Central pontine myelinolysis is defined by a symmetric area of myelin damage in the center of the basis pontis. In 10% of these cases, symmetric extra-pontine lesions of similar histological type are found in other parts of the brain. The MRI has provided information about the natural history of this demyelinating process. Disorders of plasma osmolarity appear to be a crucial factor in the pathogenesis of central pontine myelinolysis: rapid correction of hyponatremia or sometimes severe plasma hyperosmolarity. Alcoholic patients and, more generally, those afflicted with a serious debilitating illness are more susceptible to a hyperosmolar insult, absolute or relative. In such cases, the management of hyponatremia must be very cautious, owing to the risk of myelinolysis.