The question submitted by J. was “What is the difference between Chronic pain and RSD? I understand that RSD is labeled chronic pain because it's daily, but why is chronic pain not always RSD? I was asked this today by someone else with chronic pain and they were drilling me with questions about RSD physiology and why doesn't he have RSD even though it hurts all the time.”

We will try and tackle this but remember, we are not medical professionals. This is a great question. Let's start by saying first that while nearly all RSD cases are chronic pain, not all chronic pain cases are RSD. I know that seems simple on the surface but it needs to be said in the beginning. Let’s start with a few definitions first. What is Chronic Pain and how is it different from Acute Pain? 

• According to MedicineNet.com, “Chronic Pain (an unpleasant sense of discomfort) that persists or progresses over a long period of time. In contrast to acute pain that arises suddenly in response to a specific injury and is usually treatable, chronic pain persists over time and is often resistant to medical treatments. Chronic pain may be related to a number of different medical conditions including (but not limited to) diabetes, arthritis, migraine, fibromyalgia, cancer, shingles, sciatica, and previous trauma or injury. Chronic pain may worsen in response to environmental and/or psychological factors. Chronic pain is defined as pain that lasts longer than 3 months. Some experts define it as lasting longer than 6 months”.
• According to About.com, “Chronic pain is different than acute pain in that it is not easy to find the cause. Diagnosis can reveal no injury in the body at all, and yet the patient can be experiencing very debilitating pain. One way that chronic pain begins is from an injury. Scientists have found that repeated pain from an acute injury changes the way the brain lets you know you have pain. Even after the injury has healed, pain messages replay over and over again.”
• According to Uptodate.com, ”...chronic pain serves no such physiologic role and is itself not a symptom, but a disease state. It is usually defined as pain which lasts beyond the ordinary duration of time that an insult or injury to the body needs to heal. This is commonly thought of as four to six weeks, although others have chosen three months as the dividing line between acute and chronic pain [3]. The difference is more than semantic; pain that outlasts this period may be a harbinger of a serious condition, such as Complex Regional Pain Syndrome, (CRPS), in which treatment delay may lead to an irreversible and intractable condition."
• According to WebMD, “The cause of chronic pain is not clear. When you have an injury or illness, certain nerves send pain signals to your brain. With chronic pain, these pain signals continue for weeks, months, or even years after you recover. Chronic pain can develop after a major injury or illness, such as a back injury or shingles, or it can develop without a known cause. It is also possible that certain brain chemicals that usually suppress pain may not work properly. Pain that continues for 3 months or longer is considered chronic. Pain is your body's way of telling you that something is wrong. It is normal for your body to send pain signals when you are injured or ill, but pain that lingers after an illness or injury is not normal. With chronic pain, the pain continues for weeks, months, or years after you recover. Some people develop chronic pain out of the blue, with no injury or illness to trigger pain signals. Chronic pain can occur anywhere in your body and can range from mild and annoying pain to pain so severe that it interferes with your mood and ability to function. Anyone can develop chronic pain. Although it is more common in older adults, it is not a normal part of aging. Older adults are more likely to have long-term medical conditions linked to ongoing pain, such as diabetes or arthritis.1”
• You can see a simplified version of the difference between chronic pain and acute pain below. 

What Causes Chronic Pain? “The pain may be:

• Neurogenic pain, or neuropathic pain, which occurs when the peripheral nerves or central nervous system are somehow damaged. The nerves themselves cause the pain, and this kind of pain may not respond well to treatment.
• Psychogenic pain, which is pain that may be caused by an emotional or mental health issue. This pain is not caused by a disease, injury, or damage to the nervous system. Psychogenic pain is not common, but stress, depression, and other mental health factors can make the pain worse.
• Unidentifiable pain. It may be impossible to find or identify the cause of your pain. Tests may not reveal any injury, illness, or tissue change that could have triggered the pain. Some chronic pain may be due to a diagnosable anatomical problem, such as degenerative disc disease or spinal stenosis, that can cause continual pain until successfully treated. More often, the chronic pain has no clear anatomical cause, as in failed back surgery syndrome or chronic back pain without an identified pain generator. In such cases, the pain is itself the disease."

