Why would an anesthetic nerve injection produce long-term pain relief?
Updated: Jan 26, 2022
An illogical treatment that may work
One of my patients presented with chronic shoulder pain due to extensive damage of his shoulder tendons and osteoarthritis. He could not use his arm without experiencing unbearable pain, and was unable to sleep for more than 2-3 hours. Medications and physical therapy did not help, and he was under treatment for severe depression. I offered a series of four weekly local anesthetic injections of the suprascapular nerve, the major nerve supplying the shoulder. This procedure can be performed safely and quickly using ultrasound guidance (see figure and our paper).
This treatment seems illogical for chronic pain, as local anesthetics work only for few hours: once the effect wears off, the pain is expected to come back. While short duration of pain relief is unfortunately frequent, my patient experienced substantial pain relief, improvement in arm motility, sleep and mood for three months, when he eventually underwent major shoulder surgery. He is still thankful for those three months of worth-living life.
This is not an isolated case. Most pain clinicians have observed long-term improvements after local anesthetic injections (commonly called "nerve blocks"). Our former Chair John Bonica, the pioneer of Pain Medicine, used nerve blocks routinely for his patients and included a chapter on nerve blocks in the very first book on Pain Management.
Here I want to address a common question:
Considering that local anesthetics work for only few hours, why would they lead to long-term improvements?
Improvement after the injection does not necessarily mean improvement due to the injection
Some patients experience alternating periods of worsening and improvement, independent of any treatment. They may seek treatments at a peek of their pain. In this case, the pain would decrease anyway, and the improvement would be erroneously attributed to the treatment. This is called "regression to the mean".
Similarly, the natural history of a pain condition may lead to long-term improvements independent of any treatment, and therefore any intervention would unrightfully take the merit of causing the improvement.
Improvement due to the injection does not necessarily mean improvement due to the injection
This apparently awkward statement is easily explained by the "placebo effect": patient's expectation of a positive effect of a treatment leads to an improvement, even if the treatment itself is not effective. This phenomenon is normal for any individual and has nothing to do with any mental "anomaly". The placebo effect involves, among others, the release in the brain of substances that produce pain relief. It is observed with many treatments, across different areas of medicine. If interested, you can learn more here.
A placebo effect after an anesthetic injection implies that the patient would not have improved if the injection had not been performed. Therefore, the placebo effect is welcome in the management of pain, as long as the treatment is not particularly risky, or the potential benefits overweigh the risks. However, a placebo effect implies that the patient would have improved also if an inactive agent, such as a saline solution, had been injected.
The placebo effect is the reason why scientists test the efficacy of a treatment by comparing the treatment with a placebo. A "specific" effect of the treatment can be ascertained only if the treatment produces more pain relief than the placebo.
And yet, the improvement may be due to a specific effect of the injection
I am referring to my patient, but these considerations apply to any kind of pain. My patient had a severely damaged shoulder. Pain signals arising from a damaged shoulder cause profound changes in neural pathways of the spinal cord and brain that amplify the signal along its course from the shoulder to the brain, where the pain is ultimately perceived. In short, these pathways become over-excitable, causing strong pain. You can read this blog to learn more.
Injecting a local anesthetic will interrupt the flow of pain signals from the shoulder (or from any other injured area of the body). This temporary block may result in a reduced excitability of pain pathways, and ultimately long-term pain relief. In other words, the shoulder will send the same amount of pain signals, but the final signal arriving to the brain will be weaker.
Pain cannot be not be explained by just a wired system that conveys a signal from the shoulder to the brain. Pain is associated with substantial emotional, cognitive, and behavioral modifications, and such modifications contribute to pain. Poor sleep causes more pain during the following day. My patient was severely depressed, could not sleep or move his arm. Being pain-free for several hours as a result of numbing the nerve could have triggered positive cognitive and emotional processes, resulting in long-term pain relief.
Finally, the pain relief has allowed better shoulder mobility, which may have helped reduce the pain in the long term. Activity, while frequently inducing short-term increases in pain, has been shown to produce long-term improvements.
A neglected area of pain research
Despite decades of practice, nerve blocks are not performed uniformly and are not mentioned in guidelines on chronic pain treatment. This is largely due to paucity of research on their efficacy and mechanisms of action. There are several challenges associated with the plan and execution of studies on nerve blocks, which I will discuss in a future article. Such studies would represent a great service to the management of chronic pain.