Osmolar disorders.

OBJECTIVE To present a practical approach to the management of osmolar disorders by considering the mechanisms involved in their development. DATA SOURCES Articles and published peer-review abstracts on the mechanisms and management of disorders of plasma osmolality. SUMMARY OF REVIEW In health, plasma osmolality is determined largely by the concentration of sodium and its accompanying anion, and regulated by mechanisms that regulate body water. Water excess or deficiency causes hypo and hyper osmolality, respectively. Hyperosmolality may also be caused by an excess of body solutes, with the biochemical and clinical features depending on whether these compounds are impermeant (i.e. remain in the extracellular fluid) or permeant (i.e. are distributed throughout the total body water). Sodium metabolism is regulated by those mechanisms that regulate extracellular fluid volume. An increase (or decrease) in total body sodium is associated with retention (or excretion) of body water, an increase (or decrease) in the extracellular fluid (ECF) volume, and no change in plasma osmolality. The initial presentation of an osmolar disorder is usually with an abnormal plasma sodium, therefore management often focuses on treatment of hypo or hypernatraemia. However, treatment requires an estimation of all fluid and solute excesses and deficiencies, with the need or otherWise for prompt correction depending upon the nature of the disorder and time taken for it to develop. CONCLUSIONS Osmolar disorders are associated with varying changes in total body fluid and solute. Consideration of these changes, using a body fluid compartment model, is useful in understanding the abnormalities present and to facilitate management.