It may be that the oedema is due to sudden compromise of blood supply due to vasoconstriction, leading to disruption of the blood brain barrier. Given this is a vasoconstriction rather than a thrombosis, it should be reversible. However, in practice I have seen one case of permanent blindness with PRES in a young female with lupus. It was the first diagnosis of lupus in that young woman with a prior diagnosis of 'ITP' when she presented with refractory seizures. Imaging while in the ITU suggested PRES but upon extubation and regaining consciousness, she was diagnosed with cortical blindness. Now more than 4 months into her follow-up, she is in remission with immunosuppression but her vision is perception of light only.