Complex Regional Pain Syndrome (CRPS)

Populations and Risk Factors

• Approximately 26 per 100,000 person-years incidence; affects approximately 50,000 new cases annually in the United States
• Females are affected 3–4 times more frequently than males
• Peak incidence in the 40–60 age range, though CRPS occurs across all ages including pediatric populations
• Distal radius (Colles') fracture is the most common precipitating event — approximately 7% of Colles' fracture patients develop CRPS
• Surgical patients — particularly hand, wrist, ankle, and knee surgery; prolonged immobilization post-surgery is an independent risk factor
• Individuals with prior limb trauma — sprains, crush injuries, contusions; the severity of the initial injury does not correlate with the likelihood of developing CRPS (minor injuries can trigger severe CRPS)
• Immobilization — prolonged casting or splinting after fracture increases risk; early mobilization is protective
• Psychological factors — anxiety, depression, and stress are associated with increased risk, likely through their effects on central sensitization and sympathetic nervous system activation; however, CRPS is not a psychogenic disorder
• Genetic predisposition — HLA associations have been identified; familial clustering suggests a genetic component to central sensitization vulnerability
• Prior CRPS — individuals who have had CRPS in one limb are at increased risk of developing it in another limb after a subsequent injury

Causes and Pathophysiology

Type I vs. Type II Distinction

• CRPS Type I (formerly RSD): Develops after a noxious event (fracture, surgery, soft tissue trauma, immobilization) without an identifiable peripheral nerve lesion. The pain and autonomic changes extend beyond the territory of any single nerve. Accounts for approximately 90% of CRPS cases.
• CRPS Type II (formerly causalgia): Develops after a confirmed peripheral nerve injury (laceration, compression, partial transection). The pain and autonomic changes may start in the nerve's territory but typically spread beyond it. Accounts for approximately 10% of CRPS cases.

The clinical presentation of both types is identical — the distinction is made based on the presence or absence of a confirmed nerve lesion. The treatment approach does not differ between types.

Central Sensitization — The Core Mechanism

Central sensitization is the amplification of neural signaling within the central nervous system that causes the pain system to respond disproportionately to incoming stimuli. In CRPS, central sensitization develops through a cascade:

1. Initial nociceptive barrage: The inciting injury (fracture, surgery, nerve lesion) generates intense nociceptive input from the periphery. This sustained barrage of C-fiber and A-delta fiber signaling reaches the dorsal horn of the spinal cord.

1. Wind-up and NMDA receptor activation: Repeated stimulation of dorsal horn neurons produces temporal summation (wind-up) — each successive stimulus produces a progressively larger postsynaptic response. This eventually activates NMDA (N-methyl-D-aspartate) receptors, which require sustained depolarization to open. NMDA receptor activation dramatically amplifies the neuron's responsiveness.

1. Dorsal horn hyperexcitability: The sensitized dorsal horn neurons now respond to low-threshold mechanoreceptor input (A-beta fibers — normally responsible for light touch, not pain) as if it were nociceptive input. This is the mechanism of allodynia — light touch produces pain because the central neuron has been reprogrammed to interpret all input as threatening.

1. Supraspinal amplification: The sensitized dorsal horn signal propagates to the thalamus, somatosensory cortex, and limbic system. Cortical reorganization occurs — the somatotopic representation of the affected limb in the primary somatosensory cortex (S1) shrinks, and the patient may develop disturbed body perception (the limb feels foreign, swollen, or disproportionately large even when it appears normal). This cortical reorganization correlates with pain intensity.

1. Sympathetic-afferent coupling: In some CRPS patients, sympathetic postganglionic fibers (which normally innervate blood vessels and sweat glands) develop abnormal connections (ephaptic coupling) with nociceptive afferents. This means sympathetic activation (stress, emotional arousal, temperature changes) directly triggers pain signaling — hence the original term "reflex sympathetic dystrophy." Not all CRPS patients have this component; those who do are described as having "sympathetically maintained pain."

Autonomic Dysfunction — The Four Domains

The Budapest criteria organize CRPS signs and symptoms into four categories, reflecting the multiple pathophysiological pathways disrupted: Sensory domain:

• Allodynia: Pain from stimuli that normally do not cause pain — light touch, clothing contact, bed sheet weight, breeze across the skin; mechanical allodynia (touch-evoked) is most common; thermal allodynia (warm or cool stimulus evokes pain) also occurs
• Hyperalgesia: Exaggerated pain response to stimuli that are normally mildly painful — a pinprick produces severe, spreading pain; a minor bump produces prolonged aching
• Hyperpathia: Delayed onset of pain after stimulus, followed by an exaggerated, explosive pain response that outlasts the stimulus duration

Vasomotor domain:

• Temperature asymmetry: The affected limb is warmer or cooler than the contralateral limb (>1°C difference is clinically significant); in the acute "warm" phase, the limb is warmer due to loss of sympathetic vasoconstrictor tone (vasodilation); in the chronic "cold" phase, the limb is cooler due to paradoxical vasoconstriction
• Color changes: Red, blue, purple, mottled, or pale — the color may change dynamically with position, temperature, and emotional state; the affected limb may cycle between flushed (vasodilated) and cyanotic (vasoconstricted) within hours

Sudomotor/edema domain:

• Edema: Swelling of the affected limb — pitting edema in the acute phase; firm, non-pitting edema in later stages as tissue fibrosis develops
• Sweating asymmetry: The affected limb may sweat more (hyperhidrosis) or less (hypohidrosis) than the contralateral limb; sweating changes reflect disrupted sympathetic sudomotor function

Motor/trophic domain:

• Motor: Weakness (disproportionate to the degree of disuse); tremor; dystonia (sustained involuntary muscle contraction producing abnormal postures); reduced range of motion from a combination of pain avoidance, edema, and tissue contracture
• Trophic changes: Skin becomes thin, shiny, glossy; hair growth changes (initially increased, then decreased and brittle); nail growth changes (initially rapid, then slow, thickened, ridged, and brittle); subcutaneous tissue atrophy; osteoporosis in the affected limb (patchy demineralization on X-ray — Sudeck's atrophy)

Warm Phase to Cold Phase Progression

CRPS evolves through a characteristic progression, though the stages are not strictly sequential and may overlap:

Neuroplastic Changes and Body Perception

CRPS produces measurable changes in cortical representation:

• The somatotopic map of the affected limb in S1 cortex shrinks — this correlates with the intensity of allodynia
• Patients may report that the limb feels foreign, larger than it actually is, or disconnected from the body — a phenomenon called neglect-like syndrome
• These cortical changes are the rationale for mirror therapy and graded motor imagery (GMI) — interventions that retrain the cortical representation by providing visual and motor input that does not provoke pain

Signs and Symptoms

Acute Presentation (Warm Phase)

• Pain: Burning, throbbing, constant pain in the affected limb — disproportionate to the inciting injury; pain may extend beyond the initial injury site to involve the entire distal limb; pain worsens with movement, touch, temperature change, and emotional stress
• Allodynia: Light touch provokes pain — the patient cannot tolerate clothing, bed sheets, or gentle palpation on the affected limb. This is the most distressing symptom and the primary barrier to manual therapy.
• Hyperalgesia: Exaggerated response to mildly painful stimuli — pinprick produces severe spreading pain.
• Edema: Diffuse pitting edema of the affected limb, often extending from the injury site to the fingers or toes
• Vasomotor changes: The affected limb is warmer and redder than the contralateral limb; color may change with position (dependent → flushed; elevated → pale)
• Sweating changes: Increased sweating on the affected limb compared contralaterally

Chronic Presentation (Cold Phase)

• Pain: Shifts from burning to deep aching, squeezing; cold intolerance becomes prominent; pain is constant and may generalize (spread to other limbs or become widespread)
• Trophic changes: Skin becomes thin, shiny, glossy; hair loss; nails thicken and become ridged, brittle; subcutaneous tissue atrophy — the limb appears wasted despite edema
• Vasomotor changes: The limb is cooler and cyanotic; mottled appearance
• Motor changes: Increasing weakness, tremor, dystonia (abnormal fixed postures); range of motion progressively restricted by contracture
• Edema: Becomes firm and non-pitting as tissue fibrosis develops
• Bone changes: Osteoporosis visible on X-ray (Sudeck's atrophy — patchy demineralization)
• Spread: CRPS may spread from the initial limb to the contralateral limb, to other limbs, or to become more generalized — this spread pattern reflects central sensitization expanding beyond the initial spinal cord segment

Assessment Profile

Subjective Presentation

• Chief complaint: "My hand is on fire — even the air blowing on it hurts"; "I can't touch my arm — even the bed sheet is unbearable"; "My foot is swollen, discolored, and the pain is out of proportion to what happened"; the patient is often distressed and frustrated because the severity of their symptoms does not match the initial injury
• Pain quality: Burning, throbbing (warm phase); deep aching, squeezing, cold-sensitive (cold phase); pain is constant, not episodic; described as disproportionate to the injury by the patient themselves; allodynia — "even light touch hurts"; hyperalgesia — "a small bump produces severe pain that lasts for hours"
• Onset: Follows an identifiable inciting event (fracture, surgery, sprain, immobilization) — typically develops within 4–6 weeks of the event; the initial injury may have been minor (sprained ankle, minor contusion); the disproportionate pain onset is the characteristic feature; Type II follows a confirmed nerve injury
• Aggravating factors: Light touch (allodynia), temperature change (cold or sometimes heat), movement of the affected limb, dependent positioning (worsens edema and pain), emotional stress (sympathetic-afferent coupling), clothing or bandage contact
• Easing factors: Elevation of the affected limb (reduces edema); gentle movement within tolerance (reduces stiffness but must be graded carefully); warmth (in cold-phase CRPS) or cooling (in warm-phase); medications (gabapentin, pregabalin, sympathetic blocks may provide partial relief)
• Red flags: Rapidly progressive systemic symptoms → rule out infection (cellulitis, osteomyelitis), DVT, or malignancy; CRPS in a limb with no identifiable inciting event → consider vascular pathology or occult fracture; uncontrolled spreading CRPS with severe functional decline → refer for multidisciplinary pain management, which may include sympathetic blockade, intrathecal pump, or spinal cord stimulation

