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Brain imaging should not be used as a test for chronic pain, researchers from around the world have concluded after a nearly 2-year long evaluation into neuroimaging measures.

The presidential task force of the International Association of Pain was established in 2015 to examine the feasibility of a brain imaging-based diagnostic test for chronic pain, and now it’s members have delivered a consensus statement, published last week in Nature Reviews: Neurology.

“It's not possible at this point in time to say with any degree of certainty that a person does or does not have chronic pain based on brain imaging," said Karen Davis, task force chair and the head of the Division of Brain, Imaging and Behaviour-Systems Neuroscience at the Krembil Research Institute at University Health Network (Canada). “The only way to truly know if someone is in pain is if they tell you because pain is subjective and it is a complex experience. No brain scan can do that."

Currently, chronic pain affects up to 35 percent of the population, and has staggering medical care costs, as well as loss of wages and productivity. Chronic pain also finds itself in the middle of many legal disputes, as it’s an “invisible disease.”

Thus, a test for chronic pain would help all—patients wouldn’t be subjected to doubt; researchers would have actionable data; and legal officials could objectively support or challenge claims.

But, the task force concluded, the pain experience is too multidimensional for the technological brain imaging methods currently available, including fMRI, PET, EEG and MEG. Although previous imaging studies have shown stimulus-evoked pain and acute pain is associated with activity in many brain areas, most of those brain areas cannot be specifically associated with pain.

In principle at least, functional brain imaging can measure three types of activity: evoked activity; task-free resting state brain activity; and activity related to a particular attribute of ongoing clinical pain. But, the researchers found multiple problems with this when applied to chronic pain. For one, chronic pain can remain constant or vary slowly during the course of a brain imaging session, rendering it essentially invisible to fMRI. fMRI’s fail again when assessing resting-state brain activity, as any change in the pattern associated with pain during an imaging session cannot be directly attributed to such—patterns can be altered in a range of clinical conditions, so uncertainty exists.

For ongoing clinical pain, PET-based or ASL-based measurements of cerebral blood flow did prove informative, as the techniques are quantitative and do not require an external stimulus. However, as with anything brain-based, the possibility exists that the acknowledgement of the ongoing experiment caused a change in brain processing.

Based on their research, the task force recommended six criteria to drive a possible framework for neuroimaging of chronic pain:

  • Precise definition of a pain neuromarker
  • Applicability of the pain neuromarker to individuals
  • Methodological procedures used during testing must be validated
  • Measure must be internally consistent, and the equality of imaging data validate for the individual tested, using positive and negative controls
  • The neuromarker must be diagnostic for pain
  • The neuromarker must be validated with converging methods
  • The neuromaker must be generalizable to the patient group tested and to the test conditions

Due to the uncertainty rampant in this research, the researchers were forced to conclude that brain imaging methods for chronic pain cannot be used in the legal system at this time.

"Use of such tools would be inappropriate and unethical," said Davis. "This technology is not foolproof. There are vast issues of variability between people and even within a person at different times. As a result, brain imaging must not be used as a lie detector for chronic pain."

However, the researchers were more favorable in their application of brain imaging for research purposes.

“With suitable standardization and validation, several forms of brain imaging could provide objective biomarkers of key characteristics of chronic pain,” the paper reads. “Identification of brain-based markers of chronic pain might be possible with further technological advancement, large-scale data acquisition across diverse groups of individuals, and strict application of standards of evidence to the assessment of potential pain biomarkers.”

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