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Science Stories: Is chronic pain a symptom of the brain?

Two people experience the same injury.  Both receive identical treatment and rehabilitation programs – so naturally we would expect to see similar results.  Unfortunately this is not always the case. Six months later one patient has fully recovered, while the other develops a chronic pain condition.  What has caused the transition to chronic pain and why did it happen to one patient and not the other? The Robinson Institute’s Dr Ann-Maree Vallence is currently undertaking studies to understand how changes in the brain may be contributing to the development of chronic pain. 

Chronic pain conditions are common.  In Australia approximately 20% of adults suffer chronic pain of moderate to severe intensity.  For these people the condition can lead to significant social and economic consequences that affect their quality of life.  Although prevalence is high, currently the mechanisms responsible for the development of chronic pain are poorly understood. Dr Ann-Maree Vallence, a University of Adelaide Postdoctoral Fellow in the Robinson Institute, has been undertaking studies to understand how changes in the brain, termed neuroplasticity, may be responsible for the transition from acute pain to chronic pain.

Neuroplasticity is the brain’s ability to change structurally and functionally with experience and use. Neuroplasticity underlies learning and memory – making it vital during early childhood development and important for continuous learning throughout life. Broadly, Ann-Maree’s research is understanding the mechanisms that mediate neuroplasticity in the human brain and how altered neuroplasticity contributes to the pathophysiology of disease – in this case chronic pain.

“To date, most chronic pain research focusses on changes in the spinal cord.  This research is novel in that it investigates the role of brain plasticity in the development of chronic pain,” says Ann-Maree.

“While the focus of this particular study is chronic pain, developing a better understanding of the mechanisms of neuroplasticity, and characterising how changes in neuroplasticity contribute to impairment, will be useful in understanding developmental learning problems, including poor cognitive function and movement control, as well as neurological disease in adulthood, such as stroke.”

To understand the role neuroplasticity plays in the development of chronic pain, Ann-Maree conducted her study on patients with Chronic Tension-Type Headache (CTTH), a common chronic pain disorder.  CTTH is characterised by a dull, constant feeling of pressure or tightening that usually affects both sides of the head, occurring for 15 days or more per month. It is also associated with other symptoms such as sleep disturbances, irritability, disturbed memory and concentration, as well as depression and anxiety. Currently the treatment for CTTH is limited to pain medication, which often results in side effects and can induce dependency.

“People living with CTTH and other forms of chronic pain may experience reduced quality of life, as the pain often prevents them from working, amongst other things. It is therefore imperative that we understand the causes of chronic pain, not just attempt to treat the symptoms with medication,” says Ann-Maree.

Recently, the potential for non-invasive brain stimulation techniques, such as Transcranial Magnetic Stimulation (TMS), to treat chronic pain has been investigated. TMS can induce short lasting neuroplastic changes in the brain which may be helpful in reducing pain symptoms. However, while this work shows promising results for treatment, without a good understanding of the mechanisms in the brain that mediate chronic pain, the rationale is weak and targeted application of brain stimulation is not possible. Ann-Maree recognises this gap and aims to contribute to the current body of knowledge.

“Actually identifying a cause, such as an impairment in neuroplasticity, gives us a target for treatments. We can identify the region to treat and the type of non-invasive brain stimulation that is most likely to prove effective in reducing pain,” says Ann-Maree.

When designing and conducting the study, Ann-Maree worked with her supervisor Associate Professor Michael Ridding, who is an expert in non-invasive brain stimulation – having been part of the laboratory at University College London that pioneered the use of these techniques.

“The Robinson Institute’s Neuromotor Plasticity and Development (NeuroPAD) Group, which is co-led by Dr Julia Pitcher and myself, has state of the art capabilities in a range of non-invasive brain stimulation techniques which we use to investigate function and impairment in both children and adults,” says Michael.

“Ann-Maree is a valued member of the Group and her work in chronic pain is not only addressing this specific condition but contributing to our broader understanding of the brain and neuroplasticity, which may open up new possibilities in treatment and prevention.”

To determine if changes in brain plasticity contribute to the development of chronic pain, the study investigated use-dependent neuroplasticity.  CTTH patients and individuals with no prior history of chronic pain were required to undertake a motor training task consisting of moving the thumb as quickly as possible in a specific direction. The change in performance (or learning) on the task was investigated by recording how quickly subjects moved their thumb. TMS was also used to obtain a measure of neuroplasticity associated with the improvement in performance.

“Typically, when individuals undertake a motor training task such as this, their performance improves over time and this is associated with a neuroplastic change in the brain” says Ann-Maree.

“The individuals with no history of chronic pain got better at the task with training, and we observed an associated neuroplastic change in the brain. Our CTTH patients on the other hand, did not get better at the task and there was no associated neuroplastic change – suggesting impaired use-dependent neuroplasticity.”

“These results are very exciting and provide a novel and important insight into the cause of chronic pain. Additionally, this finding may help in the development of a more targeted treatment for CTTH and other chronic pain conditions.”

While these results provide some evidence for a role for impaired neuroplasticity in CTTH, more work has to be undertaken to determine whether this finding is true for chronic pain more generally.

“The next step is to determine whether our findings apply to other chronic pain groups not just CTTH. We are currently repeating the study, investigating use-dependent neuroplasticity in a group of patients with Fibromyalgia, which is a more generalised pain group,” says Ann-Maree.

“If this group shows the same result, the knowledge will be used to develop targeted treatments for reducing chronic pain.”

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