Renal & Urology

Hyponatraemia

Hyponatraemia is a low serum sodium, usually reflecting excess water relative to sodium; assess symptoms and osmolality urgently, treat severe neurological symptoms immediately, then classify the cause and control the rate of correction.

Definition

Hyponatraemia is a serum sodium concentration below 135 mmol/L. It should be classified by serum osmolality and then by the clinical and biochemical pattern of water and sodium balance; symptoms and acuity determine immediate urgency.

First principles

Serum sodium is a concentration, not a sodium-store measurement

A low sodium usually means too much water relative to total body solute. Vasopressin, thirst, renal function, intake and losses determine whether the kidney can excrete free water. The same serum sodium can therefore occur with volume depletion, normal-appearing volume status or oedema, and treatment must follow the physiology rather than the number alone.

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