Can Pain Be Measured?

     From the New England Journal of Medicine:

Persistent pain is measured by means of self-report, the sole reliance on which hampers diagnosis and treatment. Functional magnetic resonance imaging (fMRI) holds promise for identifying objective measures of pain, but brain measures that are sensitive and specific to physical pain have not yet been identified. ...

In study 1, the neurologic signature showed sensitivity and specificity of 94% or more (95% confidence interval [CI], 89 to 98) in discriminating painful heat from nonpainful warmth, pain anticipation, and pain recall. In study 2, the signature discriminated between painful heat and nonpainful warmth with 93% sensitivity and specificity (95% CI, 84 to 100). In study 3, it discriminated between physical pain and social pain with 85% sensitivity (95% CI, 76 to 94) and 73% specificity (95% CI, 61 to 84) and with 95% sensitivity and specificity in a forced-choice test of which of two conditions was more painful. In study 4, the strength of the signature response was substantially reduced when remifentanil was administered.

     Before anyone gets excited at the thought that Social Security can finally measure pain, a few caveats are in order:

• There were only 114 participants in the study. 
• The test subjects were described as healthy with a median age of just under 25 years. The study's authors caution that the results might be different in non-healthy individuals.
• The pain that was "measured" was administered by the researchers -- "thermal stimuli" to the left forearm of varying degrees of severity.
• The study's authors indicate that if the validity of the study could be shown to extent to "clinical populations", the test might be used to confirm pain in patients who cannot communicate. Also, the study could be used as a basis for other studies. The study's authors are not even suggesting that the fMRI could have forensic uses.
• The study's authors caution that the study's results would have to be validated across persons, scanning protocols and research sites.
• The study's authors caution that the results might be different depending upon the site in the body where the pain is generated, the clinical cause of the pain and the type of the pain. For instance, would visceral pain register differently than the cutaneous pain that the researchers generated in their study? How would neuropathic pain compare with the pain from arthritis?
• fMRIs are expensive. They are an uncommon test with limited uses at the moment. Probably, there would be no clinical reason to use the fMRI to evaluate pain in a person who can communicate, meaning that if used for disability determination purposes, Social Security would probably have to foot that expensive bill.