Several things! It is a disorder of central pain modulation or perception that involves the brain and spinal cord. It is not inflammatory and does not damage the joints, although it can co-exist with other forms of arthritis. Patients with fibromyalgia do not have pain in the joints per se, but in many muscular trigger points. Patients with fibro are deconditioned from the get go, so exercise programs help!
Joint Hypermobility. There is an association of mechanical spine/pelvic injuries, joint hypermobility syndrome, obesity, and musculoskeletal deconditioning with #fibromyalgia. Theories of central sensitization are theories, but without definitive proof. No one has definitively ruled out ongoing intense peripheral pain generation. See an osteopath for an in-depth musculoskeletal evaluation.
See below. Here is a nice summary on the disease from the mayo clinic: http://www.Mayoclinic.Com/health/fibromyalgia/ds00079.