The adult patient with hyponatraemia

Abstract

### Key points Sodium is the most abundant cation in extracellular fluid (ECF), accounting for 86% of plasma osmolality, and is therefore of major importance when considering the physiological effects of hyponatraemia.1 Disorders of sodium balance (usually mild) can occur in up to 30% of in-patients,2 and are associated with increased mortality and length of hospital stay. Hyponatraemia usually reflects an excess of body water relative to sodium, being classified as hypervolaemic , hypovolaemic , or euvolaemic . It has varying osmolar states and can become life-threatening if unrecognized or managed incorrectly because of a failure to appreciate changes in plasma osmolality and subsequent osmotic movement of water (below). This article describes sodium homeostasis, the pathophysiology of hyponatraemia, plus the assessment and management of hyponatraemia via clinical scenarios to illustrate educational and practical points. ### Sodium homeostasis Sodium in the ECF is normally maintained at 134–146 mmol litre−1. Total body sodium content is 58 mmol kg−1 and daily requirement is 2 mmol kg−1. Osmolarity is defined as the number of osmoles per litre of solution. Osmolality is the number of osmoles per kilogram of solvent and may be estimated by the following calculation: Osmolality=2 (Na+K) +Glucose+Urea (mOsm kg−1) (all in mmol litre−1). From the above equation, it …