Quirks & Quarks

What exactly is chronic pain?

Dr. Michael Hildebrand says chronic pain can be caused by many mechanisms.
A participant in an intensive "boot camp" to relieve chronic back pain. (M. Spencer Green/Associated Press)

Valerie Coles from North Vancouver asks: "What exactly is chronic pain? I have mild chronic pain in the tissues in my gluteal muscles. Even doctors can't explain chronic pain, which research tells me is a never-ending firing of the nervous system and the brain's pain receptors, but no one knows exactly what causes it if it remains non-injury related. So it remains unexplained." 

Dr. Michael Hildebrand, an assistant professor at Carleton University and an affiliate investigator at the Ottawa Hospital Research Institute, says there's good pain and bad pain.

Good pain occurs when you touch a hot burner and immediately pull your hand away because it hurts. We need that kind of pain to prevent further injury. In contrast, chronic pain is a bad sort of pain that's no longer adaptive. This is pain that persists for months or even years after the injury or disease that triggered chronic pain has potentially resolved. Dr. Hildebrand says, "In a way, chronic pain can actually become a disease itself." 

Chronic pain can be caused by many mechanisms, but there is a unifying theme. We have pain pathways that connect the pain receptors to the brain that ultimately encodes the pain. Nerve cells called neurons in this pathway can lose their control and excitability, so they start firing more in response to pain signals. They can even fire without any stimuli. 

In a way, chronic pain can actually become a disease itself.- Dr. Michael Hildebrand

One main mechanism that can lead to chronic pain is inflammation, so there could be a persistent inflammation that releases chemicals that sensitizes the pathway causing all sorts of firing. Or there could be damage to the nerve cells, so that even in the absence of stimuli, they can continue to send the chemical messages leading to inflammation.

Even though we think of chronic pain as one type of pain, many processes can feed into it. We have drugs that work really well for the good pain, the acute pain, to block it. When it comes to chronic pain, the drugs don't work as well partly because of the time that's needed to constantly treat it.

Scientists are trying to identify new targets that can help turn down the pain and excitability, to restore balance to the pain system. They've identified a few potential candidates that could be new targets, which they hope they can use one day to treat chronic pain.