Hyponatremia

Hyponatremia is a lower-than-normal concentration of sodium in the blood, conventionally defined as a plasma or serum sodium concentration below 135 mmol/L. Plasma sodium concentration is closely related to the balance between body water and exchangeable sodium and potassium. Hyponatremia is therefore a disorder of water and electrolyte balance rather than simply a measure of how much sodium the body contains .^1,2

Most hyponatremia is hypotonic, meaning that plasma tonicity is also reduced. It usually develops when the body retains or takes in more electrolyte-free water than the kidneys can excrete relative to the available sodium and potassium. A person with hyponatremia therefore does not necessarily have a total-body sodium deficiency. Hyponatremia can occur when extracellular fluid volume is reduced, approximately normal, or increased, depending on the underlying cause .^1,2

Not all low measured sodium concentrations represent hypotonic hyponatremia. High concentrations of an effective osmole such as glucose can draw water from cells into extracellular fluid, lowering plasma sodium concentration while increasing tonicity. A falsely low sodium result, called pseudohyponatremia, can also occur with some laboratory measurement methods when blood contains very high concentrations of lipids or proteins .²

In hypotonic hyponatremia, water moves into cells. This is particularly important in the brain, where cell swelling can produce neurological effects. The rate at which sodium concentration falls also matters because brain cells gradually adapt to sustained hypotonicity. The physiological consequences of hyponatremia therefore depend not only on sodium concentration but also on tonicity and the rate at which the condition develops .^1,2