Rheumatoid arthritis is an inflammatory illness that causes great emotional and physical pain. However, it would appear that for many rheumatoid arthritis patients, not all pain is due to inflammation: Such patients continue to report significant levels of pain, even in the setting of well-controlled disease. Research has shown the effects of depression, and the complex relationship between pain, inflammation and psychiatric difficulties-including poor sleep hygiene and mood.
The results of a recent study, published in "Arthritis Research & Therapy", demonstrated an inverse association between the inflammatory marker C-Reactive Protein (CRP) and the joint pain threshold in rheumatoid arthritis. Peripheral pain mechanisms (e.g., peripheral sensitization) are characterized by local areas of increased sensitivity to pain in response to the induction of inflammation. In contrast, central pain mechanisms have more widespread effects-involving the joint, and outside the joint. And this study demonstrated that sleep difficulties were inversely associated with pain threshold, implicating a defect in central pain processing.
Previous studies had examined non-inflammatory pain syndromes, such as fibromyalgia, and found two important mechanisms of widespread pain sensitivity: central sensitization and loss of diffuse noxious inhibitory control. Central sensitization involves an acute phase, with sensitization of pain receptors in the spinal cord; and a later phase, involving generalized changes in the brain. The loss of diffuse noxious inhibitory control is manifested by a diffuse hypersensitivity to pain due to the impairment of inhibitory neurologic responses that normally reduce pain. There is evidence of central pain processing mechanisms in rheumatoid arthritis, one study showing an altered regional blood flow in the prefrontal cortex, cingulofrontal cortex and the anterior cingulated cortex among rheumatoid arthritis patients exposed to painful heat stimuli.
It is well-known that the perfect storm of heightened pain sensitivity, poor sleep hygiene and psychiatric difficulties is common among individuals suffering from fibromyalgia, a chronic pain condition that affects almost 20% of rheumatoid arthritis patients. One might suspect that this subgroup of patients affected the study in question. However, it was found that the associations between pain threshold, CRP, sleep problems and psychiatric difficulty remained unchanged after excluding patients with 11 or more tender points from the final analysis.
Pain in rheumatoid arthritis has protean causes. Care-givers need to consider the inflammatory and the non-inflammatory factors when a patient approaches with pain complaints, and respond with multidisciplinary approaches to alleviate the pain.