Hyponatremia is a frequent cause for presentation to the emergency department (ED). While patients with chronic hyponatremia often exhibit minor symptoms, acute development of hyponatremia can lead to global cerebral edema, transtentorial herniation and death. The time course of hyponatremia development is usually not known to the treating physician at presentation. Hence, type and extent of therapy depend on the presence or absence of severe symptoms such as coma, seizures, obtundation or vomiting. It is believed that these are suggestive of increased cerebral pressure and indicative of cerebral edema. International guidance thus demands aggressive therapy with hypertonic saline at their occurrence. However, severe symptoms can also be found in patients with chronic hyponatremia (i.e., development in more than 48 hours) which are not at risk for developing cerebral edema due to adaptive counter measures in the brain. Although the percentage of patients with severe symptoms in chronic hyponatremia is considerably smaller than in acute hyponatremia, a higher incidence of chronic hyponatremia may lead to a larger overall number of severely symptomatic patients with chronic hyponatremia. The precise distribution of acute and chronic cases in the ED has not been established yet. In consequence, many patients with hyponatremia presenting with severe symptoms receive aggressive treatment in order to raise sodium levels, exposing them to the risk for overly rapid correction and osmotic demyelination syndrome (ODS). Current U.S. recommendations limit the sodium increase to a maximum of 8 mmol/L and 10-12 mmol/L in the first 24 hours in patients with high or low-to-moderate ODS risk, respectively. Similarly, the 2014 European Clinical Practice Guideline (ECPG) recommends a limit of 10 mmol/L in each 24h-period. These limits are often not accomplished in the real-world setting. In summary, weighing the risk of hyponatremia against complications associated with its (over-)correction constitutes a dilemma for the clinician. It will be strived to refine guidance on management of hyponatremia in patients with profound hyponatremia. Therefore, a better understanding of the proportions and characteristics of patients at risk for both cerebral edema and ODS is needed. The primary objective of this study is to determine the rate of hyponatremic patients who had already developed cerebral edema on admission and to identify patients who developed ODS during the index hospital stay. A secondary objective is a more detailed characterization of these subsets.