## Anatomical Distinction Between Epidural and Spinal Anesthesia **Key Point:** The defining anatomical difference is the site of local anesthetic deposition relative to the dura mater. ### Epidural vs. Spinal Anesthesia | Feature | Epidural | Spinal (Subarachnoid) | |---------|----------|----------------------| | **Site of injection** | Epidural space (outside dura) | Subarachnoid space (inside dura, CSF) | | **Onset time** | 15–20 minutes | 3–5 minutes | | **Volume of LA** | 15–20 mL | 2–4 mL | | **Motor blockade onset** | Slower, 20–30 minutes | Rapid, 2–3 minutes | | **Duration** | 1–2 hours (varies with agent) | 1–3 hours (varies with agent) | | **Spread** | Segmental, limited by epidural space anatomy | Rapid, diffuse in CSF | **High-Yield:** The epidural space is bounded by the ligamentum flavum posteriorly and the posterior longitudinal ligament anteriorly. The dura mater lies deep to this space. In spinal anesthesia, the needle penetrates the dura and arachnoid to reach the CSF-filled subarachnoid space. **Clinical Pearl:** Epidural anesthesia allows for segmental blockade with slower onset but greater flexibility in dosing and duration. Spinal anesthesia produces rapid, profound blockade but is non-adjustable once injected. **Mnemonic:** **DURA** = Dura Undermines Rapid Action (spinal is fast because it's in CSF; epidural is slower because LA must diffuse through epidural space and dura).
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