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Complex Regional Pain Syndrome
Introduction
Complex Regional Pain Syndrome (CRPS) is a neuropathic pain disorder characterized by ongoing pain disproportionate to the degree of tissue injury and persists beyond the usual expected time for tissue healing. [1] Pain is accompanied by sensory, motor, and autonomic abnormalities. Such abnormalities include allodynia, hyperalgesia, sudomotor and vasomotor abnormalities, and trophic changes. Pain does not follow a particular dermatome or myotome but is rather regional. This disabling condition often develops after a trauma, fracture, or surgery.[2][3] But some spontaneous cases have also been reported.[4]
In the 16th century, Ambroise Paré reported cases with CRPS like symptoms for the first time which developed after phlebotomy.[5] In 1864, Silas Mitchell noted this syndrome after gunshot wounds. He used the term ‘causalgia’ to describe this syndrome in 1872. James A. Evans coined the term ‘reflex sympathetic dystrophy’ to describe a similar condition where he suspected sympathetically mediated pain in 1946. [6] Finally, in 1994, the International Association for the Study of Pain (IASP) named this condition ‘Complex Regional Pain Syndrome’ and proposed a diagnostic criterion. Due to low specificity, a widely accepted revised criterion was proposed in 2010 and is commonly referred to as the "Budapest Criteria."[2][7]
CRPS has two subtypes: type I, formerly known as reflex sympathetic dystrophy, and type II, formerly known as causalgia. Type I occurs in the absence of nerve trauma, while type II occurs in the setting of known nerve trauma. Clinically they are indistinguishable and follow a regional rather than a dermatomal or peripheral nerve distribution and favor the distal extremities, though spread outside of the initially affected area commonly occurs to the proximal or contralateral limb.[6][8] CRPS is further subdivided into "warm" versus "cold," and sympathetically-maintained (SMP) versus sympathetically-independent (SIP), which may affect prognosis and treatment options. [8]
CRPS not only impacts function, sleep, and activities of daily living but also takes a significant mental and psychosocial toll on the patient.[9][10][11] Its diverse spectrum of clinical presentation and lack of any clearly defined pathophysiology poses a challenge for optimal management of this disorder.
Etiology
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Etiology
CRPS occurs as a result of varying degrees or types of tissue trauma but has even been documented in the absence of injury or due to periods of prolonged immobilization. The most common injury associated with developing CRPS is a fracture. Surgery is another common etiology. Other common inciting injuries or insults include sprains, contusions, crush injuries, and surgery. CPRS even has been reported to arise after seemingly innocuous interventions such as intravenous line placement. Increased psychological distress experienced during the physical injury associated with the onset of CRPS may affect its severity and prognosis.
Fracture
CRPS has been noted to be commonly associated with extremity fractures. A large multicenter prospective study found that 48.5% of patients developed CRPS (IASP criteria) after suffering a single fracture of the ankle, wrist, scaphoid, or the fifth metatarsal. All patients remained symptomatic at 1-year follow-up. Rheumatoid arthritis and intraarticular ankle fractures and dislocations were identified as risk factors for CRPS. However, no significant difference for disease onset was noted between fractures of arms or legs.[12] Another prospective cohort study found that CRPS developed within 8 weeks after a noxious event. Symptoms improved in many patients at 3 months, but no significant improvement was noted at about a year.[13]
Some studies in patients who developed CRPS after fracture of the distal radius identified higher age, social or psychological factors, and psychiatric comorbidities as risk factors.[14][15] But another prospective study did not find any correlation between psychological factors or depression symptoms and the development of CRPS.[16]
Surgery
Like fractures, extremity surgeries also seem to be more commonly associated with the development of CRPS. In a retrospective study of 390 patients who underwent foot and/or ankle surgeries, 4.36% developed CRPS.[17] Surgical management of fractures has been found to have a higher risk of CRPS. In patients undergoing closed reduction of distal radius fracture, 32.2 % patients developed CRPS.[18] Carpal tunnel surgeries were noted to have a 2 to 5% and Dupuytren contracture surgeries 4.5 to 40% chance of developing CRPS.[19]
Genetics
