Complex regional pain syndrome - Causes 

Even though complex regional pain syndrome (CRPS) has been a recognised medical condition for more than 150 years, its exact cause is still unclear. The condition is usually triggered by an injury or trauma (damage to the body’s tissues). For example:

• A tissue injury with no nerve damage, such as a sprained ankle, can lead to type one CRPS (triggered by a minor injury).
• A peripheral nerve injury (when a nerve in one of your limbs is damaged), such as a broken bone, can lead to type two CRPS (triggered by a more serious injury).

Why CRPS develops as a result of these injuries is still unknown. However, due to the complex nature of the symptoms of CRPS, most experts believe that it is unlikely that the condition has a single cause, and that it is more likely that the condition is caused by a combination of related factors.

Some experts believe that it may be a mistake to regard CRPS as a single medical condition, as there could be a number of different conditions that all cause the same symptoms. Some of the theories that have been suggested as possible causes of CRPS are described below.

Psychological abnormalities

A number of psychologists, including Sigmund Freud, have suggested that CRPS may be a psychological condition. CRPS could be the result of an unidentified underlying psychological trauma or difficulty that makes people think that they are experiencing pain.

However, this theory has been largely discredited because research shows that people with CRPS undergo physical changes in their nervous system, making it very unlikely that the condition is "all in the mind".

Furthermore, as most people with CRPS do not have a history of serious mental illness, it would be uncharacteristic for them to suddenly develop a psychological condition.

Sympathetic nervous system malfunction

Another theory is that CRPS is caused by a malfunction of the sympathetic nervous system.

The sympathetic nervous system is the part of the nervous system that is responsible for triggering your "fight or flight" reflex. This is a series of physical changes that your body undergoes when faced with a stressful or dangerous situation.

For example, if you were suddenly confronted by a vicious dog, your sympathetic nervous system would increase your:

• heart rate
• breathing rate
• blood pressure
• levels of certain hormones, such as adrenalin (a stress hormone)

This gives you a short-term release of energy, which can be helpful in either confronting or avoiding potential danger.

This theory suggests that an initial physical injury may cause your sympathetic nervous system to release chemicals known as catecholamines. Catecholamines are hormones, like adrenalin, that your body releases during stressful situations, as part of the "fight or flight reflex" explained above.

Due to some unknown underlying problem, the catecholamines are thought to activate pain receptors, which are special nerve endings that transmit pain signals to your brain.

This may explain why people with CRPS often experience post-injury pain that is out of all proportion to the initial injury. It is not the injury itself that is causing the problem, but your body's abnormal response to it.

As the sympathetic nervous system also helps to regulate the blood vessels in your skin, a malfunction of this system may also account for the other associated symptoms of CRPS, such as the changes in skin colour and temperature.

However, there have been some cases of CRPS, particularly type two, where there was no evidence of a problem with the sympathetic nervous system. Therefore, it may not be a factor in all cases.

Immune system malfunction

Another theory states that type two CRPS may be the result of the immune system not working properly. When an injury occurs, the immune system causes the affected area of your body to swell in order to help prevent the spread of infection. This process is known as inflammation.

The theory states that for reasons unknown, inflammation persists long after the injury has healed. This irritates the nerves in the affected area of your body, leading to severe, burning pain. Inflammation would also explain the changes that occur in the blood vessels.

Other possible causes

Other theories that may help to explain the cause or development of CRPS include:

• genetic factors – some people may be more susceptible to CRPS because of genetic factors
• limb disuse – if you are not using the affected limb enough, CRPS may be more likely to occur following an injury
• treatment related – the treatment for the initial injury or trauma may have contributed to the CRPS - for example, a plaster cast that was too tight
• oxidative stress – tissue damage that is caused by free radicals (a toxic waste product that is a natural by-product of cell activity), which are produced in large quantities by the inflammation after the injury