What is Pain? Explaining Acute VS Chronic Pain

What is pain? On the surface, this seems like a strange question and one with a simple answer. Pain is the noxious and unpleasant sensation experienced as a result of trauma. Whilst this is true for acute pain, or an acute (recent) trauma, we must also consider the more complex issue of chronic or persistent pain as well. We have discussed inflammation and tissue damage in a previous post so pop over to that one if you want to learn more. In this blog post, we will be discussing what pain is and the up-to-date research in the area of pain science.

“No one can live their entire life pain-free, but nearly everyone can live life free of pain” – Donald Murphy DC, 20161

Pain is an intricate and unavoidable aspect of the human experience. From physical injuries to emotional wounds, the sensation of pain is deeply ingrained in our lives. While often seen as a negative force, pain serves a crucial purpose in our survival and well-being. In this blog, we will delve into the multifaceted nature of pain, exploring its physical and psychological dimensions, its purpose, and how we cope with it.

The Physiology of Pain

At its core, pain is a complex physiological, psychological and emotional response to signals that present a threat to our body. Nociceptors, specialized nerve endings, detect harmful stimuli, such as heat, pressure, or chemicals, and send signals to the brain through the spinal cord. This intricate network of communication results in the perception of pain, alerting us to potential danger and prompting protective actions. Despite what some people think, there is no such thing as a “pain centre” in the brain. To experience pain, a functioning brain is required because the whole brain is involved in the experience of pain2. Some authors even discuss pain as an emotional response to the experience of noxious stimuli.

When discussing pain, we often get focused on tissue damage from a trauma. However, more recent thinking and research have revealed that pain is more than tissue damage and injury. In a nutshell, the concept of tissue injury = pain falls down within a few moments thought. If you were to pinch the tip of your finger with your other hand, you can experience pain without applying significant force. When you release your finger, you will quickly see that you have not caused yourself any tissue damage or injury. The skin of your recently pinched skin may be temporarily red, but there is no bleeding or even bruising at this point, you haven’t caused yourself any damage but you experienced pain anyway. When we apply pressure to the tip of your finger, several nerve fibres and receptors in your skin and other tissue layers register the pressure (mechanoreceptors) that gradually become more excited as we apply more pressure. These mechanoreceptors begin sending signals to your brain, telling it about the stimulus of your finger being touched and more pressure is being applied.  Once we reach a threshold of pressure, other local nerves called nociceptors are stimulated and send signals to the brain that are interpreted as pain. This gives the brain the chance to react to the stimulus, which leaps into action, being either withdrawal from the painful stimulus, or if the stimulus is great enough, or persists for long enough will trigger the fight or flight response. Acute pain like pinching the tip of your finger is an adaptive alarm system of your nervous system (including your brain) to save you from harm 3. However, persistent, or chronic pain is not that simple, we as will discuss.

While pain is often viewed negatively, it plays a vital role in our survival. Physical pain prevents further harm by prompting protective behaviours, such as withdrawing from a hot surface or avoiding dangerous situations. When we suffer an injury, be this a cut to our skin, a burn, a strain/sprain injury of a muscle or ligament, or a broken bone, the nerves in the area of the injury are triggered and fire off signals to your brain, telling you about it. The brain then learns about the pain, and how it responds to it, physically and emotionally and creates a memory of the experience. In order for this whole process to happen, the entire brain is needed to experience pain, process it, and move on.

The Biopsychosocial Model of Pain

Pain comes in various forms, classified broadly into two categories: acute and chronic. Acute pain is typically a response to a specific injury or illness, serving as a warning signal that prompts immediate action. Chronic pain, on the other hand, persists over an extended period, often lasting for months or even years, and can significantly impact one’s quality of life.

 

The Bio-psycho-social Model of Pain

The Biopsychosocial model of pain attempts to explain how pain, or our experience and perception of pain is affected by all aspects of our lives, not just the physical injury or trauma we sustained. The biopsychosocial model superseded the previous medical model of pain, that only attempted to explain pain as a result of tissue trauma (injury). This previous model ignores the concept of seeing the patient as a whole person. It operated on the assumption that the patient’s pain, must be the direct result of tissue damage found at the site of the pain. Medicine and medical practice have moved on since then, however, there are still some who fail to see the patient as a whole and ignore the fact that the patients’ pain might be referred pain from another, more remote part of their body, or it could be the result of psychological or social factors in the patient’s life. The person standing in front of you, is more than the sum of their parts.

“The human body is more than a convenient collection of systems that, just so happen to be in the same place, at the same time!”

Chronic or Persistent Pain

The definition of chronic pain, is pain that lasts greater than 12 weeks4. In some cases of chronic pain, there is no evidence of any tissue damage or injury on diagnostic imaging and scans. The lack of tissue injury doesn’t match most people understanding of pain being present as a result of an injury, which some clinicians describe as persistent pain. When discussing persistent pain, things get more complex, and we enter the field of “pain science”. I have my personal issues and criticisms of “pain science”, chief among which is that, in my experience, the concepts used to describe the psychological aspects of chronic or persistent pain to patients can be misinterpreted by clinicians, who then tell their patients; “your pain is all in your head”. Not only is this patronising to patients, especially to those who it doesn’t apply to, but this attitude doesn’t help the patient. Also, the most common advice I hear patients are given is mindfulness and meditation, which may be helpful for some patients, but others need more specialist support to meet their needs.

