The current diagnostic criteria for complex regional pain syndrome (CRPS), codified by the International Association for the Study of Pain's taxonomy committee, and newer statistically derived criteria (the “Budapest” criteria), are both deliberately based on bedside testing. Designing criteria that are accessible to any clinician, not requiring any special equipment or training, is very important for clinical diagnosis. However, that approach, albeit pragmatic, forces a very heavy reliance on the subjective (not only the subjective response of the patient, but the subjective impression of the clinician). This is very problematic scientifically and statistically. Fortunately, with some new technologies and new approaches to old technologies, significant improvements can be made not only in terms of quantification, but also in allowing significant objectification of the diagnostic data. We will initiate a discussion of some of these potentially useful approaches.
In the absence of a “gold standard test,” empirical, statistically derived yet essentially subjective diagnostic criteria are evolving for Complex Regional Pain Syndrome (CRPS) . The metrics used to measure pain are by definition fully subjective. Other diagnostic components, as indicated by principal component analysis, have deliberately been validated on the basis of “bedside testing” (so as to make these criteria accessible); but this lends further subjectivity to the data by basing it on observation (clinical impression) which in turn is biased by training, diagnostic milieu, and patient history. There is an opportunity to lend more objectivity to this diagnostic set, at least in the research context, using some newer technologies, standard traditional measures, and straightforward methods derived from treatment modalities . We can do better.
What we currently call CRPS is known by other names; commonly reflex sympathetic dystrophy and causalgia (attributed to Evans  and Mitchell , respectively). In the past, the syndrome was diagnosed using a variety of idiosyncratic criteria (e.g., Bonica ; Kozin et al. , Wilson ), each of which was derived from clinical experiences and none of which achieved wide acceptance. After much debate, the name was ultimately changed to CRPS at a consensus workshop in Orlando, Florida in 1994 [8,9], with the new name and diagnostic criteria codified by the International Association for the Study of Pain (IASP) taskforce on taxonomy (the “IASP diagnostic criteria for CRPS” (). This diagnostic entity was intended to be descriptive and general, and not imply any etiopathology (including any direct role for the sympathetic nervous system). This pivotal effort provided an officially endorsed set of standardized diagnostic criteria that had the potential to lead to improved clinical communication and greater generalizability across research samples . However, realization of this potential has been somewhat limited by the fact that these criteria were based solely on consensus. Utilization of the criteria in the subsequent literature has been sporadic at best , and certain influential groups have resisted the change (e.g., forensic, advocacy, payors). As a consequence, the full benefits of common, clearly defined criteria have not been completely realized and there has been a significant deficiency of evidence-based information about this condition since this IASP codified criteria was delineated. Consensus-derived criteria that are not subsequently validated may lead to overdiagnosis or underdiagnosis, and will reduce the ability to provide timely and optimal treatment [1,12,13]. It is therefore very difficult to apply the usual scientific tools to the problems of coherent diagnosis and therapeutic efficacy.
Results of validation studies to date suggest that the IASP/CRPS diagnostic criteria are adequately sensitive . Unfortunately, wording of the current IASP criteria permits diagnosis based solely on patient-reported historical symptoms and this contributes to overdiagnosis. The IASP criteria do not consider (ubiquitous) motor and trophic signs and symptoms, which can lead to important information being ignored that may discriminate CRPS from other syndromes [12,14].
In a series of 123 CRPS patients, signs and symptoms of CRPS factored into four statistically distinct subgroups . The first of these subgroups is a unique set of signs and symptoms indicating abnormalities in pain processing (e.g., allodynia, hyperalgesia). Skin color and temperature changes, which are indicative of vasomotor dysfunction, characterize the second subgroup. Edema and sudomotor dysfunction (e.g., sweating changes) combined to form a third unique subgroup. The finding that vasomotor signs and symptoms were statistically distinct from those reflecting sudomotor changes/edema is in contrast to the IASP criteria, which treat all three of these as diagnostically equivalent. A fourth and final separate subgroup was identified that included motor and trophic signs and symptoms [2,15]. Numerous studies have described various signs of motor dysfunction (e.g., dystonia, tremor) as important characteristics of this disorder, and trophic changes have frequently been mentioned in clinical descriptions [16–18]. Given evidence from Galer et al.  and Harden et al.  that objective signs on examination and patient-reported symptoms both provide useful and nonidentical information, the modified criteria required the presence of signs (quasi-objective) and symptoms of CRPS for diagnosis .
We postulate that by using known measurement techniques and technologies, physical and physiological sign data can be better objectified and/or quantitated, and thus would likely improve specificity of the “Budapest” criteria .
Data acquired in physical examination, lab testing, and certain methods used empirically in the therapeutic modalities hold the promise of enhancing the objectivity of the signs component. Selected examples of these tactics will be discussed in reference to each diagnostic factor (Table 1) .
