Posterior reversible encephalopathy syndrome (PRES) is a neurotoxic state that occurs secondary to the inability of the posterior circulation to autoregulate in response to acute changes in blood pressure. Hyperperfusion with resultant disruption of the blood brain barrier results in vasogenic oedema, but not infarction, most commonly in the parieto-occipital regions.
PRES is also known as acute hypertensive encephalopathy or reversible posterior leukoencephalopathy.
The term PRES can be a misnomer as the syndrome can involve or extend beyond the posterior cerebrum. Furthermore, although most cases involve a resolution of changes with the treatment of the precipitating cause and clinical recovery some patients can progress to develop permanent cerebral injury and be left with residual neurological defects.
It should not be confused with chronic hypertensive encephalopathy, also known as hypertensive microangiopathy, which results in microhemorrhages in the basal ganglia, pons, and cerebellum.
Patients present with a headache, seizures, encephalopathy, and/or visual disturbance.
Various clinical settings can precipitate the syndrome. The mechanism is not well understood but is thought to be related to the altered integrity of the blood brain barrier. Two main theories have been proposed:
- high blood pressure: leads to loss of self-regulation, hyperperfusion with endothelial damage and vasogenic oedema
endothelial dysfunction: leads to vasoconstriction and hypoperfusion resulting in cerebral ischaemia and subsequent vasogenic oedema
Hypertension is not present or does not reach the upper limits to self-regulation (150-160 mmHg) in 25% of patients.
- severe hypertension
- haemolytic-uraemic syndrome (HUS)
- thrombocytopaenic thrombotic purpura (TTP)
- systemic lupus erythematosus (SLE)
- drug toxicity
- cyclophosphamide 11
- use of L-asparaginase
- bone marrow or stem cell transplantation
Most commonly there is vasogenic oedema within the occipital and parietal regions (~95% of cases), perhaps relating to the posterior cerebral artery supply. The oedema is usually symmetrical. Despite being termed posterior, PRES can be found in a non-posterior distribution, mainly in watershed areas, including within the frontal, inferior temporal, cerebellar, and brainstem regions 2. Both cortical and subcortical locations are affected.
There are three main imaging patterns:
- holohemispheric at watershed zones
- superior frontal sulcus
- parieto-occipital dominance
Other uncommon patterns of PRES in <5% include: purely unilateral, or "central" (brainstem or basal ganglia lacking cortical or subcortical white matter involvement).
Parenchymal infarctions and haemorrhage are associated with PRES in respectively 10-25% and 15% of cases.
The presence of contrast enhancement, no matter the pattern or how avid, does not portend the clinical outcome.
The affected regions, as outlined above, are hypoattenuating.
Signal characteristics of affected areas include:
- T1: hypointense in affected regions
- T1 C+ (Gd): patchy variable enhancement. It can be seen in ~35% of patients, whether leptomeningeal or cortical pattern.
- T2: hyperintense in affected regions
- DWI: usually normal
- ADC: signal increased in affected regions due to increased diffusion
- GRE: may show hypointense signal in cases of haemorrhage
- SWI: may show microhemorrhages in up to 50%
General imaging differential considerations include:
progressive multifocal leukoencephalopathy (PML)
- periventricular and subcortical involvement, sparing the cortex
- little or no mass effect or enhancement
- severe hypoglycaemia
posterior circulation infarct
- occipital and cerebellar involvement
- acute infarct demonstrates restricted diffusion; PRES typically does not restrict
- more asymmetric
- sagittal sinus thrombosis
- hypoxic-ischaemic encephalopathy
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