for very short periods of time (e.g., 1/1000 of a second), it is also possible to block pain without creating paresthesia. Abbott, previously known as St. Jude Medical, developed a device that delivers such “burst” stimulation. We will be taking part in a study of these devices later this year.
Another change in neurostimulation devices is to target a different part of the spinal cord. Normally, the main part of the spinal cord (the dorsal column) is targeted because it gathers pain signals from all parts of the body. A newer approach involves positioning leads to stimulate the smaller branches coming from the dorsal column, called the dorsal root ganglion (DRG). By selecting the specific DRG bundles to target, it’s possible to block pain while delivering very little paresthesia, particularly to non-painful parts of the body. This appears to be helpful for pain conditions such as CRPS. We are taking part in a study with Abbott of DRG stimulation for CRPS of the lower extremities.
Another way to avoid excessive paresthesia is to make the device automatically adjust stimulation based on the position of the patient’s body. Medtronic’s RestoreSensor device uses an accelerometer in the battery to make such adjustments. For example, if the device sensed the patient lying down, it would automatically reduce the stimulation in order to avoid excessive paresthesia.
Saluda Medical is developing a device that would use the stimulator leads to get feedback directly from the spinal cord about how much of the stimulation was getting through and make millisecond by millisecond adjustments to keep the stimulation at a comfortable level. We will be taking part in a study with Saluda Medical of this device later this year.
These new technologies have the potential to deliver more effective pain relief to certain patients than current devices. If you are interested in learning more about any of these studies and whether you might qualify, you can discuss this study with your pain management providers at Arizona Pain. For additional information about this study, you can contact me directly at [email protected].
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.