Hyponatremia

Hyponatremia means a serum sodium below 135 mmol/L, and it is the commonest electrolyte abnormality in clinical practice. The critical conceptual point is that in most cases sodium stores are not depleted; rather, excess water relative to sodium dilutes the blood, usually because antidiuretic hormone is inappropriately elevated. This is why treatment is often fluid restriction rather than salt administration, and why drinking more or adding salt can worsen the situation. Causes are grouped by volume status. Hypovolaemic hyponatremia follows genuine losses through vomiting, diarrhoea or diuretics, particularly thiazides. Euvolaemic cases are dominated by the syndrome of inappropriate antidiuresis, driven by medications including SSRIs and carbamazepine, by lung and central nervous system disease, and by malignancy. Hypervolaemic cases occur in heart failure, cirrhosis and kidney disease. Exercise-associated hyponatremia deserves specific mention, since endurance athletes who overdrink plain water during long events have died from it — thirst-guided drinking rather than scheduled intake is the correct advice. Symptoms depend on both severity and speed of onset: chronic mild hyponatremia may cause only subtle unsteadiness and falls, while rapid falls cause confusion, seizures and cerebral oedema. Correction must be gradual, since raising sodium too fast risks osmotic demyelination syndrome, a devastating and often irreversible complication.

• •Nausea and headache
• •Confusion or difficulty concentrating
• •Unsteadiness and falls
• •Muscle cramps and weakness
• •Lethargy
• •Seizures in severe or rapid onset

• •Syndrome of inappropriate antidiuresis
• •Thiazide diuretics
• •SSRIs and carbamazepine
• •Vomiting and diarrhoea
• •Heart failure or cirrhosis
• •Adrenal insufficiency
• •Excessive water intake during endurance exercise

Inappropriate antidiuretic hormone secretion from medications, lung or brain disease or malignancy, thiazide diuretics, true volume depletion from gastrointestinal losses, heart failure, cirrhosis and nephrotic syndrome causing effective arterial underfilling, adrenal insufficiency, hypothyroidism, excessive plain water intake during endurance exercise, and primary polydipsia.