What Is The Cause of Chronic Pain?
To answer this question, we need to understand some facts about the nervous system.
Whatever its initial cause, pain is a function of the nervous system. Say you injure your low back. Nerves around the site of the injury detect it and sends signals that travel on a highway of nerves from the injury to the spinal cord and up to the brain. Once they get to the brain, the brain processes the signals and they register as pain in the low back. The whole highway, from the nerves in the low back to the brain, is the nervous system.
At the same time as the signals travel from the injury to the brain, the whole nervous system becomes reactive. Like a fire detector in a building sounding the alarm in response to fire, the nervous system sets off the alarm bells when in pain. Our muscles become tense forming trigger points. We guard and grimace. We cry and are emotionally alarmed. The nervous system controls all these reactions. We can think of it as the whole nervous system going into ‘red alert.’
This reactivity of the nervous system is all well and good when it comes to acute pain. It helps us to know that something is wrong. Becoming alarmed, we protect against further injury and seek help. Once the original injury or illness heals, everything about the nervous system comes back to normal.
In some people, the nervous system can stay in a persistent state of reactivity even upon healing of the original acute injury or illness. The whole nervous system becomes more and more reactive in a process called wind-up. This reactivity of the nervous system comes to maintain pain in a vicious cycle, over and above the pain of the original condition that started it all. The end state of this process is a highly reactive nervous system called central sensitization.
What is that???
The hallmarks of central sensitization are increasingly widespread pain and increasingly intense pain. Suppose you have an injury to your neck and come to have chronic neck pain. Once central sensitization sets in, you also develop pain in your shoulders and upper back as well as tension headaches. Additionally, the pain becomes so intense that even touch can hurt.
Other problems occur as well with central sensitization. Since the nervous system also controls our emotional lives, a highly reactive nervous system leads to anxiety and irritability, poor sleep, fatigue, and eventually depression. These psychological problems are secondarily stressful. The stress adds to the reactivity of the nervous system, making the pain worse. Another vicious cycle results.
The upshot of it all is that chronic pain is pain causing pain by way of central sensitization.
Central sensitization is a condition of the nervous system that is associated with the development and maintenance of chronic pain. When central sensitization occurs, the nervous system goes through a process called wind-up and gets regulated in a persistent state of high reactivity. This persistent, or regulated, state of reactivity lowers the threshold for what causes pain and subsequently comes to maintain pain even after the initial injury might have healed.
Central sensitization has two main characteristics. Both involve a heightened sensitivity to pain and the sensation of touch. They are called allodynia and hyperalgesia.
Allodynia occurs when a person experiences pain with things that are normally not painful. For example, chronic pain patients often experience pain even with things as simple as touch or massage. In such cases, nerves in the area that was touched sends signals through the nervous system to the brain. Because the nervous system is in a persistent state of heightened reactivity, the brain doesn't produce a mild sensation of touch as it should, given that the stimulus that initiated it was a simple touch or massage. Rather, the brain produces a sensation of pain and discomfort.
Hyperalgesia occurs when a stimulus that is typically painful is perceived as more painful than it should. An example might be when a simple bump, which ordinarily might be mildly painful, sends the chronic pain patient through the roof with pain. Again, when the nervous system is in a persistent state of high reactivity, it produces pain that is amplified.
Chronic pain patients can sometimes think they must be going crazy because they know intellectually that touch or simple bumps shouldn’t be as uncomfortable or painful as they experience them. Other times, it’s not the patients themselves who think they are crazy, but their friends and loved ones. Friends and loved ones can witness the chronic pain patient grimacing at the slightest touch or crying out at the simplest bump and they think that the chronic pain patient must really be a hypochondriac or something. After all, the contrast between them and the chronic pain patient is stark: the friends and loved ones can be touched or get a bump and it doesn’t send them through the roof. The difference, though, is that the friends and loved ones don’t have a nervous system that is stuck in a persistent state of heightened reactivity, called central sensitization.