Water is distributed in the body mostly in the intracellular space. Extracellular space is divided in to interstitial and plasma compartments. This is illustrated in the following diagram:
About 1.5 L of water is excreted every day via the kidneys, gut, lungs and skin. There are situations when more is lost, for example pyrexia and surgery. Intake must be balanced by an equivalent intake of water either oral or IV.
It is useful to divide patients in to those that are volume depleted and volume expanded. Volume depleted patients are losing water and sodium from their bodies either via the kidneys or some other route. Usual causes for hyponatraemia from the kidneys are diuretics, renal failure, and Addison's disease (hypocortisolism). In Addison's the adrenals do not produce enough mineralocoticoids to retain sodium. Other routes that sodium and water can be lost include diarrhoea, vomiting, fistula, small bowel obstruction and trauma. In order to discriminate between renal and extra-renal causes it is useful to measure the urinary sodium: it is above 20 mmol/L in renal causes and below 10 mmol/L in extra-renal (as the kidneys are working to preserve sodium and water).
Volume expanded patients are divided in to those who are oedematous and non-oedematous. If oedema is present the hyponatraemia is likely dilutional. Common causes include nephrotic syndrome, cirrhosis and CCF. If no oedema is present the causes can include Syndrome of Inappropriate ADH secretion (SIADH) and hypothyroidism. SIADH deserves special mention. ADH acts on the kidneys to retain water (hence the patients are fluid expanded) but unlike aldosterone it does not do this by retaining sodium. The diagnosis requires concentrated urine (>20 mmol/L) and normal or high urine osmolality in the presence of low plasma osmolality. It has a number of causes including lower respiratory tract infection (commonly), various malignancies, CNS diseases, and drugs (including opiates and SSRIs).
This depends on the cause:
If the change in plasma sodium is acute it is important to not correct it too rapidly, as fluid shifts may cause brain damage.
Thirst, confusion, neurological changes, decreased skin turgor, postural hypotension, oliguria.
Blood tests: urea, HB, albumin may be raised if dehydration (concentrational).
This is most commonly due to dehydration. Other causes include excessive IV intake of saline, hyperaldosteronism (rare) and diabetes insipidus (also rare). The latter condition can be thought of as the opposite of SIADH as there is either not enough ADH production or insensitivity in the kidneys to the hormone. ADH is produces in the pituitarys and a deficiency can arise from trauma, tumours, sarcoid, infection, vascular and autoimmune conditions. Insensitivity in the kidneys can be due to drugs (such as lithium), amyloidosis, pyelonephritis, and acute tubular necrosis amongst other causes.
Water orally if possible. If severe the IV dextrose (although saline will also bring down the sodium but more slowly).