Whew! Now that we know what chronic pain is, let’s talk about CRPS. Without going into a full-blown description of the disease let us concentrate on the four main symptoms;

• Constant chronic burning pain - also throbbing, aching stabbing, sharp, tingling, and/or crushing in the affected area.
• Inflammation - is not always present. It can take various forms, the skin may appear mottled, become easily bruised, have a shiny, dry, red, and tight look to it. An increase in sweating usually occurs as well.
• Spasms-in blood vessels and muscles of the extremities - result in a feeling of coldness in the affected extremity as well as body fatigue, skin rashes, low-grade fever, swelling (edema), sores, dystonia, and tremors. The spasms can be confined to one area or be rolling in nature; moving up and down the leg, arm, or back.
• Insomnia/Emotional Disturbance – CRPS affects the limbic system of the brain. This causes many problems that might not initially be linked to a disease like CRPS. Chief among them are Depression, Insomnia, Difficulty Concentrating, and Short-term Memory problems.

If you wish for a more involved definition of CRPS visit the main description section.
Source – American RSDHope

So now we have both Chronic Pain (CP) and CRPS defined. We know this is going to sound like an SAT question but; if we accept the premise that CRPS is a form of CP, then all CRPS patients have CP. So the original question is why don’t all CP patients have CRPS? The simple answer is that there are many things that can cause CP and CRPS is only one of them. There can be many causes for CRPS, anything from a stubbed toe, a burn, bursitis, to a problem with foot surgery, damage to a major or minor nerve, and in some cases no cause can be found. But to get a diagnosis of the disease;

• You must have most of the four main symptoms of the disease explained above.
• While there are some rare cases where burning pain isn’t present, burning pain is typically used as the chief defining symptom for CRPS. There are some other forms of CP where burning is a symptom, that is where a Doctor needs to look for the presence of the other three main symptoms.
• If you suffer from CP but don’t have at least two of the four main symptoms, chances are very slim that you have CRPS. Understand that this means over the course of months or longer and not always at the exact same time, although most patients will exhibit numbers 1, and 4 most of the time.
• An experienced physician will use a clinical diagnosis, coupled with your medical history, to make a definitive diagnosis of CRPS. Many times it takes 7 to 10 Doctors before you get the correct diagnosis unfortunately. Part of the reason is due to the progression of the disease and its ever-changing symptoms and part is due to the still-limited knowledge of the disease.
• CRPS pain is 24/7. Not all forms of CP are. There are some forms of CP that are relieved, albeit temporarily by raising your feet, or putting the limb on ice, things like that.
• CRPS cannot be treated with the application of ice and/or hot/cold contrast baths as other forms of CP almost always are without seriously damaging the myelin sheath surrounding the nerve itself and causing the CRPS to possibly spread and/or worsen. Again, there are exceptions but they are few and far between. 

Lastly, what truly sets CRPS apart from other forms of CP is the intensity of the pain. According to the McGill Pain Index, one of the most recognized Pain indexes, CRPS is ranked as the most painful form of CP that exists today. It ranks Arthritis and Fracture at an18 severe bone bruise at a19, non-terminal cancer pain at 25, and chronic back pain at a 26. CRPS pain is ranked at a whopping 42! We are sure that there are patients with CP who have yet to be diagnosed but the last thing they would want to wish for is a diagnosis of CRPS, unless that is truly what they have. It is definitely not a disease you would wish on your worst enemy and while much has been learned in the last 150 years there is a very long way to go. While the latest treatments, such as low-dose ketamine, have figured out a way to temporarily “un-stick” the pain-cycle and get it working properly again, they have just barely opened the door there.

We hope we have answered your question Jordan.
American RSDHope

PS – Remember, we not Doctors or medical professionals of any kind.