Complex regional pain syndrome (CRPS) typically starts after an injury, but the pain it produces is wildly out of proportion to the original damage. A wrist fracture, a minor sprain, or even a routine surgery can set off a chain reaction in which the nervous system, immune system, and blood vessels all malfunction together, creating persistent, severe pain in the affected limb. The estimated incidence is roughly 26 per 100,000 people per year, with women affected about 3.4 times more often than men.
What makes CRPS so puzzling is that the initial injury is often unremarkable. The real question isn’t just what starts it, but why the body’s response goes so dramatically wrong in certain people. The answer involves several overlapping systems.
Injuries That Trigger CRPS
Fractures are the single most common trigger, particularly wrist fractures. A displaced or splintered bone can damage nearby nerves directly, or a tight cast can compress them. Surgery is the second major cause: the incision itself, the instruments used to hold tissue open, stitches, and post-surgical scarring can all injure small nerve fibers.
But CRPS doesn’t require a dramatic injury. Sprains, strains, burns, and cuts can all set it off. In children, the known trigger is frequently something as minor as an ankle sprain. Even prolonged immobilization in a cast, which deprives the limb of normal sensory input like touch and temperature, has been linked to CRPS onset. In rare cases, a needle stick that pierces a superficial nerve is enough.
Doctors distinguish two types. CRPS-1, which accounts for the majority of cases, develops without a confirmed nerve injury. CRPS-2 involves a documented injury to a specific nerve. Both types produce the same constellation of symptoms, and the distinction matters more for classification than for treatment.
How the Nervous System Overreacts
After the initial injury, something goes wrong with the way pain signals are processed. Normally, inflammation around a wound calms down as healing progresses. In CRPS, the pain-sensing nerve fibers at the injury site become hypersensitive and stay that way. Inflammatory molecules and nerve growth factors keep exciting these fibers long after they should have quieted, a process called peripheral sensitization. The nerves essentially get stuck in alarm mode.
Over time, this barrage of pain signals rewires the spinal cord and brain. Neurons in the central nervous system that relay pain become more excitable, while the neurons that normally dampen pain signals lose their braking power. The result is central sensitization: the brain begins interpreting ordinary touch, mild pressure, or normal temperature changes as painful. This is why people with CRPS can experience agonizing pain from something as light as a bedsheet resting on their skin.
These changes in the central nervous system appear to be both functional and structural, meaning the brain doesn’t just misinterpret signals temporarily. It physically reorganizes in ways that sustain the pain. This helps explain why CRPS can persist for months or years, long after the original injury has fully healed.
An Inflammatory Response That Won’t Shut Off
Acute CRPS looks a lot like inflammation. The affected limb often turns red, swells, and feels warm to the touch. These are the classic signs of an inflammatory response: redness, swelling, heat, pain, and loss of function. Research shows this isn’t just a coincidence. It reflects an exaggerated post-traumatic inflammatory process that feeds on itself.
People with CRPS have elevated blood levels of pro-inflammatory signaling molecules, particularly tumor necrosis factor (TNF) and interleukin-2. TNF plays a double role: it sensitizes pain receptors directly, and it also triggers the release of a neuropeptide called CGRP, which dilates blood vessels and causes fluid to leak into surrounding tissue. This creates the visible swelling and skin color changes that characterize early CRPS. Meanwhile, these same inflammatory signals keep feeding back into the pain-sensing nerves, making them even more reactive.
Blood Flow Problems in the Affected Limb
One of the more counterintuitive features of CRPS is that the affected limb often has increased blood flow, yet the tissues are starved of oxygen. Research from the International Association for the Study of Pain suggests this paradox comes down to capillary dysfunction. After injury, high pressure in the surrounding tissue compresses the smallest blood vessels. Byproducts of blood cell breakdown cause the tiny muscles around capillaries to constrict, and inflammatory molecules make blood cells stick to vessel walls.
The combined effect is that blood rushes through the limb but can’t deliver oxygen efficiently. The body tries to fix this by dilating blood vessels further, but that actually makes the oxygen extraction problem worse. Eventually, the body resorts to constricting blood flow to protect oxygen levels, but this produces reactive oxygen species that damage the blood vessels themselves, creating a vicious cycle of poor circulation and tissue stress. This helps explain the skin color changes, temperature differences, and tissue wasting that develop as CRPS progresses.
The Autoimmune Connection
A growing body of evidence suggests that CRPS may involve an autoimmune component, at least in some patients. Researchers have found that a majority of CRPS patients carry antibodies in their blood that target and activate receptors on the autonomic nervous system, the system controlling involuntary functions like blood flow and sweating. When these antibodies were transferred to mice, the animals developed abnormal pain behaviors.
One proposed model is a “two-hit” process. The first hit is a set of autoantibodies that are already circulating in the person’s blood before the injury, causing no harm on their own. The second hit is the injury itself, which creates local conditions (inflammation, tissue damage, disrupted blood flow) that allow those antibodies to bind to targets in the affected limb and trigger a pathological response. This could explain why CRPS symptoms are concentrated in one limb even though the antibodies circulate throughout the body. Notably, researchers have found that people with CRPS also show subtle body-wide abnormalities, including mild autonomic dysfunction and enhanced inflammatory responses in their unaffected limbs.
Genetic Susceptibility
Not everyone who breaks a wrist or has surgery develops CRPS, which raises the question of genetic vulnerability. Several studies have identified associations between CRPS and specific variations in the HLA system, a group of genes that regulate immune function. HLA-DR13, HLA-DR2, and HLA-DQ1 have all been linked to CRPS or specific subtypes of the condition. The association with HLA-DR13 is particularly notable in patients whose CRPS progresses to involve involuntary muscle contractions (dystonia) or spreads to multiple limbs.
These are the same types of genetic markers found in other autoimmune diseases, which supports the idea that immune system dysfunction plays a role in at least some cases. Most of the genetic studies to date have been small, so the full picture of inherited risk is still incomplete. But the pattern is consistent: people whose immune systems are genetically primed toward overreaction appear to be at higher risk.
Other Risk Factors
Beyond genetics, several health and lifestyle factors can increase the likelihood of developing CRPS after an injury. Conditions that compromise nerve health, particularly diabetes, leave nerves less able to repair themselves. Peripheral neuropathy from any cause means that an injury that would normally heal without complications can instead trigger a disproportionate pain response.
Smoking is a significant risk factor because it directly interferes with nerve regeneration. Previous chemotherapy can have a similar effect, leaving nerves more vulnerable to lasting damage. Immune system dysfunction of any kind also appears to raise risk, given the role that inflammation and autoantibodies play in the disease process. The highest incidence occurs in women between ages 61 and 70, though CRPS can develop at any age, including in children.
How CRPS Is Identified
Because there’s no single blood test or imaging study that confirms CRPS, diagnosis relies on a standardized clinical framework called the Budapest Criteria. These criteria evaluate symptoms and physical signs across four categories: sensory (pain, heightened sensitivity), vasomotor (temperature and skin color changes), sudomotor/edema (sweating differences and swelling), and motor/trophic (weakness, tremor, changes in hair or nail growth).
To meet the diagnostic threshold, you need to report at least one symptom in three of those four categories, and a clinician needs to observe at least one sign in two or more categories during a physical exam. The criteria were updated in 2019 to clarify definitions and improve consistency between clinicians, but the core framework remains the same. Early diagnosis matters because CRPS generally responds better to treatment when caught in its initial stages, before central sensitization becomes deeply entrenched.