The impact of genetic factors in the development of CRPS is yet unclear. Human leukocyte antigen and tumor necrosis factor-alpha (TNF-α) polymorphism have been found to play a role in CRPS. These factors can lead to an earlier age of onset and more severe symptoms. Familial inheritance has been suggested by a few retrospective reports.[19]
Epidemiology
The incidence of CRPS seems to vary based on geographical location. A study by Sandroni et al. in Olmsted County reported in 2003, Minnesota found an incidence of 5.46 per 100,000 person-years for CRPS type I and 0.82 per 100,000 person-years for CRPS type II.[20] But another study by Mos et al. in the Netherlands, reported in 2006, found the incidence to be much higher at 26.2 cases per 100,000 person-years.[21] Both studies found that females were more commonly affected. The first study found that females were four times more likely to be affected than males, while the second study affirmed that this disorder was at least three times more common in females.[20][21]
The Netherlands study reported a peak incidence at 61–70 years of age, while the American study found the median age of onset to be 46 years. Upper extremities were found to be more commonly involved than lower in both studies. Both studies used the IASP CRPS criteria for diagnosis. The most common trigger for the disorder was found to be a fracture, associated with 44 to 46% of the cases. Among clinical symptoms, vasomotor symptoms of swelling, temperature, and color changes were most commonly reported.[20][21]
Among diagnostic tests, three-phase bone scans were found to be most helpful in making a diagnosis (85%). Compared to this, autonomic testing was helpful in diagnosing in 80% of cases.[20] Asthma, angiotensin-converting enzyme (ACE) inhibitor use, menopause, osteoporosis, and history of migraine are risk factors for CRPS.[21][22] Cigarette smoking also seems to increase the risk of developing CRPS.[23]
Pathophysiology
Multiple pathophysiologic mechanisms have been described in the literature so far to explain CRPS. Scientific evidence does not point to a single principal mechanism. Therefore, the underlying mechanism seems to be multifactorial. Inflammatory, immunological, central, and peripheral sensitization, as well as autonomic changes, have been studied in CRPS.[6]
Inflammatory Changes
Both the clinical presentation and elevated inflammatory laboratory markers suggest that inflammation is a key mechanism underlying the development of CRPS. The basic signs of inflammation, such as increased temperature, swelling, redness, pain, and functional impairment, are commonly associated with CRPS.[24] Elevated levels of pro-inflammatory cytokines such as TNF-α, Interleukin (IL)-1b, IL-2, and IL-6 have been found in both serum and cerebrospinal fluid of CRPS patients.[25][26][27][28] Elevated levels of neuropeptides like calcitonin gene-related peptide (CGRP), bradykinin, and substance P released from peripheral nerve endings likely as a result of tissue injury in CRPS trigger neurogenic inflammation. The elevated levels of inflammatory markers and neuropeptides lead to vasodilation and tissue extravasation.[22][29][30][31][32]
Immunological Changes
Autoimmune factors seem to play a role in CRPS pathogenesis. Autoantibodies against beta-2-adrenergic receptor, alpha -1a-adrenergic receptor, and muscarinic-2 receptor have been found in CRPS.[33][34] Goebel et al. noticed a significant improvement in pain following intravenous immunoglobulin treatment in CRPS patients. which further supports potential autoimmune pathophysiology.[35]
Peripheral Sensitization
Sensitization of the peripheral nervous system is triggered by the release of pro-inflammatory markers after the initial injury. Markers such as TNF-α released in this process reduce the stimulation threshold, leading to local sensitization and hyperalgesia in CRPS. Catecholamine sensitivity of peripheral nerve fibers has also been noted in CRPS. [6]
Central Sensitization and Neuroplasticity
Increased excitability of secondary dorsal horn neurons occurs in CRPS. Hyperalgesia and allodynia develop as a result of sensitization. The release of substance-P, bradykinin, and glutamate play an important role in this process. Persistent noxious primary afferent traffic into the dorsal horn leads to “wind up” and central sensitization.[24] Based on the response to ketamine (NMDA antagonist) infusions in CRPS patients, activation of spinal N-methyl D-aspartate (NMDA) receptors seem to play an important role in the pathogenesis.[36][37] Whereas improvement of CRPS symptoms with intrathecal baclofen suggests gamma-aminobutyric acid (GABA) involvement in sensitization.[6]
