Chronic Pain Alters Experience of Acute Pain
By Crystal Phend, Senior Staff Writer, MedPage Today Published: April 16, 2010 Reviewed by Dori F. Zaleznik, MD; Associate Clinical Professor of Medicine, Harvard Medical School, Boston and Dorothy Caputo, MA, RN, BC-ADM, CDE, Nurse Planner
Chronic pain may reverse the neural response to acute pain in some ways, researchers found.
The central nervous system sensory activation patterns and subjective sensation of pain were largely identical between healthy volunteers and chronic back pain sufferers in a brain imaging study reported in the April 15 issue of Neuron.
The difference was in how the nucleus accumbens region of the brain interpreted acute pain as a motivation for behavior, according to A. Vania Apkarian, PhD, of Northwestern University in Chicago, and colleagues
At the end of a painful heat stimulus, this portion of the brain registered phasic activity in chronic pain patients with the opposite polarity of that seen in healthy adults.
Although removal of a painful sensation usually registers as a reward, among chronic pain patients it actually appeared to reflect a predicted punishment, the researchers explained.
Acute pain robustly reduced the level of chronic pain perceived (P<0.004), such that its end predicted worsening of ongoing back pain, they suggested.
Apkarian’s group called this a potential “dysfunctional associative learning process” involved in the transition from acute back pain to chronic back pain.
The switch in the brain’s prediction and valuation of the pain — mediated by the nucleus accumbens and related circuitry — likely plays a role, not in the actual sensory perception, but in the internal state as “an integral component of the pathophysiology of chronic pain,” they wrote in Neuron.
Other changes have been shown in chronic pain in prior studies, including abnormal brain chemistry, regional gray matter atrophy, cognitive changes, and unique patterns of brain activity.
However, the phasic activity in the nucleus accumbens at the end of the painful stimulus was unique, they said.
For the study, the researchers imaged the brain using functional MRI while administering a pseudo-random series of nine thermal stimuli ranging from 47° to 51°C of 12 to 30 seconds each in 16 healthy adults and 16 patients with physician-diagnosed chronic back pain.
An additional eight chronic back pain patients rated chronic pain and pain associated with the stimuli but without MRI imaging.
During the fMRI tests, brain activity during the period when the heat was being applied and the pain reported from it was similar between groups.
But healthy individuals showed higher nucleus accumbens activity in the period when the thermal heat was decreasing and the pain was coming to an end compared with chronic pain patients (P<0.0001).
This difference at offset of pain distinguished the two groups with 100% sensitivity and 100% specificity, “implying that this signal can be used as an objective marker of chronic pain,” Apkarian’s group wrote.
Nucleus accumbens activity during this stimuli offset period appeared to be influenced from differing cortical sources between the two groups, too.
Activity in the insula best represented the magnitude of pain perception in this period among healthy controls. Meanwhile, the nucleus accumbens in chronic back pain patients was more strongly connected with activity in the medial prefrontal cortex — a brain area implicated in valuation, action selection, and pain modulation.
Chronic pain patients also had a higher poststimulus baseline activity (both P<0.0001), a difference that had 60% sensitivity and 66.7% specificity in distinguishing them from healthy volunteers.
The study was funded by a grant from the National Institute of Neurological Disorders and Stroke.
The researchers provided no information on conflicts of interest.
Primary source: Neuron Source reference: Baliki MN, et al “Predicting value of pain and analgesia: Nucleus accumbens response to noxious stimuli changes in the presence of chronic pain” Neuron 2010; 66: 149–60.