Symmetrical 'vasogenic' brain oedema.

Low attenuation of the cerebral white matter occurs with oedema and demyelination. When unilateral or asymmetrical it is usually due to a tumour, granuloma or abscess. The nidus responsible for the vasogenic oedema can be demonstrated after contrast enhancement. Bilateral demyelination occurs especially with viral infections such as herpes simplex encephalitis and progressive multifocal leukoencephalopathy but produces asymmetrical low attenuation of the deep white matter even though temporal lobes are frequently involved.