Evidence of cortical reorganization has been noted in CRPS with a reduction in the somatosensory-cortex area corresponding to the affected extremity.[38] The degree of neuroplasticity seems to correlate with the intensity of pain and severity of hyperalgesia, both of which indicate central sensitization.[39][40]
Autonomic Changes
Sympathetic-afferent coupling occurs in CRPS due to the upregulation of sympathetic receptors on nociceptive nerve fibers. As a result, sympathetic hyperactivity leads to increased pain and sympathetic sensitivity of nociceptive nerves. Local swelling, color, and temperature variations associated with this disorder suggest an involvement of the autonomic nervous system.[41] Widespread autonomic dysregulation in CRPS can affect heart rate and lead to orthostatic dysfunction.[42] In warm CRPS, vasodilation occurs as a result of reduced catecholamine release, and the opposite phenomenon occurs in cold CRPS.[6]
History and Physical
Patients may endorse sensory, motor, or autonomic symptoms. Sensory symptoms include allodynia (usually non-painful stimuli cause pain) and hyperalgesia (usually painful stimuli cause exaggerated pain). Patients can also experience autonomic symptoms, including skin color and temperature changes (vasomotor dysfunction) as well as swelling and sweating changes (sudomotor dysfunction). Motor symptoms of CRPS include weakness, reduced range of motion, tremor, and even dystonia in the affected extremity.[43]
CRPS is associated with worsening depression, anxiety, poor function, and diminished quality of life. A systematic review by Lohnberg et al. examined psychosocial factors associated with CRPS and concluded there is no support in the literature for specific personality or psychopathology predictors of CRPS.[11] However, patients with a significant comorbid psychological burden and/or poor coping mechanisms may demonstrate pain-related behavior and catastrophic thinking.
CRPS has also been associated with systemic medical issues, including neuropsychological deficits (executive functioning, memory, word retrieval), constitutional symptoms (lethargy, weakness, disruptions in sleep architecture), cardiopulmonary involvement (neurocardiogenic syncope, atypical chest pain, chest wall muscle dystonia leading to shortness of breath), endocrinopathies (impaired hypothalamic-pituitary-adrenal axis with low serum cortisol, hypothyroidism), urologic dysfunction (increased urinary frequency and urgency, urinary incontinence), and gastrointestinal dysmotility (nausea, vomiting, diarrhea, constipation, indigestion).[44][45][46][42][47][48]
Evaluation
No definite pathophysiologic mechanism for CRPS has been identified yet. Therefore, no gold standard diagnostic test for CRPS exists.[8] The diagnosis is clinical and based on the widely accepted Budapest criteria. Compared to the previous IASP criteria, the Budapest criteria have similar sensitivity (0.99) but higher specificity (0.68).[7]
Budapest Criteria[6]
A. They should report continuing pain disproportionate to the inciting event.
B. They should report at least one symptom in three of the four following categories:
- Sensory: Reports of hyperalgesia and/or allodynia,
- Vasomotor: Reports of temperature asymmetry and/or skin color changes and/or skin color asymmetry,
- Sudomotor/edema: Reports of edema and/or sweating changes and/or sweating asymmetry,
- Motor/trophic: Reports of decreased range of motion and/or motor dysfunction (weakness, tremor, dystonia) and/or trophic changes (hair, skin, nails).
C. Additionally, they must display at least one sign at the time of evaluation in two or more of the following categories:
- Sensory: Evidence of hyperalgesia (to pinprick) and/or allodynia (to light touch or deep somatic pressure),
- Vasomotor: Evidence of temperature asymmetry and/or skin color changes and/or asymmetry,
- Sudomotor/edema: Edema and/or sweating changes and/or sweating asymmetry,
- Motor/trophic: Evidence of decreased range of motion and/or motor dysfunction (weakness, tremor, dystonia) and/or trophic changes (hair, skin, nails).
D. Finally, there is no other diagnosis that better explains the signs and symptoms.
Various objective testing measures have been utilized to include thermography, triple-phase bone scan, and the quantitative sudomotor axon reflex test. While these studies provide further data, they are not necessary to make the diagnosis of CRPS. The diagnosis of CRPS is largely clinical and one of exclusion. The differential diagnosis includes small or large fiber sensorimotor neuropathy, cellulitis, erythromelalgia, vasculitis, vascular insufficiency, lymphedema, deep vein thrombosis, and Reynaud’s phenomenon. Diagnostic tests in CRPS are primarily aimed at screening for other potential differential diagnoses.
