Posterior reversible encephalopathy syndrome

Last revised by Dr David Luong on 21 Feb 2022

Posterior reversible encephalopathy syndrome (PRES), also known as acute hypertensive encephalopathy or reversible posterior leukoencephalopathy, 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 edema, usually without infarction, most commonly in the parieto-occipital regions.

The term posterior reversible encephalopathy syndrome may 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.

Presentation is somewhat variable but dominated by 16,19:

  • headache
  • seizures
  • acute confusion/altered mental state/decreased levels of consciousness
  • visual disturbance

Additional symptoms that may develop include ataxia, focal neurological defects, vertigo and tinnitus 19.

Although PRES is most commonly thought of occurring as secondary to marked hypertension, this does not appear to be a necessary or sufficient explanation, given the very large and heterogeneous clinical scenarios that precipitate the development of PRES and the fact that hypertension is not present or does not reach the upper limits of self-regulation (140-160 mmHg) in 25% of patients.

The underlying mechanisms involved are not well understood but is thought to culminate in altered integrity of the blood-brain barrier. Three main precipitant theories have been proposed, that are not mutually exclusive 19:

  • high blood pressure (breakthrough theory) leads to loss of self-regulation, hyperperfusion with endothelial damage and vasogenic edema
  • vasospasm theory results in local ischemia and hypoperfusion
  • endothelial dysfunction secondary to circulating endogenous or exogenous toxins
  • during the acute course of PRES: vasogenic edema, without inflammation, ischemia, or neuronal damage 3
  • during the late course of PRES: demyelination and myelin pallor along with evidence of ischemia, anoxic neuronal damage, laminar necrosis, or older hemorrhage in the white matter and cortex 3

Typical PRES manifests as bilateral vasogenic edema within the occipital and parietal regions (70-90% of cases), perhaps relating to the posterior cerebral artery supply.

Despite the name, however, PRES can be found in a non-posterior distribution, mainly in watershed areas, including within the frontal, inferior temporal, cerebellar, and brainstem regions 2,19. Both cortical and subcortical locations are affected.

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 hemorrhage are associated with posterior reversible encephalopathy syndrome 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.

There are signs of vasospasm or arteritis 3:

  • diffuse vasoconstriction
  • focal vasoconstriction
  • vasodilatation
  • string-of-beads appearance

Signal characteristics of affected areas usually reflect vasogenic edema, with some exceptions:

  • 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, sometimes hyperintense due to edema (T2 shine-through) or true restricted diffusion
  • ADC: usually increased signal due to increased diffusion, but restricted diffusion is present in a quarter of cases 5
  • GRE/SWI: may show hemorrhages in 9-50% 5

MRA may show patterns of vasculopathy with vessel irregularity consistent with focal vasoconstrictions/vasodilatation and diffuse vasoconstriction 3. MRV tend to be normal in posterior reversible encephalopathy syndrome 3.

Posterior reversible encephalopathy syndrome was described for the first time as a distinct entity in 1996 by an American neurologist Judy Hinchey et al 13. Although others had previously described similar reversible CT and MRI findings in hypertension back to the 1980s 14.

General imaging differential considerations include:

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Cases and figures

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  • Case 7
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  • Case 8: PRES with splenial lesion
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  • Case 9: following bone marrow transplantation
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  • Case 13
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  • Case 14
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  • Case 15
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  • Case 16: MRA with string-of-beads appearance
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  • Case 17
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