Oedema seen throughout both cerebellar hemispheres and brainstem, with vasogenic oedema ascending through the cerebellar peduncles into the internal capsule. The posterior fossa is very tight, with obliteration of the the pre pontine cisterns and inferior displacement of the cerebellar tonsils through foramen magnum. The perimesencephalic cisterns are also obliterated, as is the inferior aspect of the fourth ventricle. Resultant non-communicating hydrocephalous is noted.
In the right corona radiata a small focus of haemorrhage is demonstrated with surrounding residue oedema, unchanged compared to the prior CT.
Only a trivial amount of blood product is seen in the right occipital horn, with no blood in the third ventricle, aqueduct or fourth ventricle. In addition to the aforementioned posterior fossa changes, subcortical white matter T2 hyperintensity is seen in the occipital lobes, without haemorrhagic change. No abnormal enhancement is demonstrated in the cerebellum, brainstem or cerebral hemispheres. A few punctate regions of restricted diffusion are present within the right cerebellar hemisphere. No cortical restricted diffusion.
MR venography (not shown) demonstrates normal flow and contrast induced signal within the dural venous sinuses, without features to suggest a deep vein thrombosis.
MR angiography is unremarkable, with no focal stenosis, occlusion or beading.
Conclusion: Prominent cerebellar and brainstem oedema and swelling with right corona radiata parenchymal haemorrhage in a patient who is immunocompromised and hypertensive has a relatively broad differential including: posterior reversible encephalopathy syndrome (PRES), rhomboencephalitis (e.g. viral or Listeria) and progressive multifocal leukoencephalopathy (PML). Of these, in the setting of hypertension and occipital subcortical T2 change and a hypocellular CSF posterior reversible encephalopathy syndrome (PRES) is favoured.