Hyponatraemia (serum sodium <133mmol/L) is a common finding in the inpatient population. Levels of 130-133mmol/L are considered mild and do not require investigation. Below 130mmol/L (moderate), and particularly below 125mmol/L (profound hyponatraemia), symptoms may develop and require urgent investigation and treatment, depending on their severity. This is because severe symptoms reflect cerebral oedema and can lead to loss of neurological function, brain herniation and loss of life if untreated.
Severe symptoms: seizures, drowsiness, coma, cardio-respiratory arrest.
Moderately severe symptoms: nausea, confusion, headache.
Milder symptoms of hyponatraemia (lethargy, anorexia) and moderate hyponatraemia (125-129mmol/L) - if asymptomatic or simply without severe symptoms - should be investigated as in flow chart 1. In view of particular clinical risk around patients with serum sodium <120mmol/L, a clinical biochemist will usually phone this result through to aid with time-critical management.
Severe symptomatic hyponatraemia is a medical emergency and a senior physician must be contacted immediately. With their guidance it may be managed as in flow chart 2.
Content updated August 2020