The Reversal Sign

The reversal sign refers to the inversion of the normal attenuation relationship between gray and white matter on NECT. The result is a higher attenuation of the thalami, brainstem, and cerebellum relative to the surrounding gray matter. The finding indicates diffuse cerebral edema secondary to anoxic injury and is associated with a poor prognosis.

Several theories regarding the pathogenesis of the reversal sign exist:
1. Transtentorial herniation relieves pressure on the central structures which improves tissue perfusion in the posterior fossa.
2. Hypoxia induces neovascularity in regions of normally high metabolism and capillary density.
3. Hypoxia causes petechial hemorrhages in the regions of relatively increased attenuation.
4. Increased intracranial pressure causes venous outflow obstruction and distention of deep medullary veins.
5. Elevated serum (brain) glucose levels during anoxia causes preferential damage to the cortex with relative preservation of the thalami and brainstem.

REFERENCES

Han BK, Towbin RB, De Courten-Meyers G, et al. Reversal sign on CT: effect on anoxic/ischemic cerebral injury in children. AJNR Am J Neuroradiol 1989;10:1191-1198.
Kavanagh EC. The reversal sign. Radiology 2007;245:914-915.