The many faces of posterior reversible encephalopathy syndrome
The classic imaging findings of posterior reversible encephalopathy syndrome (PRES) are of bilateral parietal and occipital subcortical vasogenic oedema, and are well established in the literature. As experience with PRES grows, varied and atypical presentations are being increasingly described. This pictorial review illustrates the variable presentations of PRES, including cases with atypical imaging findings. We illustrate cases of PRES with varying distributions of vasogenic oedema as well as cases with atypical imaging findings, such as variations of haemorrhage and restricted diffusion. Atypical imaging findings should not dissuade the diagnosis of PRES in the appropriate clinical situation, and knowledge of the varied appearance and atypical findings of PRES allows the radiologist to make this diagnosis.
Posterior reversible encephalopathy syndrome (PRES) describes a neurological syndrome characterised by a variety of symptoms, including headache, altered mental status, visual disturbances and seizures, and is accompanied by a unique, potentially reversible imaging pattern [1,2]. Causes of PRES are diverse and include hypertension, eclampsia/pre-eclampsia, sepsis, immunosuppressive agents, chemotherapy, collagen vascular disease and renal failure . Although the pathophysiology of PRES remains unknown, the currently preferred explanation relates to hypertension, impaired autoregulation and hyperperfusion . The classic imaging findings are of vasogenic oedema in the subcortical white matter of the parietal and occipital lobes . As experience with PRES grows in the literature, atypical presentations of PRES are being increasingly described. Authors have demonstrated cases of PRES with atypical vasogenic oedema patterns of distribution, such as frontal lobe, cerebellum, basal ganglia or brain stem involvement [3-6]. Unilateral cases of PRES have also been demonstrated . In addition, cases of PRES with haemorrhage  and reversible restricted diffusion  have been described. Knowledge of the different presentations of PRES is important, as atypical imaging findings should not dissuade a diagnosis of PRES in the correct clinical context. The purpose of this review is to demonstrate the diverse imaging features of PRES. The cases illustrated in this review demonstrate clinical and imaging presentations characteristic of PRES, with the characteristic clinical and imaging reversibility associated with this diagnosis.
Hinchey et al  originally described a posterior hypertensive encephalopathy syndrome with a predominant posterior distribution of imaging findings on CT or MRI. Typically, PRES presents as widespread, usually reversible vasogenic oedema, predominantly in the subcortical white matter of the occipital and parietal lobes (Figure 1). In large retrospective studies, this is the most common pattern of oedema [3,4,6]. The predilection for the posterior brain is thought to be a result of the better-developed sympathetic regulation in the anterior circulation, with the posterior circulation more susceptible to impaired autoregulation and vasogenic oedema in the setting of hypertension .
In addition to the occipital and parietal symmetric pattern of vasogenic oedema, wide variations of distribution have been described in the literature. The pattern of oedema in PRES can be widespread, and can involve the entire brain (Figure 2). Although typically symmetrical and bilateral, asymmetrical or unilateral distributions of vasogenic oedema in PRES can also be seen  (Figure 3). Many authors have described cases of PRES with basal ganglia (Figure 4), cerebellar (Figure 5) or brain stem (Figure 6a–d) involvement [2-4,6].
Intracranial haemorrhage is known to occur in PRES with an incidence of approximately 15% . The pathological mechanism is still not well understood but is thought to relate to hypertension and hyperperfusion or vasculopathy with resulting hypoperfusion. Varying patterns of haemorrhage have been identified, including large focal haematomas (Figure 6d–f), subarachnoid haemorrhage (Figure 7) or multiple minute foci of haemorrhage (Figure 8).
Restricted diffusion as an associated finding has been described in PRES and has been shown to be potentially reversible . Benziada-Boudour et al  demonstrated a case of PRES with foci of restricted diffusion that resolved on follow-up imaging with no abnormal imaging sequelae. Similar reversibility of restricted diffusion abnormalities has been seen in patients with transient cerebral ischaemia  and vasospasm after subarachnoid haemorrhage . The patient in Figure 9 demonstrates a region of restricted diffusion that resolved on follow-up imaging. Another patient demonstrates restricted diffusion in the splenium of the corpus callosum, which resolved on follow-up imaging (Figure 10).
Findings of PRES on catheter angiography include focal or diffuse vasoconstriction, vasodilation or a “string-of-beads” appearance. This vessel irregularity can also be seen on CT angiography or MR angiography . These findings may be confused for other diagnoses, such as vasospasm or vasculitis . The patient in Figure 4d,e demonstrates these characteristic angiographic findings.
As demonstrated, PRES can present with a variety of imaging findings on CT, MRI and angiography. Although originally described as a bilateral symmetrical pattern of vasogenic oedema in the posterior parenchyma, other distributions of vasogenic oedema can be seen, including a unilateral pattern and involvement of the cerebellum and brain stem. The typical findings of patchy discontinuous vasoconstriction on angiography are well recognised. Furthermore, haemorrhage is increasingly being recognised in PRES and does not exclude the diagnosis. This can be anatomically located in the brain parenchyma as a focal haematoma or petechial gyral pattern, or in the subarachnoid space. Restricted diffusion is an atypical finding in PRES, but has been described and should not preclude the diagnosis of PRES in the appropriate clinical context.
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