What causes the swirl sign on unenhanced CT of the head?
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Answer:
The swirl sign is a region of low or mixed attenuation within an otherwise hyperdense acute extra-axial or intraparenchymal haematoma, giving a whorled appearance. The hypodense component is unclotted, actively extravasating blood, and its presence therefore indicates ongoing haemorrhage at the time of scanning rather than a chronic or mixed-age collection. In acute epidural haematoma it is a recognised marker of rapid expansion and is associated with a larger haematoma volume, greater midline shift, a worse Glasgow Coma Scale score and higher mortality, and it is one of the findings that pushes management toward immediate surgical evacuation rather than observation. The same principle applies to spontaneous intracerebral haemorrhage, where heterogeneity of density, the black hole sign, the blend sign and the CT angiographic spot sign all describe active bleeding and predict haematoma growth. The main differential for low attenuation within a hyperdense collection is a subacute or chronic component in a repeat bleed, which is usually layered rather than swirled, and severe anaemia or coagulopathy in which the whole collection is relatively hypodense. In a patient on anticoagulants or with a known coagulopathy the sign should prompt urgent reversal as well as neurosurgical referral.
Why is it called so?
The mixture of clotted high-density and unclotted low-density blood produces a swirled or marbled pattern within the haematoma.
Pathophysiology
When a vessel, usually the middle meningeal artery in an epidural haematoma, is torn, blood accumulates faster than it can clot. Clotted blood retracts, its haematocrit rises and it attenuates strongly at about 60 to 90 HU, whereas fresh liquid blood that has not yet formed fibrin retains a lower attenuation similar to unclotted serum and plasma. The two coexist within the expanding collection until bleeding stops. Because clot formation takes minutes, the presence of unclotted blood shows that the scan captured the haematoma while it was still enlarging, which is why the sign is a marker of activity and not merely of size.
Alternative names: Swirled appearance; whirl sign of active haemorrhage
Other associated named signs: Spot sign on CT angiography, the blend sign and the black hole sign of haematoma expansion
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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