Good question! I think because they are both chronic pain syndromes, a lot of the sx are similar - affected sleep, cognitive difficulties, TMJ, etc. The main difference is fibro is a muscular pain syndrome, where chronic pain affects the muscles, tendons and joints, as well as fatigue. Other sx follow, including nerve tingling/numbness occur as a result of the muscular pain/stiffness/cramping.

RSD's cause is still not clearly understood - but is thought to be either due to direct nerve injury (CRPS II) or microtrauma to nerve or soft tissue (CRPS I). It is a chronic neurological pain condition that is chiefly characterized by burning, shooting, or acid-like pain, along with abnormal sympathetic and autonomic nervous system responses. (Temperature change, allodynia, color change, hair loss, sweating, joint stiffness, skin lesions, etc).

There are similar sx, but separate causes.