A 68-year-old man with chronic alcoholism presents with progressive confusion and headache over 3 weeks. CT head shows a subdural hematoma. Which imaging feature would most reliably distinguish this chronic subdural hematoma from an acute extradural hematoma in the same location?

Explanation

## Chronic Subdural vs Acute Extradural Hematoma: Temporal Evolution ### Density Evolution Over Time **Key Point:** The density characteristics of intracranial hematomas change predictably with time. Acute extradural hematomas remain hyperdense for 1–2 weeks, while subdural hematomas evolve through isodense (1–3 weeks) and hypodense (>3 weeks) phases. ### Comparative Imaging Features by Age | Temporal Phase | Extradural Hematoma | Subdural Hematoma | |---|---|---| | **Acute (0–3 days)** | Hyperdense, lens-shaped, sharp margins | Hyperdense, crescent-shaped, concave margins | | **Subacute (1–3 weeks)** | Remains hyperdense; may become isodense | Becomes isodense; difficult to see | | **Chronic (>3 weeks)** | Rare; usually evacuated or resolves | Hypodense; mixed signal on MRI; septations | | **MRI signal** | T1: hypointense; T2: hypointense (acute) | T1: mixed (methemoglobin); T2: mixed; hemosiderin rim | ### Why Chronic Subdural Shows Isodense/Hypodense with Mixed MRI Signal **High-Yield:** A 3-week history in this patient indicates **chronic subdural hematoma**. By this time, the hematoma has undergone: 1. **Lysis of RBCs** → breakdown of hemoglobin 2. **Conversion to methemoglobin** → isodense on CT (1–3 weeks) 3. **Further degradation** → hypodense on CT (>3 weeks) 4. **Hemosiderin deposition** → hypointense T2 signal (hemosiderin rim) 5. **Neomembrane formation** → enhancement on contrast-enhanced imaging **Clinical Pearl:** Chronic subdural hematomas frequently appear isodense on CT, making them easily missed ("isodense SDH"). MRI with T2 and FLAIR sequences reveals the mixed signal intensity and hemosiderin rim, which is pathognomonic. ### Extradural Hematoma Remains Acute Extradural hematomas, being epidural, are under higher pressure from arterial bleeding (middle meningeal artery). They: - Remain **hyperdense** for 1–2 weeks - Have **sharp, well-demarcated margins** (confined by dural attachments) - Are typically **evacuated surgically** before reaching chronic phase - Rarely progress to chronic stage in clinical practice ### Mechanism of Density Change ```mermaid flowchart TD A[Acute Hematoma: RBCs intact]:::outcome --> B[Hemoglobin deoxidation]:::action B --> C[Methemoglobin formation<br/>1-3 weeks]:::outcome C --> D[Hemoglobin breakdown<br/>Hemosiderin deposition]:::action D --> E[Chronic phase: Hypodense<br/>Hemosiderin rim on MRI]:::outcome A -->|Extradural| F[Remains hyperdense<br/>Sharp margins<br/>Usually evacuated]:::urgent A -->|Subdural| G[Progresses through phases<br/>Becomes isodense then hypodense]:::outcome ``` [cite:Robbins 10e Ch 28; Neuroradiology Essentials]