Central pontine myelinolysis (osmotic demyelination syndrome) results from overly rapid correction of chronic hyponatremia (>8-10 mEq/L/24h). Osmotic stress causes oligodendrocyte apoptosis with demyelination in the central pons and extrapontine structures. Prevention: limit sodium correction to <=8 mEq/L/day. Treatment is supportive. PRES (posterior reversible encephalopathy syndrome) involves vasogenic edema from endothelial dysfunction due to severe hypertension, eclampsia, or immunosuppressive agents. MRI shows bilateral white matter edema in parieto-occipital regions. Management: aggressive blood pressure control and removal of inciting agent.