A 68-year-old woman with suspected complex regional pain syndrome (CRPS) Type 1 of the right hand presents 3 months after a wrist fracture. Clinical findings include swelling, color changes, temperature asymmetry, and allodynia. Which investigation is most specific for confirming CRPS and assessing microvascular dysfunction?

## Investigation of Choice for CRPS — Three-Phase Bone Scan ### Clinical Context Complex regional pain syndrome (CRPS) Type 1 is diagnosed primarily on **clinical criteria (Budapest/IASP criteria)**. The patient here meets these criteria: post-traumatic onset, vasomotor changes (swelling, color changes, temperature asymmetry), and sensory dysfunction (allodynia). Objective investigations serve as **supportive** tools — they do not independently confirm or exclude CRPS. ### Three-Phase Bone Scan: Most Specific Supportive Investigation **Key Point:** Three-phase technetium-99m bone scintigraphy is the **most specific objective investigation** currently available for supporting a CRPS diagnosis. It is the only imaging modality formally recognized as a supportive investigation in IASP guidelines. It evaluates three distinct physiological phases: - **Phase 1 (Blood Flow, 0–5 sec):** Increased radionuclide flow to the affected limb - **Phase 2 (Blood Pool, 2–5 min):** Periarticular increased blood pooling reflecting microvascular dysfunction - **Phase 3 (Delayed, 2–4 hours):** Increased osteoblastic activity indicating abnormal bone metabolism Specificity for CRPS when all three phases are positive: **>85–90%** (per published literature and Harrison's Principles of Internal Medicine). ### Why Not Infrared Thermography? Infrared thermography detects skin temperature asymmetry (>1°C difference between limbs), which is a **diagnostic criterion** for CRPS and reflects microvascular dysfunction. However, its **specificity is lower (~60%)** because temperature asymmetry occurs in many other conditions (vascular disease, nerve injury, inflammation). It is a useful screening/monitoring tool but is **not included in IASP supportive criteria** and is less specific than bone scan. ### Comparison of Investigations | Investigation | Specificity for CRPS | Microvascular Assessment | IASP Supportive Criteria | |---|---|---|---| | Three-phase bone scan | >85–90% | Yes (indirect) | **Yes** | | Infrared thermography | ~60% | Yes (direct temperature) | No | | X-ray | Low (osteopenia late) | No | No | | Quantitative sensory testing | Variable | No | No | **Clinical Pearl:** X-ray may show patchy osteopenia (Sudeck's atrophy) in late CRPS but is neither sensitive nor specific. QST quantifies sensory thresholds but does not assess microvascular dysfunction or confirm CRPS. ### Important Caveat **High-Yield:** A **negative bone scan does NOT exclude CRPS** — clinical diagnosis per Budapest criteria remains the gold standard. The bone scan is most sensitive when performed **2–6 months** after symptom onset; early scans may be falsely negative. The stem asks for the investigation "most specific for confirming CRPS and assessing microvascular dysfunction" — among the four options, three-phase bone scan has the highest documented specificity and is the only one with formal IASP supportive status. *Reference: Harrison's Principles of Internal Medicine, 21st ed.; IASP CRPS Diagnostic Criteria (Budapest Criteria, 2003/updated 2012)*