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Posterior reversible encephalopathy syndrome

Posterior reversible encephalopathy syndrome (PRES), also known as hypertensive encephalopathy, is a neurotoxic state that occurs secondary to the inability of posterior circulation to auto-regulate 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.

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.

Clinical presentation

Patients present with headache, seizures, encephalopathy and/or visual disturbance. 

Pathology

The syndrome can be precipitated by various clinical settings. The mechanism is not well understood but is thought to be related to 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.

Aetiology

Radiographic features

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:

  1. holohemispheric at watershed zones
  2. superior frontal sulcus
  3. 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.

CT

The affected regions, as outlined above, are hypoattenuating.

MRI

Signal characteristics of affected regions include:

  • T1: hypo intense 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%

Differential diagnosis

General imaging differential considerations include:

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