HealthDay News — Functional magnetic resonance imaging (fMRI) can be used to as a sensitive and specific tool to assess pain, data from several studies indicate.
Two studies measured neurological signatures on fMRI to see if it was possible to distinguish painful heat from nonpainful warmth. Results from the first study showed fMRI was able to distinguish between the two with sensitivity and specificity of 94% and greater (95% CI 89% to 98%).
Results from the second study were similar, with fMRI yielding a 93% sensitivity and specificity (95% CI 84% to 100%), Tor Wager, PhD, of the University of Colorado Boulder, and colleagues reported in New England Journal of Medicine.
In a third study, fMRI was able to discriminate between physical and social pains with 85% sensitivity (95% CI: 76% -94%) and 73% specificity (95% CI: 61%-84%), and had a 95% sensitivity and specificity in determining which of the two pains was greater, the researchers found.
Subjective self reporting has historically limited physical pain assessment, particularly among vulnerable populations such as the very old, very young, low-conscious patients and those with cognitive impairment.
To determine the ultility of fMRI to assess pain, Wager and colleagues evaluated an fMRI-based measure of pain intensity in four studies with a total of 114 healthy participants. In each study, the researchers applied thermal stimuli of varying intensities to participants’ forearms during fMRI scanning.
In the first two studies, they analyzed fMRI activity across brain regions associated with heat-induced pain. These included the thalamus, the posterior and anterior insulae, the secondary somatosensory cortex, the anterior cingulate cortex and the periaqueductal gray matter.
The third study assessed response to heat versus painful heat among participants who recently experienced a romantic breakup, while they viewed images of ex-partners or friends. In this study, comparing ex-partner vs. friend and pain vs. warmth “yielded similar levels of self-reported negative affect, and overlapping portions of many regions related to pain intensity were activated,” the researchers noted.
The fourth trial assessed pain responses among participants who received two IV infusions of the opioid agonist, remifentanil — one infusion that the participant was aware of and another where the participant was not made aware of dosing. On a forced-choice test of pain vs. warmth, the sensitivity was 95% (95% CI: 86%-100%) and specificity was 62% (95% CI: 43%-79%). When remifentanil was administered, the strength of the signature response decreased considerably.
“This signature consisted of interpretable, stable patterns across regions known to show increased activity in association with experimentally induced pain, hyperalgesic or allodynic states, experimentally induced acute pain in patients, and experimentally induced tonic pain in healthy persons,” the researchers wrote.
In an accompanying editorial, Assia Jaillard, MD, PhD, and Allan H. Ropper, MD, praise the studies for addressing “the question of whether particular regions of the brain are specific for physical pain and whether activity in these regions can be quantified,” adding that “these results may be of great practical importance, because physical pain is the most common reason for consultation with a physician.”
However, they emphasized that caution is necessary in interpreting the findings for several reasons:
- The studies only assessed cutaneous pain and may not be generalizable to pain in the context of disease or chronic pain
- Using photos of an ex-partner to assess emotional pain may prompt many other emotions that can affect activity of reward centers in the brain
- Low sensitivity of the 1.5-T fMRI system used in the study “may have led to the misidentification of small deep-brain structures that contributed to the neurologic signature response for pain.”
“Further studies in diverse clinical circumstances with the use of more-sensitive MRI acquisition techniques will be necessary to validate any pain biomarker,” Jaillard and Ropper wrote.
The researchers acknoweldged that further studies are necessary to determine whether the findings are generalizable to unhealthy populations in clinical settings.