Persistent pain is clearly not the same as acute pain described above, an adaptative alarm system of your nervous system. It is a maladapted system that is constantly on false alarm3. This maladapted system also causes hypersensitivity, meaning that even normal, everyday stimuli are interpreted as noxious by the patient’s brain. Studies into complex regional pain syndrome (CRPS) have observed global disorganisation between the white and grey matter in the brain of CRPS patients4. Similar observations regarding grey/white disorganisation and loss of grey matter have been made in patients with chronic lower back pain, fibromyalgia, chronic fatigue syndrome and posttraumatic stress disorder5–8. Milder changes in brain disorganisation have been observed in patients with sleep deprivation9,10.

Therefore, it is not simply “all in your head” in the conventional sense that, you are making it up. It is all in your brain. Disorganisation of the grey and white matter in your brain leads to inappropriate connections within neural networks and an imbalance of neural transmitters, the chemicals used by brain cells to communicate with each other. Some of these cells and neural networks become hypersensitive and over excitable. This results in miscommunication between brain regions and the easy triggering of networks and pathways in the brain to stimuli. Maladaptive changes are, therefore, the over excitable children in the classroom who start making noise and disturbing the class when something boring and uninteresting is happen outside in the playground. They get riled up easily and drag everyone else into mischief when nothing is really happening.

Dealing with chronic pain requires a combination of physical and psychological strategies. Medications, physical therapy, and other medical interventions can alleviate physical pain, while the psychological aspects of the patients’ pain may benefit from therapy, support systems, and self-care practices. Developing healthy coping mechanisms is essential for navigating the complex landscape of pain. Improving sleep and patterns of sleep may also help with pain management, along with mood stabilisation and cognitive function.

Chronic pain, whether physical or emotional, can take a toll on mental health. Conditions like depression and anxiety often coexist with persistent pain, creating a challenging cycle that requires a comprehensive and integrated approach to treatment. Having a psychologist on board for referrals for patients with chronic or persistent pain can be a game changer for patient management. Having your psychological needs met by a trained professional who understands persistent pain is essential.

Conclusion:

Pain is an inevitable part of the human experience, serving as a crucial signal for potential threats and opportunities for growth. Understanding the intricacies of pain, both physical and psychological, empowers individuals to navigate its complexities and work towards recovery or long-term management strategies.

For more information about the complex phenomenon that is pain, there is the British Pain Society and the Pain Faculty of the Royal College of Chiropractors.

 

References:

  1. Murphy D. Clinical Reasoning in Spine Pain Volume II: Primary Management of Cervical Disorders Using the CRISP Protocols. An Evidence-Based Guide. CRISP Education and Research LLC; 2016.
  2. Apkarian AV, Bushnell MC, Treede RD, Zubieta JK. Human brain mechanisms of pain perception and regulation in health and disease. Eur J Pain. 2005;9(4):463. doi:10.1016/j.ejpain.2004.11.001
  3. Woolf CJ, Ma Q. Nociceptors-Noxious Stimulus Detectors. Neuron. 2007;55(3):353-364. doi:10.1016/j.neuron.2007.07.016
  4. Geha PY, Baliki MN, Harden RN, Bauer WR, Parrish TB, Apkarian AV. The Brain in Chronic CRPS Pain: Abnormal Gray-White Matter Interactions in Emotional and Autonomic Regions. Neuron. 2008;60(4):570-581. doi:10.1016/j.neuron.2008.08.022
  5. Kuchinad A, Schweinhardt P, Seminowicz DA, Wood PB, Chizh BA, Bushnell MC. Accelerated brain gray matter loss in fibromyalgia patients: Premature aging of the brain? J Neurosci. 2007;27(15):4004-4007. doi:10.1523/JNEUROSCI.0098-07.2007
  6. De Lange FP, Kalkman JS, Bleijenberg G, Hagoort P, Van Der Meer JWM, Toni I. Gray matter volume reduction in the chronic fatigue syndrome. Neuroimage. 2005;26(3):777-781. doi:10.1016/J.NEUROIMAGE.2005.02.037
  7. Okada T, Tanaka M, Kuratsune H, Watanabe Y, Sadato N. Mechanisms underlying fatigue: a voxel-based morphometric study of chronic fatigue syndrome. BMC Neurol. 2004;4(1). doi:10.1186/1471-2377-4-14
  8. Villarreal G, Hamilton DA, Petropoulos H, et al. Reduced hippocampal volume and total white matter volume in posttraumatic stress disorder. Biol Psychiatry. 2002;52(2):119-125. doi:10.1016/S0006-3223(02)01359-8
  9. Killgore WDS. Effects of sleep deprivation on cognition. Prog Brain Res. 2010;185(C):105-129. doi:10.1016/B978-0-444-53702-7.00007-5
  10. Mroczek M, de Grado A, Pia H, Nochi Z, Tankisi H. Effects of sleep deprivation on cortical excitability: A threshold-tracking TMS study and review of the literature. Clin Neurophysiol Pract. 2024;9:13-20. doi:10.1016/j.cnp.2023.12.001
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