The simple model of pain as a signal and measure of physical damage has had its day.
As long ago as ancient Greece, philosophers thought of pain as an emotional construct, often describing it as a “passion of the soul”.
Today, we could think of the Greek view of pain as lacking basic physiology and anatomy understanding. Indeed, Aristotle’s insistence that pain is experienced in the heart, not the brain, shows that all the pieces of the puzzle were far from coming together.
But the fact that Aristotle practised the theory of holism in his medicine tells us that the Greeks had understood the importance of treating the whole person, taking into account the psyche and social influences rather than looking at just the diseased part.
In the 1960s, George Engel, an internist and psychiatrist at the University of Rochester Medical Center in Rochester, New York, introduced the biopsychosocial model of illness. The model rejected the prevailing idea that a disease is a separate entity from the human body and that the patient can be cured if the diseased part is removed.
In his landmark 1977 paper, he described the dynamic interconnection between psychological, social and pathophysiological factors. He highlighted the hypothesis that the mind could affect the body as much as the body could affect the mind.
But it wasn’t until the 1980s that the model reached the pain field when John Loeser, a neurosurgeon and pain management specialist, introduced the “onion skin model” of pain and suffering. His onion included five rings: nociception, attitude and beliefs, suffering, pain escape behaviour, and social environment.
The biopsychosocial model pioneers found a lot of resistance in the medical world, where a biomedical approach was predominant. But Associate Professor David Butler, a physiotherapist and pain scientist at the University of South Australia and director of the NOI group (Neuro Orthopaedic Institute Australasia), says that science has backed up their perspective time and time again.
The biomedical model most health professionals are familiar with describes pain as a direct consequence of tissue damage – the more severe an injury is, the stronger the pain is. This vision supposes that pain provides an accurate indication of the state of the tissues, and it can be cured by “fixing” the tissue or providing pain relief. But decades of research in pain science have overturned this concept.
Scientists have discovered that pain is not an accurate measure of tissue damage. In 1995, the British Medical Journal published an astonishing report about the hospitalisation of a 29-year-old builder. He had accidentally jumped onto a 15cm nail that pierced through his steel-toed boot. He was rushed to the hospital in agonising pain that even multiple doses of morphine would not alleviate. But when the doctors took off his boot, they were amazed to see that the nail had never touched his foot.
On the flip side, another construction worker unknowingly shot himself in the head with a nail gun. He experienced a mild toothache and a bruise under his jaw, so he thought he had escaped relatively unscathed. He went to the dentist six days later, when an X-ray revealed a 10cm nail embedded in his head. The man was indeed in great danger, but contextual cues failed to alert his brain. Despite the severe injury, his pain system remained quiet.
Another fundamental discovery was that pain is a vital, protective mechanism that happens in the brain and not in the tissues. When we get hurt, pain receptors, also known as nociceptors, send a “possible threat” signal to the brain. The brain evaluates how dangerous such a threat is by drawing information from current and past experiences and the state of the mind.
Where are we? What are we doing? What can we see, smell, hear? Have we been here before? What happened last time? How did we solve it? Are we frightened? Or are we relaxed? These are all inputs the brain uses to decide on how dangerous the situation is.
If we are frightened, anxious or in a social circumstance that makes us feel threatened, our brain can perceive the situation as dangerous and produce pain to protect us. If the brain believes that there is no need for protection, it will not cause pain.
In 2003, an 84-year-old war veteran discovered from a routine X-ray that he had had a bullet in his neck since World War II. At the time of being shot, his brain had decided that pain would not have been a helpful cue. At that moment, running for his life was more valuable than worrying about his injury.
“The fact that you ring up and make an appointment with a health professional can often ease pain, and you’ve done nothing but taken away some of the threat,” says Professor Butler.
Besides the anecdotal examples, there is plenty of scientific evidence that the psychological and social context influences our perception of pain. Lab experiments that manipulate the psychological context of a painful stimulus have demonstrated that factors that change the meaning of pain – such as anxiety, fear and expectation of pain – affect how much pain we feel.
In one experiment, researchers placed an ice-cold rod on the back of volunteers’ hands and then showed them either a red or blue light. Although the rod was at the same temperature each time, those who saw the red light – the typical colour of danger – reported feeling more intense pain than those who saw the blue light.
In another study, researchers asked volunteers to put their heads inside what they thought was a “head stimulator”. In front of them was the intensity knob of the stimulator. The volunteers reported levels of pain that correlated with where the knob was in the intensity setting, although the stimulator was doing nothing at all.
All this suggests that pain is a response to perceived danger more than to real danger or physical damage. This means that psychosocial factors that alter our perception of threat have an essential role in the level of pain we feel.
That is more so when pain persists for months or years. When pain is chronic, it does not reflect the state of the tissues because most injuries generally heal within a few months. Chronic pain is less about physical damage and more about a pain system that has become excessively sensitive to possible threats. Therefore, for a patient who has had back pain for years, watching someone bending over to pick up a heavy box might be a strong enough stimulus to make their brain think there is a need for protection, resulting in pain.
“Everything matters in the construction of pain: your thoughts, your feelings, places you’ve been, your idea of the future, everything matters,” says Professor Butler. “But that is liberating because if there are so many things constructing it, there are so many ways you can start to deconstruct it when [your pain system] becomes overprotective.
“The biopsychosocial thinkers would say that pure biomedical thinking can fit into biopsychosocial thinking. But particularly when something is chronic, particularly when more than one thing is contributing to it, particularly when pain depends on what a person thinks and does about it, then you get the power of the biopsychosocial approach.”
