• Consider when limb injury causes unexpectedly severe or prolonged distal pain.
  
• The complex regional pain syndrome (CRPS) diagnosis also requires regional microvascular dysfunction (edema or abnormal skin color or temperature).
  
• Posttraumatic neuralgia (PTN) does not require microvascular dysfunction.
  
• CRPS often devolves through PTN as it heals.
  
• Variably present symptoms not required for the diagnosis of CRPS include disordered movement, sweating, and posture (eg, dystonia).
  
• Symptoms can be mild and transient, moderate, or severe and prolonged. Severe and prolonged symptoms are uncommon. Cases that are not associated with trauma may have an internal cause that requires attention. Such cases comprise a minority of CRPS cases.

The cardinal symptom of CRPS is chronic pain (neuralgia) in a region influenced by one or more damaged nerves. Full CRPS usually only develops in limbs because of their circulatory constraints; PTN can develop anywhere. Onset is usually immediate or within days of the injury. Nerve damage can be disproportionate to the visible injury, such as when routine venipuncture for phlebotomy or insertion of an intravenous catheter transects nerve twigs that encircle blood vessels. Approximately 80% of CRPS patients are female. The median age at onset is approximately 40, and the condition is rare in young children and the elderly. Contrary to widespread belief, most CRPS patients and virtually all children with this disorder recover spontaneously. Patients with prolonged or severe illnesses are uncommon and in many cases have complicating endogenous factors that impede healing (eg, smoking).

Different names have been used in the past for CRPS, depending on whether or not nerve injury was evident. Causalgia was first described in wounded Civil War soldiers with major nerve injuries. Reflex sympathetic dystrophy (RSD) described patients with seemingly trivial injuries without overt nerve damage. Other names included algodystrophy and Sudeck atrophy. In 1994, these terminologies were renamed CRPS types I (RSD) and II (causalgia). However, the use of this divided nomenclature is fading because later studies found subtle nerve injuries in CRPS I/RSD; the concept of “sympathetically maintained pain” is also fading, since it had little clinical utility. CRPS may be a complex form of PTN that involves neurogenic inflammation as well as pain. Both CRPS and PTN trigger extensive abnormalities of neural processing in the spinal cord and brain, including emotion and learning; fortunately, these seem to reverse during recovery. CRPS/PTN often spreads beyond classic single-nerve territories. These were mapped from myelinated motor and sensory axons, but CRPS/PTN primarily involves the thinly myelinated (A-delta) and C-fibers that often have far larger receptive fields and innervate bone and blood vessels outside the traditional nerve-map areas. Their axon terminals can couple electrically to adjacent neurons. Thus, injured axons can influence uninjured neighbors within nerve trunks, roots, and the spinal cord, explaining how tiny lesions can trigger regional dysfunction.

There is new appreciation of other contributors to the symptoms, including tissue ischemia likely initiated or maintained by malfunctioning microvessels. Neurogenic inflammation further recruits immunocytes to the area that perpetuate and extend inflammation and injury. Patients’ intrinsic biology seems to influence the development and persistence of symptoms even more than does the inciting injury. New epidemiologic data link CRPS with asthma and other hypersensitivity syndromes, and perhaps even with autoimmunity.

Accidental injuries, including fractures, sprains, and strains, are the most common cause of CRPS/PTN. About half occur on the job, and some trigger lawsuits. Medical (iatrogenic) injuries are a close second. It may not be the actual sprain that initiates CRPS, but the arthroscopy or casting used to treat it. The best prevention is to avoid unnecessary medical procedures and to remember that CRPS/PTN may possibly develop after any procedure involving cutting or punctures (Table 70–1). Tight casts are another well-documented cause. Limb edema, whether from the injury or from early CRPS, can create a compartment syndrome under a cast. Too many patients report being unable to get a constricting cast urgently removed. Early CRPS responds better to treatment than established cases, so any suggestion of early CRPS requires prompt evaluation and treatment.

Symptoms and Signs

The 1994 consensus definition developed by the International Association for the Study of Pain (Table 70–2) needs updating. More rigorous definitions have been proposed for research use.