Hyponatraemia

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What is Hyponatraemia?

Hyponatraemia is defined as low levels of sodium in the serum, that is less than 135mmol/L (normal range is between 135mmol/L and 145mmol/L). Sodium is the predominant positively charged ion (cation) in the body's extracellular fluid. Severe hyponatraemia is when the serum sodium levels are below 125mmol/L.

Who gets Hyponatraemia?

Hyponatraemia occurs in approximately 1% of hospitalised patients. Incidence rates can be much higher in cancer patients, for example between 15-50% in small cell lung cancer. Decreased serum sodium levels in cancer patients usually occur due to fluid overload, which is caused by inappropriate levels of hormone that prevents excretion of free water (antidiuretic hormone - ADH). This is known as the syndrome of inappropriate secretion of ADH (SIADH), and is easily the most common cause of hyponatraemia in cancer patients.

Predisposing Factors

Malignant illness can cause hyponatraemia in a variety of ways. These include:

Decreased kidney (renal), adrenal or thyroid function (insufficiency)
Factors relating to medical treatment (iatrogenic factors)
SIADH
Central nervous system (CNS) lesions, lung disease or nausea causing physiologic increases in ADH secretion.
Adaptive circulatory mechanisms such as low blood pressure, heart failure or liver cirrhosis.

SIADH is the most common cause of hyponatraemia in patients with cancer.

Other Mechanisms that can cause hyponatraemia include:

Increased thyroid hormones (hyperthyroidism) and adrenal insufficency.
Excessive salt excretion (salt-wasting) in drug-induced kidney damage, adrenal insufficiency or use of specific medications that increase urine excretion (thiazide diuretics).
Chemotherapy.
Chemotherapy and associated nausea and vomiting.
Overhydration with fluids containing less sodium than normal blood (hypotonic fluids).

Progression

Hyponatraemia is usually caused by an excess of total body water relative to sodium levels. It is most often caused by water retention combined with excessive administration of fluids.

Water retention usually occurs as a result of increased ADH secretion due to a reduced volume of blood within the vessels (intravascular volume). This decreased intravascular volume may be due to excessive swelling when the body's fluid has shifted outside of the blood vessles (oedematous state) or true volume depletion (e.g. from blood loss).

The syndrome of inappropriate ADH secretion (SIADH) involves the inability of the kidney to dilute urine even though the plasma has become less concentrated. It is defined as an inappropriately high urine concentration (osmolality) at the same time as decreased serum osmolality.

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