Conditioned pain modulation equals less spinal cord activity: study

Researchers have found conditioned pain modulation (CPM), a measure of the brain’s ability to regulate pain, decreases activity in the human spinal cord as visible via functional magnetic resonance imaging (fMRI).

“This study confirmed our hypothesis that CPM results in significant
reductions in spinal dorsal horn activity in humans.” said lead study
author Ian Mackey, of Stanford University in Palo Alto, Calif. The
results were presented in a scientific poster at the American Academy of
Pain Medicine’s 30th Annual Meeting.

Recent research has
focused on the brain’s powerful ability to regulate the pain experience,
a capacity that is reduced in patients with chronic pain. CPM is one
way scientific investigators test endogenous analgesia in this setting.
CPM is when pain produced by a (conditioning) stimulus at one body part
results in reduced pain perception to a second pain-producing (test)
stimulus at a distant body part. The CPM effect is the reduced pain
rating of the second or test stimulus, also described as “pain
inhibiting pain.”

Low CPM efficiency, which indicates a low
capacity to inhibit pain through the body’s endogenous systems, has been
associated with the development of various pain syndromes, including
irritable bowel syndrome, temporomandibular disorders, fibromyalgia and
tension type headache and – in some reports – with neuropathic pain
(Yarnitsky, Curr Opin Anaesthesiol 2010;23(5):611-5).

The
current study set out to measure whether the same spinal basis first
described in rats also could be characterized in humans. Institutional
Review Board approval was obtained for the study, which was funded
through the National Institutes of Health and the Redlich Pain
Endowment.

Working with the hypothesis that CPM would reduce
fMRI activity in the human spinal dorsal horn, the investigators used a
standard CPM task in which the test stimulus of heat pain was applied to
the left forearm twice, once with and once without the CPM component (a
foot immersed in a cold water bath of 12 degrees Celsius). The
investigators collected two separate fMRI scans.

What the
investigators found was significant activity within the ipsilateral
dorsal horn; fMRI activity was significantly less during the test
stimulus plus CPM compared with the test stimulus alone. Furthermore,
changes in spinal cord activity correlated with the changes in perceived
pain reduction due to CPM.

Subjecting healthy subjects to fMRI
scans during noxious stimuli has led to researchers’ ability to create
high-resolution, neuronal activation maps of the human cervical spinal
cord, after correction for physiologic noise (Nash et al, Pain
2013;154(6):776-81). Imaging techniques that create objective evidence
of pain transmission are contributing to a growing body of scientific
literature to help scientists better understand the changes that occur
in the central nervous system during acute and chronic pain.

“We
hope these findings will serve as a platform for investigation of
abnormal spinal pain inhibitory systems in human chronic pain conditions
– and will help with targeting effective therapies for chronic pain,”
Mackey said.