Pain is not just in the brain of the beholder

It’s a bright Saturday morning and you are cooking breakfast. The hashbrowns are finished, and you put on your oven mitt to take them out. As you grab the pan, a searing pain shoots through your hand and up your arm. You immediately let go and apply ice. Unfortunately, a small hole in the oven mitt introduced itself that morning.  

While pain is, well, painful, it is essential to perceive as it allows us to avoid damage to our bodies. In the above example, it would be detrimental to keep holding the pan, which could damage your hand beyond use. Almost everyone is familiar with this short-lasting and visceral pain, but what happens if that sensation of pain does not dissipate? 

Chronic pain is typically defined as pain which lasts longer then 3 months and is recognized by the World Health Organization as a disease on its own and not just a symptom of something else. Approximately 8 million, or 21% of Canadians suffer from chronic pain. 

Pain, whether visceral or chronic, is typically framed and researched as a process occurring within the brain. Indeed, we have a general understanding of exactly which regions of the brain are involved in perceiving pain. It is to these regions that most research and solutions to chronic pain have been targeted. However, this appears to be an isolated view of pain perception as it separates brain and body. The intermediary between the two is the spinal cord, which acts as the highway for pain signal propagation from the body to the brain. While it is a critical part of propagating pain signals, less focus is placed on the spinal cord’s role in chronic pain perception. Is the spinal cord important for perceiving pain? Should it be considered when a patient with persistent pain presents at a clinic?

A 2022 publication and commentary co-authored by researchers at Western University have asked these questions, attempting to highlight the role of the spinal cord in chronic pain processing and how this may translate into the clinical realm of chronic pain. In particular, the publication focuses on the dorsal horn of the spinal cord, which is thought to be the site where pain information from the body enters the spinal cord. By collating recent and previous research, they argue that the spinal cord is a key component of the pain response, which goes beyond the classic clinical notion that “pain is in the brain”. 

The term central sensitization refers to the perception of pain that is not equal to the thing that caused it. That is, there is an increased responsiveness of the body to pain. While this is normal directly following an injury, it can become a problem when this sensitivity persists over time (i.e., becomes chronic). The dorsal horn of the spinal cord is highly connected with the body and other spinal cord regions. With sensitization to pain, the amount of information that a single dorsal horn cell can collect from the body increases. This then makes these pain receptors even more sensitive, even without input from the brain. For example, following a whiplash injury it was found that the activity of the cells in the dorsal horn was increased, which coincided with increased pain perception and lower pain tolerance in patients with central sensitization. Thus, the spinal cord is a key structure in explaining why some individuals may experience pain that is greater than expected based on an injury. 

A consequence of the dorsal horn being highly connected is that pain information can get sent to multiple areas of the spinal cord, which coincides with increases in reported pain intensity. Furthermore, it is thought that the amount of pain propagation throughout the spinal cord may increase in the presence of central sensitization. That is, when someone becomes more sensitive to perceived pain, the activity in the spinal cord may increase when future painful events are encountered. This can also contribute to the perception of pain becoming chronic, as the dorsal horn becomes more sensitive and likely to propagate pain signals. Finally, the increased connectivity of the dorsal horn can make it challenging to localize the source of pain symptoms and provide treatment in clinic. As a result, those with chronic pain usually struggle to describe the location of their symptoms. Given that the pain information can travel to multiple regions of the spinal cord, it can then appear that the area of perceived pain is present across multiple body areas. 

The pain-related processes in the dorsal horn are important to consider for clinical practice. For example, using the sensitivity of the skin to determine where pain is originating from ignores the dynamic way that the spinal cord may be activated and affect pain perception. Furthermore, the location of pain may be more indicative of how the brain and spinal cord have changed in response to the painful stimulus, rather than being a direct marker for which region of the body is apparently damaged. Overall, the spinal cord plays an essential role in the dynamic response of the body and brain to pain. Fully appreciating how the spinal cord contributes to the complex nature of pain processing can allow clinicians to better understand the limitations of common clinical tests for measuring pain, and to take a more holistic view of pain treatment. It seems that pain is experienced beyond just the brain of the beholder. 

Original article: Margerison, S. M., Westlake, K. P., & Seminowicz, D. A. (2022). Beyond pain in the brain: A clinician’s guide to interpreting the Spinal Cord’s role in the pain experience. Musculoskeletal Science and Practice, 62, 102664. https://doi.org/10.1016/j.msksp.2022.102664