Understanding Complex Regional Pain Syndrome

Understanding Complex Regional Pain Syndrome

Mimoun Azizi ¹, Ibrahim Krenawi *²

1. Chief Physician, Senior Consultant Neurologist and Neurogeriatrician, Klinikverbund Südwest, Sindelfingen, Germany.
2. Consultant Neurologist, Ain Alkhaleej Hospital, Al Ain, UAE.

*Correspondence to: Ibrahim Krenawi, Consultant Neurologist, Ain Alkhaleej Hospital, Al Ain, UAE.

Copyright

© 2026 Ibrahim Krenawi, This is an open access article distributed under the Creative Commons Attribution  License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Received: 13 July 2026

Published: 01 August 2026

DOI: https://doi.org/10.5281/zenodo.21714704

Abstract

Complex Regional Pain Syndrome (CRPS) is a multidimensional disorder of pain that occurs most frequently following limb trauma or surgery and presents with disproportionate pain, sensory deficiencies, vasomotor instability, sudomotor changes, and trophic or motor dysfunction. No longer dichotomized into CRPS-I and CRPS-II, modern studies emphasize that it has different phenotypes: warm or cold, neuropathic or nociplastic mechanisms, and motor-dominant or dystonic subtypes, more representative of its clinical heterogeneity. The quality of diagnostic tests has improved with the Budapest criteria using quantitative sensory measurements, neuroimaging, and new biomarkers of immunological diagnosis. According to epidemiological data, it is more common in women, especially after distal radius and ankle fractures. In 2018, the Royal College of Physicians stressed multidisciplinary, phenotype-informed, and early care as the key to management. The interventions have been incorporated in treatment procedures to include anti-inflammatory interventions, medication of neuropathic pain, use of interventional neuromodulating, and functional rehabilitation. CRPS requires the early identification, personalised treatment, and continued study of the mechanistic pathways to better patient outcomes.

Keywords: Complex Regional Pain Syndrome (CRPS), CRPS diagnosis and treatment, Warm vs cold CRPS phenotypes, Budapest criteria for CRPS, CRPS rehabilitation and recovery, CRPS pain management strategies, Dorsal Root Ganglion (DRG) stimulation in CRPS.

Abbreviations

Introduction

Complex Regional Pain Syndrome (CRPS) is a chronic, disproportionate pain condition that typically arises after trauma, surgery, or fracture and is characterized by a constellation of sensory, vasomotor, sudomotor, and motor/trophic abnormalities. In comparison with a simple post-traumatic pain, CRPS develops as a localized disorder with impaired functioning that goes beyond the original place of injury.

Historically, CRPS was classified into two groups: CRPS-I (without demonstrable nerve lesion) and CRPS-II (associated with a definable nerve injury) [1], [2]. However, modern evidence shows that such binary classification is insufficient to deal with its clinical heterogeneity. Harden et al. 2010 in Pain Journal confirmed the validity of the Budapest Criteria, as they are the international gold standard of diagnosis because they have highly appropriate specificity relative to the past IASP definitions [2], [3].

In 2018, on the Royal College of Physicians site, the authors noted that CRPS is a complex interaction of peripheral and central sensitization, autonomic dysregulation, and immune-mediated mechanisms, and a multidisciplinary and phenotype-based approach to management is necessary [4].

Over the past decade, been a growing focus on the role of phenotypic subtypes of CRPS, such as “warm” versus “cold” CRPS, neuropathic versus nociplastic pathogenesis, and motor dominant or dystonic phenotypes that are more reflective of pathophysiology and respond to treatment [4]. It is important to understand these dimensions to recognize them on time, diagnose them, and formulate appropriate treatment strategies.

History and Definition of CRPS

Complex Regional Pain Syndrome (CRPS) has been a concept that has developed over the last two centuries. The syndrome was described in the mid-19th century by American physician Silas Weir Mitchell, who termed the condition “causalgia”- a severe burning pain that appeared after injury to peripheral nerves (in general, in soldiers who had been wounded during the Civil War) [5]. Later, in the 20th century, the term “reflex sympathetic dystrophy” emerged to highlight the presumed role of the sympathetic nervous system in sustaining pain and trophic changes.

In 1994, these descriptions came under the modern umbrella of Complex Regional Pain Syndrome; subclassified by the International Association of the Study of Pain (IASP):

• CRPS-I: A clinical diagnosis without a demonstrable nerve lesion.
• CRPS-II: Defined by a similar clinical picture but associated with a confirmed peripheral nerve injury.

Harden et al. (2010) in Pain Journal wrote that the Budapest diagnostic criteria are more specific than the previous definitions of the IASP, and hence, it is the standard that should be used in both research and clinical practice [3]. The combination of sensory, vasomotor, sudomotor/edema, and motor/trophic changes is highlighted in these criteria as an effective method of making a diagnosis.

