Although complex regional pain syndrome (CRPS) is a recognised medical condition, its exact cause is still unclear.
The condition usually seems to develop within a month of an injury, either minor or more serious.
These can include:
Most people recover from these types of injuries without any significant long-term effects, but people with CRPS develop pain that's much more severe and long-lasting than usual.
The pain can spread beyond the original injury site, usually affecting an entire limb.
For example, CRPS may affect your whole arm after an injury to your finger or hand.
In some cases, more than one area of the body can be affected.
CRPS has also been known to occur after surgery to a limb or after part of a limb has been immobilised (for example, in a plaster cast).
It's not known why some people develop CRPS after an injury.
Because of the complex nature of the symptoms, it's unlikely the condition has a single, simple cause.
Some people even believe CRPS should not be regarded as a single medical condition, as the symptoms could be the result of several different conditions.
One of the main theories is that CRPS is the result of a widespread abnormal response to an injury that causes several of the body's systems to malfunction, including:
These systems are responsible for many of the body's functions that are often affected in people with CRPS, such as:
It's also been suggested that a person's genes may play a part in them developing CRPS after an injury.
But the precise role genes play in CRPS is unclear and it's very unlikely that other members of your family will be affected if you have CRPS.
In the past, some people believed CRPS may be a psychological condition that makes people think they're experiencing pain. But this theory has been largely disproven.