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It's not clear why some people develop fibromyalgia. The exact cause is unknown, but it's likely that a number of factors are involved.

Here are some of the main factors thought to contribute to the condition.

One of the main theories is that people with fibromyalgia have developed changes in the way the central nervous system processes the pain messages carried around the body.

This could be the result of changes to chemicals in the nervous system.

The central nervous system (brain, spinal cord and nerves) transmits information all over your body through a network of specialised cells.

Changes in the way this system works may explain why fibromyalgia results in constant feelings of, and extreme sensitivity to, pain.

Research has suggested genetics may play a small part in the development of fibromyalgia, with some people perhaps more likely than others to develop the condition because of their genes.

If this is the case, genetics could explain why many people develop fibromyalgia after some sort of trigger.

Fibromyalgia is often triggered by an event that causes physical stress or emotional (psychological) stress.

Possible triggers include:

• a serious injury, such as after a car accident
• an infection, such as Epstein-Barr virus or Lyme disease
• having a major operation
• significant emotional trauma, such as after serving in the armed forces during a war

Sometimes, fibromyalgia does not develop after any obvious trigger.

There are several other conditions often associated with fibromyalgia.

Generally, these are rheumatic conditions (affecting the joints, muscles and bones), such as:

• osteoarthritis – when damage to the joints causes pain and stiffness
• rheumatoid arthritis – when the immune system mistakenly attacks healthy cells in the joints, causing pain and swelling
• ankylosing spondylitis – pain and swelling in parts of the spine
• temporomandibular disorder (TMD) – a condition that can cause pain in the jaw, cheeks, ears and temples