Chronic pain in osteoarthritis changes the brain and imparts a unique “signature” of morphologic or functional characteristics in the brain that may have future clinical implications, according to research presented at the 2011 ACR/ARHP Annual Scientific Meeting held in Chicago in November.
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A. Vania Apkarian, PhD, a pain researcher and professor of physiology at Northwestern University Feinberg School of Medicine in Chicago, reported his research findings during a presentation entitled “Pain, the Brain, and Osteoarthritis.” [Editor’s note: This session was recorded and is available via ACR SessionSelect at www.rheumatology.org.] He described an interaction between knee pain and the brain in patients with this chronic disease. “The anatomy of the brain in osteoarthritis patients is changing profusely, and it is changing at a rate that we can, in fact, identify,” said Dr. Apkarian. “The brain as a whole network is reorganizing simply by living with pain.”
Using functional magnetic resonance imaging (fMRI) in quantitative psychophysical studies, Dr. Apkarian’s research team studied brain activity in patients with osteoarthritis. The investigational use of this brain-mapping technique captured brain activity while a mechanical pressure stimulus occurred at the position on the knee where patients complained of pain. Patients also underwent fMRI while they subjectively rated their pain using a visual rating task (a finger signal). The investigators conducted the same experiment in healthy participants.
Surprisingly, the researchers found that the knee joint in arthritic patients was not sensitized to the pressure stimulus. The pressure pain ratings, and the brain activity during those ratings, were only minimally different between the patients with arthritis and the healthy people, they observed.
However, fMRI showed less than a 10% overlap in the regions of brain activity for ratings of pressure pain compared with those for ongoing pain such as chronic back pain, said Dr. Apkarian.
Various Pain Types Affect Different Brain Regions
The researchers also studied brain activity while patients rated ongoing osteoarthritis knee pain using well-validated pain scales, including the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) and the McGill Pain Questionnaire. Their findings, Dr. Apkarian suggested, indicate that ongoing chronic pain and spontaneous acute pain may have separate brain networks.
Other fMRI studies reported by Dr. Apkarian show different regions of brain activity for osteoarthritis knee pain and for other types of pain, such as allodynia, thermal pain, and fibromyalgia-related pain. In a study of patients with chronic back pain, complex regional pain syndrome, or knee arthritis, he and his coworkers found that gray matter decreased in density in different brain regions depending on the condition. Dr. Apkarian theorized this could mean that, “there is a very specific signature in the brain that differentiates osteoarthritis pain from other pain conditions.”