What is the difference between acute and chronic pain?

Key take aways:

• Acute and chronic pain differ not only in terms of duration, but also in brain activation.
• Once pain transitions into a chronic state it no longer depends on the external stimulus of an injury.
• The cycle of chronic pain can be broken and long term pain relief is possible.

In the previous blog, we established that lower back pain does not correlate with the presence of anatomical spine abnormalities seen in radiological imaging. We were left with the question: what then, is behind chronic back pain, if not issues with our spines? 

In this blog, we will explore some of the factors that are known to explain pain experienced in our backs - as well as in other body parts. 

In order to begin to understand the factors that cause our pain, we must first look at the important distinction between two very different kinds of pain: acute pain and chronic pain. 

Dr. Vania Apkarian has been studying pain since 1992. During these 30 years of research, he has gained an extensive understanding of its mechanisms, from a variety of angles, and at different stages on its timeline. Summating these years of pain research, a theory on the course of chronic pain has emerged, most comprehensively published in 2019 in the journal Pain. This theory forms the basis for much of the modern academic understanding on chronic pain, with a wealth of evidence to support it (Chronic Pain and the Emotional Brain: Specific Brain Activity Associated with Spontaneous Fluctuations of Intensity of Chronic Back Pain, Brain Morphological Signatures for Chronic Pain, Corticostriatal functional connectivity predicts transition to chronic back pain, Abnormalities in Hippocampal Functioning with Persistent Pain, Brain white matter structural properties predict transition to chronic pain, ‍Shape shifting pain: chronification of back pain shifts brain representation from nociceptive to emotional circuits, Reorganization of hippocampal functional connectivity with transition to chronic back pain, ‍The Emotional Brain as a Predictor and Amplifier of Chronic Pain, ‍Shape shifting pain: chronification of back pain shifts brain representation from nociceptive to emotional circuits).

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Dr. Apkarian’s theory of chronic pain dynamics is described in the above figure. Healthy individuals have varying levels of underlying risk for developing chronic pain, which are shaped by genetic and environmental influences (11). If a person is injured, they will experience acute pain. If this acute pain continues for 2-3 months, a certain percentage of patients will undergo a transition period where the pain shapeshifts into something completely different. This transition stage is marked by emotional learning of brain pathways, which sensitize a person’s nervous system to the sensory signals that give rise to pain. Essentially, the brain turns up the volume on pain. 

Once the threshold period of chronification has been crossed, the pain has become a self-perpetuating brain state. The important distinction is that once here, the pain no longer requires the external stimulus of injury, but instead has become a self-perpetuating cycle wholly dependent on nervous system function. This state is then maintained and strengthened by many of the same factors that contribute to the chronification in the first place.

The difference between acute and chronic pain is not only measurable in duration, but in brain activation

In order to gain a better grasp of what goes on in pain chronification, let’s look deeper into the differentiating elements between acute and chronic pain.

Acute and chronic pain are commonly differentiated by the length of time one has experienced them: acute being most often defined as pain that has lasted for anywhere less than 3 months, and chronic being most often defined as pain that has outlasted 3-6 months. (Pain somewhere in between these two definitions, is sometimes labeled as subacute pain.) 

In the past, classifying pain based only on duration made sense. We had little else to base our understanding of the distinctions between these types of pain. However, clinicians have known for a long time that pain tends to follow differing laws of nature, depending on its length of duration. For example, acute and chronic pain often respond differently to medications and other pain interventions, and their likelihood of spontaneous recovery differs drastically. 

This can now be explained by research from Dr. Apkarian’s group, which depicts the difference between acute and chronic pain in very concrete terms. In a seminal paper published in the journal Brain, he showed that completely different brain regions light up in acute versus chronic pain, when imaged using functional MRI – an advanced research technology that allows neuroscientists to view electrical activity within the brain.

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The differing activation patterns seen in the two types of pain give scientists information on what brain processes underlie these experiences.

In acute pain, the brain lights up in areas known for processing sensory information: the insula, thalamus and anterior cingulate cortex. In chronic pain however, a completely different set of regions of the brain known for emotion, emotional learning and reward, light up: namely, the amygdala, hippocampus and the medial prefrontal cortex.

The finding that emotional regions of the brain play an especially important role in chronic pain, has now been replicated in multiple subsequent studies. This new understanding has also contributed to the development of a new diagnosis called Chronic primary pain, included in the ICD-11.

Chronic pain is cyclical in nature

Given that acute and chronic pain have completely different activation signatures in the brain, we can confidently say they are actually two completely different phenomena within the brain. 

Let’s look even a little closer into this difference. 

Acute pain is a direct result of injury to the body’s tissues – when we sprain an ankle, break a bone, or burn our hand, for instance. In these cases, the peripheral nervous system (which runs throughout our bodies) senses a danger signal in a process called nociception, which is necessary to protect us from physical threats in the external world. Such physical threats may include dangerous levels of heat, pressure, or chemical irritants, which our nervous systems translate into the experience of acute pain. Acute pain is therefore closely related to our ability to sense the environment and protect us from dangers that may present themselves in our surroundings.‍

Chronic pain, however, is markedly different. We typically enter chronic pain through a transition from an initial phase of acute pain. In chronic pain, the peripheral nervous system plays a much less important role, while the role of the brain becomes central to keeping the pain ongoing. At this stage, the pain is no longer dependent on the existence of a normally painful stimulus, such as tissue injury. 

At the chronic pain stage, pain has become increasingly dependent on adaptations of the nervous system – the ones we saw imaged by functional MRI in Figure 2 – as well as other more subtle changes throughout the nervous system. Chronic pain is therefore much less in touch with the actual state of one’s body parts in relation to the environment, and is instead a self-sustaining cycle that keeps gaining fuel from the emotional importance we place on our pain. Chronic pain is sometimes also referred to as neuroplastic pain, because its development requires changes in the brain that are a result of the brain’s ability to reorganize, a phenomenon known in neuroscience as neuroplasticity.

Contributors to the chronic pain cycle

Once the windmill of neurological events required for chronic pain have been set in motion – in other words, once we have crossed from acute pain to a chronic pain state – a number of factors will feed into the cycle and enable it to pick up speed. 

Biochemical factors, including inflammatory mediators, neurotransmitters, and stress hormones, are all important biological mechanisms enabling a bridge from acute to chronic pain. Once chronic pain is established, these biochemical factors can provide more fuel to the fire. Stress, which can come in the form of either emotional or physical stress to the body, leads to an activation of the sympathetic nervous system – increasing momentum of the chronic pain cycle. Learning, also referred to as neuroplasticity, eventually follows. The interplay of these factors then summate to form a self-sustaining brain state that amplifies pain. 

This increased volume of pain is experienced as a sensory AND emotional phenomenon, and its development is closely related to reward learning.

Breaking the cycle of chronic pain with Aivo

The groundbreaking research pioneered by Dr. Apkarian brings us a message of hope: we now have a clear understanding on a topic, that has previously been misunderstood for decades and subsequently mistreated. 

We are here to change that. Our world class pain management specialists are here for you, and want to engage with you in our digital clinic in order to understand the intricate and unique details of your pain, and thus find just the correct personalized and proven solutions for it. ‍

The Aivo treatment reverses chronic pain by laying out new brain pathways through mindfulness, CBT, exercise, and other lifestyle changes. We will be here to support you every step of the way – just a message or click away, in your pocket. 

To start your journey towards pain relief, we welcome you to join us for a free consultation. Just click the button below.