Sodium is responsible for maintaining the extracellular fluid volume. Serum sodium concentration is kept within range by number of various mechanisms which adjust the amount excreted in the urine or increase the intake of water to prevent a rise in serum sodium levels.
Hypernatraemia
This is a level > 145mmol/L. The most common causes include
- Inadequate intake causing dehydration.
- Excessive GI loss without fluid replacement through severe vomiting and/or diarrhoea.
- Loop diuretic use which increases urine output.
- Thirst impairment (elderly, dementia patients).
- Diabetes insipidus.
- Dermal skin losses – burns, excessive perspiration in athletes, eg long distance runners.
- Renal dysfunction.
- Incorrect IV fluid replacement (excessive saline).
- Osmotic diuresis for diabetic coma.
- Primary aldosteronism.
- Hypernatraemic dehydration in breastfed infants.
Hyponatraemia
Hyponatremia is a low plasma sodium level < 135mmol/L, and is usually caused by a failure to excrete water normally. It can be categorised as mild (Na 125 – 134mmol/L), moderate (120 – 124mmol/L) or severe (<120mmol/L). Normally the release of ADH regulates the water balance with excess water being excreted in the kidneys.
The causes of hyponatremia are typically classified according to a person’s body fluid status. There are 3 types: hypovolaemic, euvolaemic or hypervolaemic hyponatraemia.
Hypovolaemic hyponatraemia
This occurs when there is depletion of both total body water and sodium with a relative excess of sodium loss. Common causes include:
- Medications – particularly thiazide & loop diuretics.
- GI loss: prolonged vomiting, chronic diarrhoea, pancreatitis, peritonitis, bowel obstruction.
- Addisons Disease.
- Congenital adrenal hypoerplasia.
- Renal impairment – nephropathy.
- Haemorrhage.
- Exercise induced – excessive perspiration with prolonged endurance type of exercise.
- Rhabdomyolysis.
- Severe burns.
Euvolaemic hyponatraemia occurs when there is relative absolute increase in body water with a concurrent reduction in serum sodium, with no clinical oedema. Common causes include:
- SIADH.
- Adrenal insufficiency.
- Hypothyroidism.
- Excessive water intake.
- Normal physiologic change of pregnancy.
Hypervolaemic Hyponatraemia occurs with both sodium and water content increase. Common causes include:
- Congestive cardiac failure.
- Liver cirrhosis.
- Renal dysfunction particularly nephrotic syndrome.
- Hyperglycaemic states.
Drugs that may cause hyponatraemia: Diuretics (thiazide, combination, loop diuretics), anti convulsants (carbamazepine), ACE inhibitors, Psychotropics – antidepressants: SSRI, MAOIs; antipsychotics, mood stabilisers, sedatives, hypnotics, COX 2 inhibitor, Sulphonylureas , PPIs, Desmopressin, oxytocin
