Currently, diagnosis is based on patient history and tender point sensitivity. 18 sites on the body have been classified as "Tender Points" . Extreme sensitivity upon palpation of at least 11 of these 18 points as well as a history of widespread, chronic body pain for at least 3 months provides the most definitive diagnosis at this time.
There are theories as to what may cause fibromyalgia, but at this point there is not enough evidence to support any single cause. Some theories suggest that fibromyalgia may be related to oversensitive nerve cells, an imbalance of chemicals in the brain (neurotransmitters), or an inability of the brain or spinal cord to dampen pain signals.