The underlying cause or causes of fibromyalgia are still unknown, but there are many theories. Research in brain imaging and neurosurgery are exploring these theories as well as identifying new ones. Possible causes include:

• Neuroplasticity. Faulty synaptic connections (the connections between the nerves). If prolonged, it can result in pain despite the absence of a source for that pain. One of the most classic examples of neuroplasticity is phantom limb phenomenon, where an individual feels touch and pain sensations in a missing limb after amputation. It is believed that the pain felt by fibromyalgia patients, which has no identifying cause at the pain site, may have some link to neuroplasticity. Just what this link is or how extensive it may be is not known.
  
  
• Neurotransmitters. Brain chemicals that help transmit and amplify pain signals. Fibromyalgia patients often have abnormal levels of these chemicals. Specifically, levels of substance P (responsible for initiating pain signals) are high, and levels of serotonin (responsible for reducing intensity of pain) are low. This is believed to be responsible for the hypersensitivity (increased sensitivity) to pain demonstrated by patients with fibromyalgia.
  
  
• Changes in muscle metabolism. Deconditioned muscles, such as those caused by a general lack of fitness, may play a role in causing fibromyalgia. Poor fitness can result in decreased blood circulation in the muscles, cell damage within muscle fibers, and decreased voluntary muscle strength. It is unknown whether the decreased strength and increased fatigue of such muscles are merely symptoms of fibromyalgia or linked to a cause of the disorder.
  
  
• Infections. Viral infections (e.g., hepatitis, HIV, coxsackie virus) and bacterial infections such as Lyme disease (Borrelia burgdorferi) have preceded the onset of fibromyalgia in a large number of cases. These infections may cause abnormalities in the endocrine system that may lead to fibromyalgia.
  
  
• Physical or emotional trauma. According to the American Academy of Pain Management, trauma may play a role in causing fibromyalgia in some cases. Emotional trauma may lead to changes in brain chemistry, and physical trauma may affect the central nervous system. Either of these conditions may trigger fibromyalgia. It takes at least three months after a trauma incident for fibromyalgia to evolve, according to criteria of the American College of Rheumatology.
  
  
• Biological agents. Some physicians and researchers believe that there may be a link between specific biological agents (e.g., bacteria and toxins) and fibromyalgia. There is not a great deal of evidence to support this theory yet, but research is continuing.
  
  
• Genetics. Speculation remains as to how an individual’s genes regulate the way the body processes pain stimuli. Fibromyalgia may be associated with a gene or genes, but no such genes have yet been identified. However, the condition is more common among individuals closely related to fibromyalgia patients.
  
  
• Hormone changes. Some researchers believe that female reproductive hormones may be involved in the increased sensitivity to pain characterizing fibromyalgia. This may explain why the condition is far more prevalent in women.
  
  
• Sleep disturbances. Disrupted sleep patterns are a major symptom of fibromyalgia. Recent research indicates that these disturbed sleep patterns may be a cause rather than just a symptom.

Several factors lead to an increased risk of developing fibromyalgia. These include:

• Sex. Women are affected more often than men, although men may be underdiagnosed.
  
  
• Age. Onset of fibromyalgia occurs more often in early to middle adulthood than in childhood or late adulthood.
  
  
• Rheumatic disease. Fibromyalgia is found in increased numbers in individuals affected by rheumatic diseases.
  
  
• Family medical history. Individuals related to fibromyalgia patients have increased chances of developing fibromyalgia.
  
  
• Psychological and social factors. Fibromyalgia is found in increased numbers in individuals with histories of physical and sexual abuse, alcoholism or depression in their families.

In addition, the U.S. Institute of Medicine has found a higher prevalence of certain conditions in veterans deployed during the Gulf War of the early 1990s. These include fibromyalgia, chronic fatigue syndrome and a rare nerve disease called amyotrophic lateral sclerosis (ALS, also known as Lou Gehrig’s disease).