Unusual Presentation of a Posterior Reversible Encephalopathy Syndrome With Brainstem Involvement and Subarachnoid Haemorrhage.

Posterior reversible encephalopathy syndrome is an increasingly recognized disorder characterized by a headache, visual disturbances, and seizures. It is a reversible neurotoxic state, with multiple risk factors in which endothelial injury and compromised brain perfusion are the common characteristics. Diagnosis is usually made by cerebral magnetic resonance imaging that typically shows early-stage bilateral symmetrical parieto-occipital hyperintensities on T2 and fluid-attenuated inversion recovery (FLAIR) sequences. However, other locations have been described where the disease may appear less frequently. We describe the case of a 62-year-old man, with a medical history of hypertension, who presented with anisocoria with mydriatic non-reactive pupil and ptosis of the left eye. CT head showed a slightly hypodense brainstem, in relation to vasogenic edema. This was confirmed with magnetic resonance imaging. The angiography did not identify cerebral artery aneurysms. The symptoms and radiological findings were almost completely reversible after improving the patient's blood pressure. This case highlights a rare single presentation of posterior reversible encephalopathy syndrome associated with subarachnoid hemorrhage. A high index of suspicion, careful examination, and exploration with imaging techniques were essential to reach this diagnosis.