Hyponatraemia

Definition

Serum sodium<135mmol/L (<120mmol/L is severe).

Epidemiology

It is the most common electrolyte abnormality. 4.4% of postoperative patient have it. Almost 100% will have had an operation.

Pathophysiology

Broadly speaking there are four groups that the causes of hyponatraemia fall into.

Hypovolaemic

• Kidney problems - Addison's, renal failure, acute tubular necrosis, chronic pyelonephritis, diuretics and others
• Non-renal - diarrhoea and/or vomiting, burns, pancreatitis, trauma, fistula, small bowel obstruction, endurace sport event, heat exposure

Not hypovolaemic

• Oedmatous - nephrotic syndrome, heart failure, cirrhosis, renal failure
• Normovolaemic - SIADH, water overload, severe hypothyroidism, glucocorticoid deficiency

Clinical Features

• Neuro - headache, decreased level of consciouness, cognitive impairment, personality change seizure, brain stem herniation (fixed unilateral dilated pupil, decorticate or decerebrate postuiring, respiratory arrest)
• GI - nausea, vomiting
• General - lethargy, muscle cramps and weakness
• If hypovolaemia - dry mucous membranes, tachycardia, reduced skin turgor
• If hypervolaemia - crackles, third heart sound, raised JVP, peripheral oedema, ascites

Investigations

Dehydration

• U+Es are useful for sodium (obviously) but also assessing dehydration (urea and creatinine).

Further diagnosis

Firstly determine their hydration status. Then the most important things are:

• Urinary sodium
• Urine osmolality

The reason being the help to determine which of the causes is most likely. In hypovolaemia:

• Urinary sodium >20mmol/L - renal cause
• Urinary sodium <20mmol/L - non-renal cause

In normovolaemia:

• Urine osmolality >100mOsmol/kg - SIADH

Otherwise, it must be something else. See pathophysiology.