In Mayo Clinic Proceedings, in 2015, Bruehl wrote that CRPS is, in fact, not a disorder but a syndrome with overlapping mechanisms, consisting of inflammation, autonomic dysfunction, central sensitization, and, in some subpopulations, autoimmune [6].

Therefore, the current conceptualization of CRPS is a multidimensional regional pain syndrome, usually after trauma or surgery, in which the manifestation is disproportional to the precipitating cause and in which several areas of the nervous and autonomic systems are involved.

Epidemiology and Risk Factors

Complex Regional Pain Syndrome (CRPS) is regarded as a relatively rare, yet clinically important, pain disorder. Its incidence is estimated at 5-26 cases per 100,000 person-years in population-based studies. De Mos and colleagues (2007) made the same claim in the BMJ, noting that CRPS was most common in women in midlife [7]. On the same note, Sandroni et al. (2003), in Neurology, determined a ratio of females to males to be higher, and this highlights the possibility of hormonal or genetic factors [8].

The most conducive precipitating factor is fractures. Fractures of the distal radius and ankle injuries have the highest risk, and cohorts of wrist fractures show the highest short-term incidence. Other well-documented risk contributors include intra-articular injuries, dislocations, post-traumatic immobilization, severe acute pain intensity, and post-menopausal female status.

This epidemiology profile indicates biological predispositions and triggers of injury, especially the need to recognize vulnerability in the population at an early stage to minimize chronic disability.

Risk Factors for Complex Regional Pain Syndrome (CRPS)

Table 1: Risk Factors for (CRPS)

Clinical Features and Symptomatology

Complex Regional Pain Syndrome (CRPS) is a syndromic disorder with sensory, vasomotor, sudomotor/ edema, motor, and trophic disruptions beyond the normal healing process of the original trauma. Budapest clinical criteria are considered the diagnostic keystone and must include persistent disproportionate pain together with objective manifestations in more than one domain [9].

Trophic Disturbances

Patients often develop altered hair and nail growth, changes in skin color or temperature, glossy or atrophic skin, and patchy osteoporosis on imaging. The evolving nature of trophic features was reported by Agten et al. (2020) in Skeletal Radiology, who found that up to 50% of known CRPS would not have marrow edema [10].

Sensory Disturbances

The syndrome blends gain-of-function phenomena (allodynia, hyperalgesia) with loss-of-function features (hypoesthesia). In 2012, in Pain, Gierthmuhlen et al. categorized mechanical and thermal hyperalgesia as common symptoms, which very often reach beyond the localities of the trauma [11].

Motor Disturbances and Dystonia

Motor abnormalities include reduced range of motion, tremor, weakness, and dystonia. In Movement Disorders, Munts et al. (2011) described the systemic nature of dystonic patterns in CRPS, which are associated with a lack of sensorimotor integration and unfavorable prognosis [12].

Distal Spread and Mirror Patterns

CRPS may spread contiguously within a limb, mirror contralaterally, or become multifocal. Rommel et al. (2001) in Pain have demonstrated hemisensory impairment in CRPS-I that contributes to disease propagation using central nervous system contributions [13].

These various clinical manifestations combined reflect that CRPS is not a homogeneous entity but a multisystem syndrome, and patterns of phenotyping inform prognosis as well as treatment.

Subtypes and Phenotypes of CRPS

Although traditionally divided into CRPS-I (without definable nerve lesion) and CRPS-II (with demonstrable nerve injury), this binary classification often fails to capture the syndrome’s true heterogeneity [1], [2]. Contemporary literature focuses on separate phenotypes that mirror inherent processes, clinical progression, and treatment outcome.

CRPS-I and CRPS-II (Historical Classification)

CRP was originally designated as type I (no nerve lesion) and type II (causalgia, with proven nerve damage). Although it remains useful in a medico-legal and prognostic context, this dichotomy fails to illuminate the wide clinical variation. Harden et al. (2010) validated the Budapest criteria in the article Pain Journal in 2010, and they are still the best at diagnostic specificity [3].

Warm vs Cold CRPS

One of the most reproducible clinical distinctions is warm versus cold CRPS. In Bruehl et al. in 2016 introduced evidence that during the early stages of CRPS, warm CRPS prevails over cold CRPS, with redness, warm, swelling and a higher level of inflammatory markers, whereas in chronic stages, the opposite occurs, with cold CRP being the dominant type, and a pallid skin, cold skin, sensory loss, and a higher level of central reorganization [14]. This polarisation indicates an over-time transition of inflammatory to centralized processes.

Mechanism-Driven Phenotypes

It has suggested mechanistic groupings to organise treatment with biology:

• Inflammatory-predominant CRPS: elevated local cytokines (IL-1, IL-6, TNF-α).
• Peripheral neuropathic CRPS: nerve lesion or small-fiber dysfunction with neuropathic pain features.
• Nociplastic/centralized CRPS: widespread hyperalgesia, cortical neuroplastic changes, and bilateral sensory disturbances.

