Previous studies have demonstrated that MMP-9 plays an important role in inflammatory and painful processes, and the Escolano-Lozano F study found that elevated serum MMP-9 levels correlated with the severity of complex regional pain syndrome [35], and several other studies found that MMP-9 dysregulation was associated with acute migraine [36] and lumbar spinal stenosis pain [37]. Here, MMP9 is linked to complex regional pain syndrome.