The common dominator of all the factors contributing to pain is their effect on the meaning of the pain. “Nerve pain after having part of your tongue removed because of cancer – it’s not a surprise,” says Dr Tim Hucker, a cancer pain specialist at the Peter MacCallum Cancer Centre, Melbourne. “It means dealing with disfigurement, not being able to speak properly, not being able to return to work or feeling guilty because you were a heavy smoker.”
Dr Hucker says post-cancer-treatment pain is common, yet often under-reported and under-treated. As in non-cancer chronic pain, the assessment of pain in cancer survivors is multifaceted because social, psychological and physical domains all contribute to it.
“There is a strong link between mood and pain management,” says Dr Hucker. “With chronic non-cancer-pain patients, we stress very heavily to people that not getting their mood managed is an obstacle to getting their pain managed. The same applies to cancer survivors.”
As cancer therapies become more effective, the odds of surviving cancer have drastically increased in the past decades. Many cancers have today become long-term illnesses and require prolonged treatments, with implications for pain.
“People can be on immunotherapy for years, and they more closely resemble the model of chronic non-cancer pain than an end-of-life care model, despite having active cancer,” says Dr Hucker.
Thus, a biomedical model such as the WHO analgesic ladder, more appropriate for end-of-life care, is not suitable for cancer survivors, he says. “Using strong painkillers such as opioids and keeping on upping the dose when the pain gets worse does not work. We must take into account how they feel about being disfigured, for instance.”
A multidisciplinary and multimodal approach is essential for optimal outcomes in cancer survivors who suffer from chronic pain. That might include a balanced combination of pharmacological and non-pharmacological techniques, including psychological support.
Dr Hucker says there is a general lack of understanding of the dangers of pain medications such as opioids. High doses of morphine can increase the risk of sudden death, depression and physical dependence, and therefore “we really want to avoid using opioids in cancer”.
“You can’t take out cancer from the whole opioid issues. It’s not about being restrictive. It’s about providing better pain management,” says Dr Hucker. That in turn can affect the patient’s prognosis, he says, as people make better treatment decisions when their pain is better managed.
Flipping the paradigm
Dr Daniel Lewis, a rheumatologist and chronic pain specialist, says a biopsychosocial approach is “absolutely essential for effective treatment, because no pain is purely nociceptive. It’s always a mixed picture of nociception, neuropathic and nociplastic pain.”
(The International Association for the Study of Pain defines the last as “Pain that arises from altered nociception despite no clear evidence of actual or threatened tissue damage causing the activation of peripheral nociceptors or evidence for disease or lesion of the somatosensory system causing the pain”.)
In conditions such as rheumatoid arthritis, it is necessary to separate the inflammation, the function and the experience of pain, says Dr Lewis. He says drugs are essential to put out the inflammatory fire, but that does not mean people will recover function or be free from ongoing pain and chronic fatigue. “So, we need to know what we’re trading. In the biomedical model, it’s fairly simplistic – we get rid of the inflammation, and everything will be fine. But that’s not the experience most people have.”
When looking at an osteoarthritic knee, he says, most clinicians stop at the X-ray and fail to consider the complex nature of a person’s condition. “It’s kind of a focal myopic view of the health condition,” he says. Thus, about 20% of patients with an osteoarthritic knee who have a knee replacement report having persistent pain after surgery. “If someone had taken into account the nociplastic aspect of osteoarthritis, then [complementary] treatments could have been administered before the surgery to improve the outcomes.”
“All of the things that lower the sympathetic nervous system response will dampen the pain response as well,” says Dr Lewis. He suggests that many different strategies can dim the sympathetic response and decrease the odds of developing post-surgery chronic pain, such as breathing exercises, relaxation methods and good sleep strategies. “Taking fear away lowers the likelihood of a high pain experience,” he says.
Dr Lewis says clinicians need to build up a network of health professionals to refer their patients to offer a multidisciplinary, multimodal approach to pain management. But he admits that it requires time and effort.
The way scientists understand pain today has implications for clinical practice. Investigating the biological reasons a patient is in pain is no longer sufficient. When someone suffers from chronic pain, finding a biological cause might not be possible at all.
For decades, scientists have urged clinicians to broaden their skills and knowledge beyond anatomy and biomechanics when dealing with chronic-pain patients. That means to use a diagnostic and treatment approach based on the definition of pain as a reaction of the brain to the threat rather than to physical damage. Such an approach would consider how each factor – biological, psychological and social – affects the patient’s perception of threat.
“The biomedical approach is simplistic and does not account for the complexity of an individual’s pain experience,” says Associate Professor Meredith Craigie, a staff specialist in the pain management unit at the Queen Elizabeth Hospital in Adelaide.
“There’s an argument we should flip the biopsychosocial paradigm to a more socio-psycho-biomedical paradigm, particularly when pain persists,” says Professor Craigie. “We see that fluctuations in pain experience are related to what’s going on socially, culturally, psychologically and even spiritually and, with time, the original biomedical event becomes less and less important.”
Another implication is that patients have an active role in chronic pain treatment. Teaching patients about modern pain biology helps them alter their beliefs and attitudes about pain and increases their pain threshold. When education about pain biology is incorporated into physiotherapy management of patients with chronic pain, pain and disability are reduced.
“If you understand something, you don’t worry about it so much,” says Professor Craigie. “Knowing what’s going on allows us to have control. Humans like to be in charge of their lives. We like predictability. It makes us feel safe.”