Parkitny et al. (2013) in Neurology confirmed the presence of more inflammatory mediators in the affected limbs, which supports the inflammatory subtype [15].

Motor-Dominant and Dystonic CRPS

A subgroup is fixed with dystonia and contracted motor with severe impairment. Still, in Movement Disorders (2011), Munts et al. described typical abnormal postures and associated dystonia with chronicity and adverse outcomes [12].

Sympathetically Maintained vs Sympathetically Independent Pain

Other patients develop pain that, without sympathetic blockade, would continue to be painful, historically referred to as sympathetically maintained pain [16]. Nevertheless, predictive value is not perfect, and not every patient responds in this way. Sympathetic contributions are more likely to be involved in early warm CRP.

Spreading and Multifocal CRPS

CRPS often spreads to contiguous regions, mirrors the contralateral limb, or becomes multifocal. Rommel et al. (2001) gave a description of contralateral spread and disturbance of the hemisensory in 2001 in pain, in which the authors suggested that the central nervous system plays a role in the development of the disease [13].

Diagnostic Criteria and Investigations

Budapest Clinical Criteria

The Budapest criteria have been the world standard in the diagnosis of CRPS. These require:

• Prolonged pain, which is not proportional to the causative agent.
• One or more symptoms in three or four (sensory, vasomotor, sudomotor/edema, motor/trophic) categories.
• Examine at least one of two or more of these categories.
• Other diagnosis that more aptly accounts for findings.

In 2010, in Pain Journal, Harden et al. demonstrated that the Budapest criteria outperform the original IASP definitions in specificity and diagnostic reliability [17].

Instrumented Diagnostics

CRPS is a clinical diagnosis, but studies can help determine or exclude imitators:

• Triple-phase bone scintigraphy (TPBS): Can demonstrate more periarticular uptake in early, warm CRPS, but sensitivity and specificity are inconsistent. Wertli et al. (2017) wrote in PLoS One that TPBS cannot be applied on its own [18].
• Thermography: detects temperature abnormalities, but it is not specific; it is not necessary.
• MRI: May show bone marrow edema or soft tissues. Agten et al. (2020) detected no signs of marrow edema in ~50% of confirmed CRPS in 2020 in Skeletal Radiology [10].

Quantitative Sensory Testing (QST)

Standardized QST procedures (e.g., DFNS battery) are used to help phenotype the patients by recording thermal and mechanical abnormalities. Rolke et al. also determined reference values in 2006, in Pain, where CRPS patients tend to show paradoxical thermal changes, dynamic allodynia, and pressure pain hypersensibility [19].

Neuroimaging and Neurophysiology

In both somatosensory and motor regions, cortical reorganization is reported using functional and structural imaging, especially in chronic or cold CRPS. Transcranial magnetic stimulation research also substantiates a change in cortical excitability. PNS tests are useful in differentiating CRPS-II and focal neuropathies.

Biomarkers and Immunological Insights

Emerging evidence highlights inflammatory cytokines (IL-1β, IL-6, TNF-α) and autoantibodies against β2-adrenergic and muscarinic-2 receptors in subsets of patients. In 2011, in Pain, Kohr et al. reported that in experimental models, such antibodies caused nociceptive sensitization, implying an autoimmune subgroup [20].

Differential Diagnoses

CRPS should be distinguished from such conditions as deep vein thrombosis, infection, entrapment of peripheral nerves, lymphedema, inflammatory arthritis, small-fiber neuropathy, vascular insufficiency, and functional neurological disorders. Guidelines on the StatPearls (NCBI) platform promoted systematic exclusion of these mimics as a critical component of the diagnostic procedure in 2025 [21].

Pathophysiological Mechanisms of CRPS

CRPS is a multifactorial pain syndrome whose biological processes overlap and change as time progresses. There is no single mechanism that can encompass all the cases, but rather, pathophysiology is a dynamic interaction between peripheral, central, immune, and psychosocial factors.

Post-Traumatic Inflammation

Early CRPS is often marked by neurogenic inflammation with local elevations of cytokines such as IL-1β, IL-6, and TNF-α. A meta-analysis was conducted by Parkitny et al. in 2013 in Neurology and confirmed that there are more inflammatory mediators in affected limbs, especially in warm CRPS [22]. Acute vasomotor instability, edema, and redness are due to these changes.

Peripheral and Central Sensitization

Sensitization of peripheral nociceptors is one of the causes of spontaneous pain, hyperalgesia, and allodynia. With time, central sensitization will arise, which is manifested in temporal summation and extensive hyperalgesia. Konopka et al., in PLoS One in 2012, have shown bilateral sensory dysfunction despite unilateral CRPS, supporting a central involvement [23].