Hyponatraemia (low sodium): symptoms, causes and treatment

NICE CKS says most people with hyponatraemia have no symptoms, particularly if it is mild and gradual. Symptoms, if present, are often non-specific and relate to how low the sodium is, how fast it fell, how long it has been low and any other illnesses. Rapid changes or severe hyponatraemia can cause vomiting, headache, drowsiness, seizures, coma and cardio-respiratory arrest. Chronic hyponatraemia can lead to gait disturbance, a risk of falls, reduced concentration and other cognitive deficits.

A Royal Berkshire NHS Foundation Trust patient leaflet explains that symptoms depend on the sodium level and how quickly it falls. In acute hyponatraemia there is a sudden fall over hours, whereas in chronic hyponatraemia it falls over days or weeks. If levels are only mildly abnormal you may have no or only mild symptoms, but if the drop is sudden even mild hyponatraemia can cause significant symptoms, while severe chronic hyponatraemia may cause none because the brain adapts.

Sodium is a salt in the blood whose level is tightly controlled by the body. The Royal Berkshire leaflet says hyponatraemia is a low level, and explains that hormones such as antidiuretic hormone and cortisol, and organs including the brain and kidneys, are involved in controlling sodium and fluid balance.

NICE CKS classifies it as mild (130 to 135 mmol/L), moderate (125 to 129 mmol/L) or severe (less than 125 mmol/L), and as acute (under 48 hours) or chronic (48 hours or more). The Royal Berkshire leaflet uses a different severe threshold of under 120 mmol/L and says a typical normal range is 135 to 145 mmol/L, which varies slightly between laboratories.

The leaflet describes three types by body fluid level: normal, reduced (the most common cause being dehydration) and increased (for example heart failure leading to fluid retention).

NICE CKS says the cause is often multifactorial, involving medications (most commonly thiazide diuretics), the syndrome of inappropriate antidiuresis (SIADH), and underlying conditions such as heart failure, kidney disease and liver disease.

The Royal Berkshire leaflet adds that people more at risk include older adults, who may be less likely to say they are thirsty and may take medicines that cause hyponatraemia as a side effect, and people with kidney or liver disease or heart failure.

NICE CKS says assessment of someone with hyponatraemia includes asking about recent illness or surgery, fluid intake, thirst and urine output, medications, other conditions, exercise, diet and alcohol, and any previous episodes. Tests include repeat sodium, urea and electrolytes, serum osmolality, and urine osmolality and sodium to help identify the cause.

Treatment depends on the cause and severity. NICE CKS says management includes arranging emergency hospital admission for acute-onset, severe or symptomatic hyponatraemia or signs of low fluid volume, discussing asymptomatic moderately severe cases with an endocrinology specialist, referring to a specialist depending on the likely cause, managing any acute illness, stopping or reducing medication that may contribute, and monitoring sodium in primary care where appropriate.

The Royal Berkshire leaflet describes follow-up for people discharged after diagnosis, with regular telephone contact and return for blood tests until levels are satisfactory. It describes a fluid restriction, only if your medical team has advised it: tracking all drinks and counting half the volume of high-fluid foods such as soup, using smaller cups, spreading fluids across the day, and avoiding salty and spicy food to reduce thirst.

NICE CKS warns of osmotic demyelination syndrome from over-rapid correction of sodium in chronic hyponatraemia, which is why treatment in hospital is monitored. It also lists cerebral oedema as a potentially life-threatening complication of severe or acute-onset hyponatraemia, and notes increased risk of fragility fractures and osteoporosis.

The Royal Berkshire leaflet says to seek help via the team, NHS 111 or 999 for new or worsening symptoms of loss of appetite, headache, feeling or being sick, personality change or confusion, muscle cramps and weakness, lack of muscle co-ordination, and drowsiness or fits. Confusion, drowsiness and fits are on that list, so they should not be left waiting for a routine appointment.

NICE CKS says that emergency hospital admission is arranged if there is acute-onset, severe or symptomatic hyponatraemia, and rapid changes or severe hyponatraemia can cause seizures, coma and cardio-respiratory arrest.

Hyponatraemia is usually found on a routine blood test arranged by a GP or in hospital, and managed in primary care if mild and in hospital if symptomatic. Specialist endocrine and kidney input may be involved. Private clinics can run the same blood tests, but acute or symptomatic low sodium needs hospital care.

Do not stop or change prescribed medicines, such as diuretics, without medical advice; NICE CKS lists medication review as part of management.