Observation

• Local inspection: The affected limb displays visible asymmetry compared to the contralateral limb — color changes (red, blue, mottled, pale), edema (diffuse swelling), skin quality changes (shiny, glossy, thin in chronic cases), hair and nail changes (increased growth early, decreased and brittle later); the patient may guard the limb protectively, holding it away from contact; in severe cases, dystonic posturing may be visible (wrist or ankle held in fixed abnormal position)
• Posture: The patient protects the affected limb — holds it close to the body, avoids contact with surfaces or clothing; may refuse to use the limb entirely (learned disuse); compensatory patterns develop in the contralateral limb and trunk from overuse
• Gait: If a lower extremity is involved — antalgic gait with reduced weight-bearing on the affected foot; may refuse to bear weight entirely; if upper extremity — reduced arm swing, guarded posture

Palpation

• Tone: The affected limb may show generalized increased muscle tone from protective guarding and dystonia, or may feel hypotonic and wasted from chronic disuse. Tremor may be palpable. Proximal muscles (shoulder girdle or hip girdle) may be hypertonic from compensatory overuse. Palpation of the affected limb must be approached with extreme caution — allodynia means even light touch may produce severe pain.
• Tenderness: Generalized tenderness throughout the affected limb — not confined to a single nerve, dermatome, or anatomical structure. This non-dermatomal distribution is a defining characteristic of CRPS. Allodynia testing: lightly stroke the skin with a cotton swab or fingertip — if this produces pain, allodynia is confirmed. Compare to the identical stimulus on the contralateral limb. Do not perform aggressive palpation assessment if allodynia is present — document it and modify all subsequent treatment accordingly.
• Temperature: Temperature asymmetry between the affected and contralateral limb — palpate with the dorsum of the hand for comparison. Warm phase: affected limb is warmer. Cold phase: affected limb is cooler. A difference >1°C is clinically significant. Temperature may change during the session from vasomotor shifts.
• Tissue quality: Edema is pitting in the warm phase and firm/non-pitting in the cold phase. Skin feels smooth, glossy, thin, and adherent from subcutaneous tissue loss. Nails may be ridged and brittle. Hair may be absent or sparse. The tissue feels distinctly different from the contralateral limb — a combination of swollen, taut, and fragile.

Motion Assessment

• AROM: Significantly reduced in the affected limb — limited primarily by pain and protective guarding in the warm phase; limited by contracture, fibrosis, and dystonia in the cold phase; the patient may refuse to move the limb voluntarily; when movement does occur, it may be accompanied by tremor; strength is reduced disproportionate to disuse — CRPS produces true weakness beyond what immobilization alone would cause
• PROM / end-feel: In the warm phase — PROM exceeds AROM (limited by pain guarding, not contracture); end-feel is muscle guarding or empty (the patient stops the movement due to pain before a mechanical limit is reached); in the cold phase — PROM may be limited by contracture (firm/capsular end-feel); periarticular fibrosis produces genuine mechanical restriction; perform PROM cautiously — passive movement of an allodynic limb may provoke severe pain and a flare response
• Resisted testing: Weakness is present but difficult to assess accurately because pain inhibits maximal effort; grip strength dynamometry (upper extremity) or functional testing (lower extremity) provides objective measurement; weakness may be distributed across multiple muscle groups, not following a single nerve or myotomal pattern

Special Test Cluster

The special test cluster for CRPS is oriented toward confirming the Budapest criteria and differentiating CRPS from conditions that mimic it, rather than direct orthopedic confirmation.

Diagnostic note: CRPS is a clinical diagnosis made by applying the Budapest criteria. There is no single confirmatory test. The criteria require: (1) continuing pain disproportionate to the inciting event; (2) at least one symptom in 3 of 4 categories; (3) at least one sign in 2 of 4 categories (clinical) or 3 of 4 (research); (4) no other diagnosis better explains the signs and symptoms.

Differential Diagnoses