A 28-year-old woman with a closed fracture of the tibia 6 weeks ago now presents with severe pain, swelling, skin changes (shiny, cyanotic), and reduced range of motion at the ankle. You suspect complex regional pain syndrome (CRPS). Which investigation is most appropriate to confirm the diagnosis?

## Diagnosis of Complex Regional Pain Syndrome (CRPS) **Key Point:** Complex regional pain syndrome (CRPS) is primarily a clinical diagnosis based on Budapest criteria, but **three-phase bone scan (technetium-99m scintigraphy)** is the most sensitive imaging investigation for confirming the diagnosis — it shows characteristic patterns of blood flow and bone uptake abnormalities. ### Three-Phase Bone Scan Findings in CRPS **Phase 1 (Blood flow / Angiographic phase):** - Increased blood flow to the affected limb (hyperemia) - Visible within seconds of injection **Phase 2 (Blood pool / Soft tissue phase):** - Increased uptake in soft tissues surrounding the affected joint - Occurs at 2–5 minutes **Phase 3 (Delayed / Bone phase):** - Periarticular uptake (around the joint) — **most characteristic** - Diffuse or patchy increased uptake in the affected bone - Occurs at 2–4 hours post-injection **High-Yield:** The combination of **increased blood flow (Phase 1) + increased soft tissue uptake (Phase 2) + periarticular bone uptake (Phase 3)** is highly sensitive (>80%) for CRPS and helps differentiate it from other post-fracture complications. ### Comparison of Investigations | Investigation | Sensitivity in CRPS | Specificity | Role | |---|---|---|---| | **Three-phase bone scan** | **>80%** | **Good** | **Gold standard imaging** | | MRI | Variable (50–90%) | Moderate | Shows edema, atrophy; non-specific | | X-ray | Low | Low | Excludes fracture non-union; shows osteopenia | | EMG/NCS | Very low | Very low | Excludes neuropathy; CRPS is not a nerve disorder | **Clinical Pearl:** CRPS has two types: Type 1 (without confirmed nerve injury) and Type 2 (with documented nerve injury). Both present with pain disproportionate to the injury, autonomic dysfunction (color/temperature changes, sweating), motor dysfunction (weakness, tremor), and trophic changes (skin, hair, nail changes). Early recognition and multidisciplinary management (physiotherapy, pain control, psychological support) are crucial to prevent progression and disability. **Mnemonic: CRPS Diagnostic Approach** — **B-C-I-M** - **B**udapest criteria (clinical diagnosis) - **C**linical signs (pain, autonomic, motor, trophic changes) - **I**maging — three-phase bone scan (confirmatory) - **M**ultidisciplinary management (PT, pain, psychology)