Proposed Clinical Diagnostic Criteria for CRPS (modified from Harden et al. )
General Definition of the Syndrome:
CRPS describes an array of painful conditions that are characterized by a continuing (spontaneous and/or evoked) regional pain that is seemingly disproportionate in time or degree to the usual course of any known trauma or other lesion. The pain is regional (not in a specific nerve territory or dermatome) and usually has a distal predominance of abnormal sensory, motor, sudomotor, vasomotor and/or trophic findings. The syndrome shows variable progression over time.
To make the clinical diagnosis, the following criteria must be met:
1. Continuing pain, which is disproportionate to any inciting event.
2. Must report at least one symptom in three of the four following categories:
____ Sensory: Reports of hyperesthesia and/or allodynia.
____ Vasomotor: Reports of temperature asymmetry and/or skin color changes and/or skin color asymmetry.
____ Motor/Trophic: Evidence of decreased range of motion and/or motor dysfunction (weakness, tremor, dystonia) and/or trophic changes (hair, nail, skin).
4. There is no other diagnosis that better explains the signs and symptoms.
NOTE: For research purposes, diagnostic decision rule should be at least one symptom in all four symptom categories and at least one sign observed at evaluation in two or more sign categories.
The concept of “disproportionate pain” is in most aspects subjective. Time since onset is a measurable component, and should always be recorded as per subject or, ideally, record. The type of precipitating lesion or trauma (if known) should be listed. Importantly, the pattern or distribution of pain and symptoms, with attention to dermatomal or peripheral nerve patterns, should be documented. CRPS type I is thought to be nondermatomal, while CRPS type II will often conform, loosely, to the peripheral nerve distribution of lesion. This is best done with a subject-generated “pain drawing” and careful and methodical physical examination. Patient drawings can be quantified with a “pixel overlay” (count the pixels over the painful area).
Pain report should be quantified, by 100 mm visual analog scale if practical . The qualities of the pain should be documented using an appropriate metric such as the McGill short-form . Allodynia and hyperpathia should be quantitated using the available quasi-objective technologies . Temperature allodynia can be quantified by a standard Peltier-type device . Mechanical allodynia can be quantified using a variety of standard methodologies such as von Frey testing and weighted pin . Deep mechanical sensitivity can be measured by algometer, over muscle and joint . Many old and new methods of sensory testing may be considered , but functional imaging provides the best data: a fully objective correlate with evoked pain (and hyperalgesia) and new analytic techniques that allow for assessment of spontaneous pain [24–26].
Laser Doppler is a direct and fully objective measure of vasomotor tone . Limb temperature serves as an indirect, yet objective measure of cutaneous and subcutaneous blood flow. Methods in order of objectivity and quantification: Infrared telethermography, thermistors, thermometers, and temperature tape [28,29]. Vasomotor tone can be manipulated and measured experimentally, using prototype “space suits”.
Sudomotor function can be objectively measured using quantitative sudomotor axon response testing . Sudomotor function can be indirectly measured (and patterned) using bioimpedance (skin conductance testing) and skin potential fluctuations (sympathetic skin response testing) . Edema can be quantitated using volumetry [2,33].
Weakness can be subjectively quantitated by scores, such as the 0–5 “Canadian Score.” Features of certain motor signs can be measured, such as bradykinesia  and general activity (e.g., accelerometer). There are currently no reliable scores or metrics for myoclonus, athetosis, dystonia, or contracture. Likewise, skin, nail, or hair trophic changes use subjective measures. Range of motion can be quantified using a Goniometer. Bone density (Sudeck's atrophy) can be measured. Finally, small nerve density can be quantitated, perhaps measuring small fiber dropout  or small fiber ischemic lesion, especially in the context of CRPS type II (causalgia).
In a field of research endeavor that must by definition include subjective data (pain metrics), it is scientifically problematic to add more data elements that are also fully subjective (patient report) or weakly objective (practitioner observation). Hypothetically, objectifying and quantifying as many criteria as possible will enhance diagnostic accuracy and research efficiency; of course, this postulate must be proven. In a syndrome that is characterized by so much measurable physiologic change, it is important that objective measures be added and validated when possible [21,34]. The complicated, expensive, and time-consuming task of validating specific and more objective outcomes will likely vastly improve diagnostic and research precision.
. The reflex sympathetic dystrophy syndrome III: Scintigraphic studies, further evidence for the therapeutic efficacy of systemic corticosteroids, and proposed diagnostic criteria. Am J Med 1981;70:23–30.
. Case report and hypothesis: A neglect-like syndrome may be responsible for the motor disturbance in reflex sympathetic dystrophy (complex regional pain syndrome-1). J Pain Symptom Manage 1995;10:385–91.