Treatment / Management
Although there is a possibility that patients with CRPS may improve spontaneously, considering the debilitating nature of this syndrome, it is prudent to institute aggressive management as soon as possible as a delay may result in an unfavorable outcome. Moreover, compared to chronic CRPS, early CRPS is less resistant to treatment and therefore has a better prognosis.[49] The goal of treatment is not only improvement in pain and discomfort but also functional restoration and prevention of disability. Therefore, the most optimal management would include an interprofessional approach including physical and occupational therapy, pharmacotherapy, behavioral therapy, and interventions.[6](B3)
Physical and Occupational Therapy
Manual therapy and exercises are not the only CRPS treatments under this section. Other therapy modalities include transcutaneous electrical nerve stimulation, ultrasound, laser, pain education, mirror therapy, and graded motor imagery (GMI). Multiple mechanisms of action of physical therapy have been proposed with no clear definitive theory. Manual therapy and exercise improve range of motion, function and reduce disability through endorphin release as well as other central and peripheral analgesic mechanisms.[3][50] Pain education influences pain perception and behavior by improving understanding of pain pathophysiology in such patients.[3] Whereas mirror therapy and GMI remediate maladaptive cortical neuroplastic changes associated with chronic pain conditions like CRPS.[51](A1)
A 2016 Cochrane review found that among the different physiotherapy modalities, mirror therapy and GMI may improve pain as well as function in CRPS. However, the quality of evidence was poor. Two clinical trials each for GMI and mirror therapy have demonstrated improvement of pain and function at 6 months. Very low-quality evidence was also found for improvement of impairment in CRPS with multimodal physiotherapy.[3](A1)
Pharmacotherapy
Multiple pharmacotherapeutic agents are used in the management of CRPS. The commonly used therapeutic options in this category include anti-inflammatory medications, anticonvulsants, antidepressants, transdermal lidocaine, opioids, NMDA antagonists, and bisphosphonates. Using a multimodal pharmacologic regimen that combines several different classes may lead to superior outcomes.
Anti-inflammatory Medications
Oral corticosteroids and non-steroidal anti-inflammatory drugs (NSAIDs) have been used in CRPS as inflammation is thought to play a role in disease pathogenesis. Based on three trials that compared oral corticosteroids to placebo in CRPS, a 2013 Cochrane review concluded that oral steroids do not significantly reduce pain. This was supported by very low-quality evidence. The review also found that oral corticosteroids seem to improve composite pain scores.[52] Another study found that compared to piroxicam (NSAID), oral prednisone seemed to be more effective in improving composite CRPS scores in post-stroke patients.[53] A more recent study found that 2-month treatment with low-dose oral prednisone was safe and effective in post-stroke CRPS.[54](A1)
Bisphosphonates
This class of medication is used routinely in bone-related problems as it inhibits osteoclastic activity. Several mechanisms of action of bisphosphonates in CRPS have been proposed. The more commonly accepted mechanisms include inhibition of bone marrow cell proliferation and migration as well as inflammation modulation.[55] A 2017 meta-analysis concluded that bisphosphonates seem to reduce pain in CRPS I.[56] A Cochrane review in 2013 found that low-quality evidence also seemed to suggest the same response in CRPS, more so in those with concomitant evidence of osteopenia or osteoporosis.[52](A1)
Anticonvulsants and Antidepressants
Gabapentin is the most widely studied medication in this class. It works by inhibiting the alpha 2-delta subunit of voltage-gated calcium channels. Despite its widespread use in treating CRPS, very low-quality evidence suggests that gabapentin is ineffective in the treatment of CRPS I.[52] In 2016, a study compared amitriptyline and gabapentin for CRPS I and pediatric neuropathic pain. Both the medications were found to reduce pain intensity and disability significantly. However, no significant difference in effect was noted between the two.[57](A1)
Opioids
The effectiveness of opioids has not been studied in CRPS, and therefore no evidence-based conclusions can be drawn.[41]
NMDA Antagonists
NMDA receptor antagonists like ketamine have been hypothesized to reverse central sensitization and maladaptive cortical neuroplastic changes in CRPS.[24] Low-quality evidence suggests that intravenous ketamine infusion may improve pain in CRPS for up to 4-11 weeks. [52][52] However, side effects and psychomimetic properties of ketamine have prevented widespread use.[24](A1)
