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🧠 Subarachnoid Hemorrhage Volume: Key Points

Estimating the volume of a subarachnoid hemorrhage (SAH) is trickier than for intraparenchymal or intraventricular bleeds due to its irregular, diffuse distribution in the subarachnoid space. Here’s a comprehensive look at how it's approached:

🔬 1. Why It's Hard to Measure

  • SAH spreads along cisterns, sulci, and fissures, making it non-confluent and irregular.
  • There’s no standard “ABC/2” method for SAH like there is for ICH.

🧮 2. Methods of Estimation

A. Visual Grading Systems (most commonly used)

➤ Fisher Scale (Classic) Used to predict vasospasm, not volume per se:

  • Grade 1: No SAH
  • Grade 2: Diffuse thin SAH (<1 mm)
  • Grade 3: Localized clot and/or vertical layer >1 mm
  • Grade 4: Any SAH with intraventricular hemorrhage

➤ Modified Fisher Scale More detailed, still semiquantitative, and slightly better correlated with vasospasm risk.

B. Hijdra Score

A more granular scale scoring 10 basal cisterns and sulci from 0 (no blood) to 3 (completely filled with blood), total score 0–30.

C. Volumetric Estimation Using Imaging Software

Manual or semi-automated segmentation of hyperdensities on CT using tools like:

  • 3D Slicer
  • OsiriX
  • ITK-SNAP

Pros: → Results in volume in mL

Cons: → Time-consuming → Not routinely done in clinical practice → Mainly used in research

📊 3. Clinical Reference Values

SAH Classification Estimated Volume Prognostic Value
Small/localized < 5 mL Lower risk
Moderate 5–15 mL Intermediate risk
Large >15–20 mL Higher risk of vasospasm, poor outcome

🛠️ 4. Radiological AI Tools

Some modern centers use AI-based algorithms to:

  • Automatically segment blood in SAH
  • Estimate volumes
  • Correlate with outcome scores