Most of us think we have a pretty good idea about pain. We experience it throughout our lives and so we should kind of know how it works, right? WRONG! The basic model of pain we have been taught over the past one of two thousand years was described by Descartes. This model of pain says that tissue damage = pain. It is based on an old view of pain that is the prevalent model of pain used in medical and physical therapy schools. This is called a biomedical model of pain. It says that when there is pain then there is a tissue damage or some type of pathology causing the pain and if we find it and fix it the pain will go away. Here’s the problem with the Biomedical model. It is incomplete. It doesn’t explain why some people have pain without any tissue damage (like phantom limb pain or people totally normal imaging). It doesn’t explain why some people who are completely healed from surgery or an injury still have pain. It doesn’t explain why some people have significant tissue injury but don’t experience pain (such as in life-threatening situations). There are so many situations that it doesn’t explain that it quickly becomes clear that the biomedical model of pain fails to adequately explain pain. For the estimated 50-100 million of Americans who suffer from persistent pain this model has failed them.
Let me describe an alternate model of pain. This model is more complete and explains how pain really works in the body in a way that helps us understand some the situations I listed above. This is what has been described as a biopsychosocial model of pain. The biopsychosocial model takes into account other factors that can influence our experience of pain including our emotions, mood, environment, culture, expectations, attention, heredity, nervous system chemicals and structure, immune system, and past experiences with pain. Scientist have now shown that there isn’t a single part of the brain that controls or responds to pain but rather many parts of the brain are active during the pain experience and that this is different for each person. So yours and my pain experiences will be different and influenced by many factors I listed above.
An important fact to understand is that injured tissues heal in predictable and consistent time frames. Assuming the tissues healing is not being interfered with, it will heal completely during these time frames. We would expect pain to resolve as tissues heal, but for some people this does not happen. For example, the majority of disc bulges heal in 9 months. Tendons heal in 3 to 6 months, ligaments heal in 6 to 9 months. But many people still have pain after these tissues have healed. When you consider that all tissues heal in a predictable way, why do so many people have pain that persists even after healing? Current research indicates the problem lies in the nervous system and how the brain responds to the information it receives from the body.
Let’s review what happens when injury occurs in the body. Throughout the body are special sensors called nociceptors. The nociceptors have the job of sending messages to the brain when there is damage or danger in the tissues. All our nerve cells are like little alarms. They have a little bit of electrical current running through them at any point in time. This tells us we are alive. When they are stimulated this electrical activity increases to a point we call the firing level. If and only if the electrical current in the nerve reaches this level does the nerve transmit the message to the spinal cord and onto the brain. So when damage occurs danger messages are sent to the brain. As the brain receives the message it has to make a choice whether this message means there is a threat. If the brain determines there is threat the brain usually will give us pain to warn us. It rings the alarm, we feel pain and we know we need to do something about it. So if we step on a nail we take action, we remove the nail, clean the wound, maybe get a tetanus shot, bandage it up, etc. As the wound heals the electrical activity in the nerve reduces back to its normal level, pain goes away and everything is ready to go for the next time an injury occurs.
However in about 25 percent of people as the body heals the alarm does not go back down. The alarm (nervous system) stays extra sensitive. So if pain last beyond the normal healing time, it is likely due to an extra sensitive alarm system. We now believe this extra sensitive alarm system is the reason for chronic pain, limited movement and sensitivity that many people develop.
What does this mean for people with chronic pain? It means that before their injury, surgery or disease process started their nervous system allowed them a lot of room for activity. They could do all the things they love, their hobbies, sports, recreation and work without pain. Now their nervous system has become like a helicopter parent, WAY TOO PROTECTIVE. They now get pain with normal activities that aren’t dangerous or damaging. They have little room to for activity, for their life. It’s like home security system going off any time a leaf tumbles by outside.
Why does this happen in 1 in 4 people? Why does the alarm system stay extra sensitive? It has to do with everything that people may go through during their pain experience. For example, being in pain everyday can cause stress at home and work. Treatments that fail can contribute to fear and stress. Different explanations of what is wrong can confuse and contribute. When people are stressed, afraid or confused during recovery the alarm system can become extra sensitive.
In the next post in this series I will delve deeper into some of the other factors that contribute to developing a sensitive nervous system and what can be done about it. For those of you who would like to learn more about the sensitive nervous system and chronic pain may I recommend a book for you to buy. Why Do I Hurt by Adriann Louw was written to describe this process to the lay person.
The pictures and information in this post are taken from “Therapuetic Neuroscience Education: Teaching Patients About Pain” by Adriaan Louw and Emilio Puentedura.