Behavioral Therapy
Elevated levels of catecholamines associated with depression can worsen CRPS by inducing central sensitization through adrenergic mechanisms. Reversal of this effect is one of the proposed mechanisms of action of psychotherapy in CRPS. Apart from case reports and case series, only one small trial has been done evaluating the efficacy of behavioral interventions in CRPS. Despite the lack of clear evidence supporting their use in CRPS, behavioral therapy has been recommended part of comprehensive treatment.[58]
Interventions
Sympathetic Blocks
Sympathetic hyperactivity is believed to be an underlying pathophysiologic mechanism of CRPS.[41] Therefore, lumbar sympathetic nerve blocks are routinely used in the treatment of lower extremity symptoms, and stellate ganglion sympathetic blocks are used for the management of upper extremity symptoms of this syndrome. A 2013 Cochrane review found that sympathetic blocks with local anesthetic were ineffective at reducing CRPS related pain, but the quality of evidence was low.[52] A more recent Cochrane review in 2016 failed to draw any definitive conclusions on the efficacy of such treatment in CRPS due to paucity of evidence.[59](A1)
Spinal Cord Stimulation
Spinal cord stimulation (SCS) involves delivering electric stimulation to the dorsal column of the spinal cord by the placement of electrodes in the epidural space. The electrodes are usually connected to an implanted pulse generator to power the electrode, but some devices use an external pulse generator. Multiple mechanisms of action of SCS have been proposed, which include inhibition of nociceptive neural conduction in the spinal cord, adrenergic inhibition, vasodilation, and reversal of cortical maladaptive neuroplastic changes. A systematic review in 2017 studied the effectiveness of SCS in CRPS. The authors concluded that a high level of evidence supports the use of SCS for improvement of pain scores, quality of life as well as the perception of pain relief in CRPS.[59](A1)
Dorsal Root Ganglion Stimulation
Targeting the dorsal root ganglion (DRG) instead of the spinal cord is a relatively new and novel neuromodulation modality for the management of chronic pain. This enables a more focused application of neurostimulation than traditional SCS. DRG stimulation was approved by United States Food and Drug Administration in 2016 for treatment of lower extremity pain in CRPS. A recent pooled analysis study concluded that DRG stimulation was safe and effective for CRPS with a 4.9-point mean reduction of pain intensity in CRPS-I.[60] The ACCURATE study compared SCS and DRG stimulation in 152 subjects with CRPS, and study results were published in 2017. This multicenter randomized trial found that DRG stimulation was more effective than traditional SCS in reducing pain and improving quality of life in CRPS.[61](B2)
Differential Diagnosis
- Arterial insufficiency
- Gillian Barre syndrome
- Hysteria
- Monometric amyotrophy
- Multiple sclerosis
- Peripheral atherosclerotic disease
- Phlebothrombosis
- Porphyria
- Poliomyelitis
- Tabes dorsalis
Staging
Bonica, in 1990 had proposed 3 stages of CRPS. Bruehl et al. studied the validity of stages in a series of 113 patients and found no significant difference in duration of symptoms among the stages. This research suggested that clear generalized disease stages don't exist in CRPS.[62]
Prognosis
The prognosis of CRPS can be variable. Both spontaneous remission and refractory clinical presentation have been seen. But early treatment may improve the prognosis.
Complications
Dystonia, cognitive executive dysfunction, adrenal insufficiency, gastroparesis, and irritable bowel syndrome are a few of the complications associated with long-standing CRPS.
Deterrence and Patient Education
Oral supplementation of vitamin C has been hypothesized to lower the risk of development of CRPS after fractures due to its antioxidant properties. A meta-analysis of 3 trials in 2015 found that available evidence failed to demonstrate a definitive preventive role of vitamin C in CRPS development after distal radial fractures, although the level of evidence was low.[63] Another meta-analysis and systemic review in 2017 evaluated the efficacy of vitamin C in the prevention of CRPS development after wrist fractures. 500 mg vitamin C daily therapy for 50 days seemed to reduce the risk of CRPS at 1 year in this study.[64]
Enhancing Healthcare Team Outcomes
An interdisciplinary team approach to this problem is required to maximize recovery and limit disability. Social service, pharmacology, nursing, and physical therapy, along with early advanced pain management, are key to improved outcomes.
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