What causes the reversal sign on unenhanced CT of the brain?
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Answer:
The reversal sign is inversion of the normal attenuation relationship between grey and white matter, so that the cerebral white matter appears denser than the overlying cortex and the deep grey nuclei, with the thalami, brainstem and cerebellum standing out as relatively dense. It follows severe diffuse hypoxic-ischaemic injury, classically in infants after cardiorespiratory arrest, drowning, non-accidental injury or status epilepticus, and usually becomes apparent after about 24 hours rather than immediately. It is a marker of irreversible injury and, like the white cerebellum sign with which it overlaps, predicts death or severe neurological disability in the great majority of cases. Care is needed not to overcall it: partial volume averaging in a small head, beam-hardening artefact in the posterior fossa, the normally high water content of unmyelinated neonatal white matter and technically poor low-dose studies can all mimic reduced differentiation. Quantification with the grey-to-white attenuation ratio, measured at the level of the basal ganglia and the centrum semiovale, is more reproducible than visual assessment, and a ratio below about 1.1 to 1.2 indicates severe injury. MRI with diffusion-weighted imaging and MR spectroscopy showing a lactate peak provides earlier and more accurate prognostic information and should follow when the child survives the acute phase.
Why is it called so?
The normal density relationship of grey and white matter is reversed, with white matter appearing brighter than cortex.
Pathophysiology
Global hypoxia-ischaemia causes energy failure and cytotoxic oedema. Cortical and deep grey matter neurons, being metabolically the most active and richest in excitatory glutamate receptors, swell first and lose attenuation, while the immediate loss of density in myelinated white matter is smaller. Additional factors are transtentorial and subfalcine compression of the deep venous system, which produces venous congestion and further raises water content in the cortex, and redistribution of intravascular blood into the deep white matter and central structures. The net effect is that cortical attenuation falls below white matter attenuation, and the comparatively spared cerebellum, brainstem and thalami appear dense, giving a picture that is best understood as diffuse cerebral oedema with regional selectivity rather than as true hyperdensity anywhere.
Alternative names: Reversal of grey-white differentiation
Other associated named signs: White cerebellum sign and the pseudo-subarachnoid haemorrhage sign in the same setting
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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