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The mechanism of action in reflex sympathetic dystrophy (RSD) is not well understood and is the subject of extensive debate. Much of the confusion is due to the fact that RSD is clearly not a condition caused exclusively by the sympathetic nervous system and many experts believe that there must be a more complex reaction occurring in response to precipitating events that cause RSD.

There are at least two possible origins of pain in reflex sympathetic dystrophy: sympathetically-maintained pain which is pain caused by some "malfunction" in the sympathetic nervous system and sympathetically-independent pain.

Sympathetically-Maintained Pain

The sympathetic nervous system (SNS) regulates involuntary responses to stress such as increased heart rate and constriction of peripheral blood vessels as well as some of the body's initial response to any injury. Research indicates that the sympathetic nervous system also plays a role in neuropathic and inflammatory pain. In patients with RSD, there may be evidence of more widespread impairment of sympathetic nervous system function which is not necessarily limited to the affected extremity.

Until recently, it was thought that RSD was characterized by sympathetically-maintained pain where the SNS basically overreacted to an injury. Typically, after an injury occurs, the sympathetic nervous system is activated. It mobilizes the body's inflammatory response with the release of certain substances in order begin the process of healing the wound. The sympathetic response typically decreases within minutes or hours after the initial injury. When the inflammatory response continues unchecked, even when the stimulus is no longer present, the pain becomes sympathetically driven and the condition known as RSD develops. When treatment is directed towards interrupting the sympathetically-maintained pain, the patient experiences relief from pain.

Sympathetically-Independent Pain

With sympathetically-independent pain, the pain is caused by a combination of factors that interact with the SNS, such as the peripheral and central nervous systems. Treatments directed towards the SNS do not bring relief to people experiencing this type of pain.

Reflex sympathetic dystrophy appears to be a combination of both the sympathetic nervous system as well as other systems such as peripheral, central, immune, and vascular. In effect, what may be happening with RSD is that a vicious cycle is created: the sympathetic response leads to chemical changes which then activate the response of other systems (e.g., central nervous system) which leads to more pain, which leads to more chemical changes, and so on.

Some of the evidence that points away from RSD being exclusively related to the sympathetic nervous system includes:

• While some symptoms of RSD can be traced to the sympathetic nervous system, such as pain, or changes in sweating of the affected limb, other symptoms, such as warming of the limb, or swelling, are caused by substances released from other sources such as damaged blood vessels and not the sympathetic nervous system.

• Sympathectomy, a procedure which interrupts the flow of the sympathetic nervous system, is effective for individual patients, however larger clinical studies have shown the procedure to be no more effective than a placebo.