Imaging description

Posterior reversible encephalopathy syndrome (PRES) is a clinicoradiological entity characterized by encephalopathy and symmetrical parieto-occipital edema. The most common imaging appearance of PRES is that of focal areas of symmetrical vasogenic edema [1]. The parietal and occipital lobes are affected most commonly, although involvement of frontal lobes, inferior temporo-occipital junction, and cerebellum may also be noted (Figs. 46.1, 46.2, 46.3). The involved areas demonstrate increased water content and there is T2 and FLAIR hyperintensity that typically reverses with resolution (Fig. 46.1). Diffusion-weighted imaging reveals that a vast majority of lesions do not display reduction in apparent diffusion coefficient (ADC) values, especially in the early stages. In fact, development of restricted diffusion (Fig. 46.2), which is encountered in 11–26% of patients, may signify an adverse outcome [2].

The distribution of PRES is reminiscent of involvement of watershed zones of the brain, with lesions favoring the cortex and subcortical white matter. Topographically, the lesions are often located between the MCA and ACA and MCA and PCA watersheds. Although symmetry of lesions and predominant supratentorial involvement are most common, other patterns do exist and may pose greater challenges to the diagnosis. The lesions may be asymmetric and could involve the deep gray matter (basal ganglia, thalamus) or white matter (internal/external capsules) or even the brainstem. Diffusion restriction may occur, signifying developing cytotoxic injury. Hemorrhagic manifestations (parenchymal hematoma, subarachnoid hemorrhage) have been reported in approximately 15% of patients (Fig. 46.3) [1].