Animal studies involving ligation of the L5 spinal nerve in rats demonstrate that these animals develop similar symptoms (allodynia) as CRPS patients. Subsequent surgical sympathectomy reportedly showed significant reversal of these symptoms, demonstrating a sympathetic component.72 There are studies, however, involving L5 nerve ligation of rats noting that neuropathic pain behavior does not depend on the sympathetic nervous system.73 Other studies show neuropathic pain behavior possibly resulting from a mechanical injury to a peripheral nerve.74 CRPS may also be triggered by the arrival in the central nervous system of a transmission like injury discharge, produced by traumatized tissues (ligaments/tendons) with or without nerve injury.75 CRPS may involve damage to small diameter nociceptive fibers.76 A nociceptor is a sensory receptor that sends signals that cause the perception of pain in response to a potentially damaging stimulus. Nociceptors are silent receptors and do not sense normal stimuli. Only when activated by a threatening stimulus do they invoke a reflex, as would occur when a ligament or soft tissue is injured.77 (See Figure 8.)
| Figure 8. Nociceptors and CRPS. Any noxious or painful stimuli can activate nociceptors which sets up a cascade involving the central nervous system (CNS) and sympathetic nervous system (SNS) resulting in CRPS. |
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It is also proposed that there is a genetic predisposition to CRPS.78 The conclusion set forth after years of patient observation and research on humans and animals, is that CRPS is a complex neurological disease, involving the brain at several integrative levels.79
TESTING FOR CRPS
There are no specific diagnostic tests for CRPS which can reliably confirm or exclude the diagnosis.80, 81 Diagnostic criteria is purely clinical.82-84 It is based on history and physical examination and is not determined by test results, since the utility of diagnostic tests has not been demonstrated.85-87 Although there is no definitive test for CRPS, physicians try to use such tools as volumetry to measure edema, thermometry to measure skin temperature differences, and resting sweat output (RSO) to measure sweating, however it is not clear that objective measurement is precise. This instrumentation is used to measure clinically apparent signs like those included in the diagnostic criteria, however due to the confounding nature of CRPS, ie. changing body temperature, time of day, and exact placement of the device, it is unclear if objective measurement is even practical.88 Testing may be invasive or noninvasive, but data is not available for sensitivity or specificity of the tests.89 In a study comparing testing to clinical diagnosis, instrumentation added little to the overall accuracy of diagnosing CRPS type I, while no single test identifies all persons with CRPS.90, 91
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