What causes the white cerebellum sign on unenhanced CT of the brain?
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Answer:
The white cerebellum sign is the appearance of a relatively dense cerebellum and brainstem against a diffusely hypoattenuating cerebrum after severe global hypoxic-ischaemic injury. The posterior fossa structures are not truly hyperdense; they appear so because the supratentorial brain has lost attenuation through cytotoxic oedema, so the sign is one of relative contrast. It is seen after cardiac arrest, drowning, strangulation, status epilepticus and non-accidental head injury in infants, and it carries a very poor prognosis, with most reported patients dying or surviving in a vegetative state. It should be reported together with the other CT markers of severe injury: loss of grey-white matter differentiation, a reduced grey-to-white attenuation ratio measured at the basal ganglia and centrum semiovale, effacement of the sulci and basal cisterns, and the reversal sign, in which white matter becomes denser than cortex. Two mimics must be excluded before the sign is called: the pseudo-subarachnoid haemorrhage appearance of severe cerebral oedema, in which effaced cisterns and engorged superficial vessels simulate subarachnoid blood, and normal physiological hyperdensity of the neonatal cerebellum. MRI with diffusion-weighted imaging is more sensitive and more accurate for prognosis, particularly between 24 hours and 5 days after the insult.
Why is it called so?
The cerebellum stands out as strikingly white against a dark, oedematous cerebrum.
Pathophysiology
In global hypoxia-ischaemia, failure of the sodium-potassium ATPase allows water to move into neurons and glia, producing cytotoxic oedema and lowering the X-ray attenuation of affected tissue. The cerebral cortex, basal ganglia and hemispheric white matter, with their high metabolic demand and watershed vulnerability, are affected earliest and most severely. The cerebellum and brainstem are relatively resistant, partly because of their different metabolic requirements and partly because posterior circulation perfusion is comparatively preserved during systemic hypotension. Their attenuation therefore remains near normal while everything above the tentorium falls, and the deep venous structures and dural sinuses appear correspondingly prominent, which is the same mechanism that produces pseudo-subarachnoid haemorrhage.
Alternative names: Reversal of cerebellar and cerebral density; dense cerebellum sign
Other associated named signs: Reversal sign, the pseudo-subarachnoid haemorrhage sign, and loss of the grey-white matter differentiation
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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