Background/Purpose: An important aspect in chronic pain is central sensitization (CS) which involves increased responsiveness of the central neurons and it is clinically expressed like hyperalgesia and allodynia. CS is common in patients with OA and may reduce anti-inflammatory treatment effects and increase clinical severity. The aim of this study was to typify brain changes related to pain sensitization by functional MRI (fMRI) in patients suffering from chronic knee OA.

Methods: We designed a cross-sectional, single blind study and compared OA patients following clinical and radiological ACR criteria vs healthy controls. All participants were selected during one year and a half in the OA Unit in the Hospital del Mar (Barcelona). Presence of CS was assessed in OA group. CS was clinically defined based on the evidence of regional spread of pain (spreading sensitization assessed by an extended version of the Arendt­Nielsen peripatellar map¹) and increased pain response to repeated stimulation (temporal summation). The fMRI paradigm included 3 painful test: direct painful stimulation of the knee (articular interline) using a pressure of 2.5kg/cm², painful stimulation on the anterior tibial surface of the leg (sensitized site) by exerting a pressure of 4kg/cm², and painful heat stimulation on the forearm using 45º Celsius peaks.

Results: We included 60 OA patients (66.7 +/­ 7.8yrs) and 30 controls (62.8 +/­ 7.7yrs). A total of 33 patients showed some evidence of CS which 19 met all criteria of CS. At interline test, there was no difference on fMRI outcomes. At tibial test we found significant differences on brain activity which involved greater activation, in sensitized patients, in primary somatosensory area, supramarginal gyrus, sensorymotor cortex and basal ganglia. Correlation between brain response and clinical CS assessment was significant on somatosensory cortex, suppramarginal gyrus, anterior cingulated cortex and ventral putamen nucleus, bilaterally. No significant differences were found on brain activation between groups on the painful heat stimulation.

Conclusion: The presence of pain CS in chronic knee OA patients was very frequent. The pressure at medial interline has shown not to be an appropriate test to discriminate sensitized patients, since it is a maneuver with direct impact on the damaged structures in the disease. In contrast, relevant clinical pain in sensitized patients and increased brain response was produced by the pressure stimulation on the anterior tibial surface of the leg. Pain brain sensitization was related to a widespread activation of sensory cortices suggesting that sensitization is expressed mostly as an enhanced sensory phenomenon. At correlation maps, the changes in fronto­subcortical structures may speculatively suggest that pain CS also involves alterations of elements implicated in associative pain­related learning (e.g., associations of pain with everyday contexts). Finally, negative results in the painful heat stimulation test suggest that sensitization is not a general phenomenon that may not implicate superior limbs or the processing of heat­elicited pain.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/pain-central-sensitization-assessed-by-functional-magnetic-resonance-imaging-in-patients-with-knee-osteoarthritis/