Hyponatraemia

Plasma sodium of less than 135 mmol/l. Caused by:

• Water excess
• Excessive intake (Urine [Na} < 10 mmol/l)
• iv administration of Na deficient fluids
• TURP syndrome
• Water intoxication
• Reduced excretion (unine [Na}>20 mmol/l:
• SIADH
• Drugs e.g. chlorpropramide, oxytocin have ADH effect.
• Water excess with smaller sodium excess
• Cardiac failure
• Hepatic failure
• water defieciency with greater sodium deficiency
• Renal loss (urine [Na} > 20 mmol/l
• Diuretic therapy
• Hypoadrenalism
• Cerebral salt wasting syndrome
• Salt-losing nephritis
• Renal tubular acidosis
• Post-relief of urinary obstruction
• other loss
• D&V
• Panceatitis
• Redistribution of sodium/water
• Sick cell syndrome in teminally ill patients (Impaired Na/K ATP-ase function)
• Water shift from intra to extracellular compartments e.g. in hyperglycaemia where corrected sodium concentration = [Na]+{Glu/4]
• Pseudohyponatraemia - e.g. in hyperlipidaemia, older analysers included the salt-poor lipid component given artifically low results.

Features

Symptoms (headache, nausea, confusion, coma and convulsions) thought to be due to water entering cells by osmosis. Pre-menopausal women are at increased risk and may suffer fits/respiratory arrest with a sodium of less than 130 mmol/l. In other groups this might not be expected with a [Na] above 115-120 mmol/l

Treatment

Treat underlying causes. Too rapid a correction may result in subdural haemorrhage, central pontine myelinosis or cardiac failure and a rate of 5-10 mmol/day has been suggested, although rates of 2 mmol/l/hr may be tolerable while the [Na] is < 120 mmol/l

Total sodium deficit may be calculated according to the formula:

Deficit = (125 - [Na]) * 0.6 * body weight