Low sodium (hyponatraemia) — a case-based approach

The illustrative case

Our imagined patient is a 78-year-old who over a few days has become increasingly confused, wobbly on their feet, and sleepy, and has been sick once or twice. They take a water tablet for blood pressure and have been drinking large amounts of water because a friend told them it was healthy. A clinician does not assume this is simply old age. They ask about medicines, especially water tablets, some antidepressants and other drugs that can lower sodium, about how much fluid the person has been drinking, and about recent illness, diarrhoea or vomiting. In real life a simple blood test measures the sodium level, and further tests find the cause. Here we use the story to show the questions and the careful thinking involved.

What sodium does and why low is a problem

Sodium is one of the body's key salts. Together with water, it keeps the right amount of fluid inside and outside cells and helps nerves and muscles work. The body normally keeps sodium within a narrow range. Hyponatraemia means the sodium level in the blood is too low. This is often not because there is too little salt, but because there is too much water diluting it. As sodium falls, water moves into cells, including brain cells, causing them to swell. Because the skull is a fixed space, brain swelling is what makes low sodium dangerous. How ill someone becomes depends both on how low the level is and, importantly, on how quickly it has fallen.

Common causes

There are many causes, and finding the right one matters because it guides safe treatment. Water tablets (diuretics) used for blood pressure or heart failure are a common culprit, as are some antidepressants and other medicines. Illnesses causing vomiting and diarrhoea lose both salt and water. Drinking very large amounts of water, sometimes during endurance exercise or from a mistaken belief that more is always better, can dilute the blood. Heart, kidney and liver problems can cause the body to hold on to too much water. A condition called SIADH, in which the body makes too much of a water-retaining hormone, can be triggered by chest infections, some cancers and certain drugs. Older people are more prone to low sodium and to its effects.

Warning signs and safe correction

Mild low sodium may cause few symptoms, or vague ones like tiredness, headache, feeling sick, muscle cramps and poor concentration. As it worsens or falls quickly, people can become confused, drowsy and unsteady, and severe cases can cause seizures or loss of consciousness — a medical emergency needing 999. Correcting low sodium must be done carefully and under medical supervision. If it is raised too quickly, it can cause a rare but serious brain injury. That is why treatment is tailored to the cause and the speed of onset: sometimes simply limiting fluid, sometimes changing a medicine, and in severe cases giving carefully controlled fluids in hospital with frequent blood tests. This is never something to attempt to fix alone at home.

What the case teaches

This made-up story shows that new confusion, unsteadiness and drowsiness in an older person should never be dismissed as just ageing — a simple blood test can reveal a treatable cause such as low sodium. The clinician's approach is to measure the level, ask carefully about medicines and fluid intake, and find the underlying cause before correcting it slowly and safely. The practical lessons are that more water is not always better, that water tablets and some other medicines can lower sodium, and that severe symptoms such as seizures are an emergency. Do not stop prescribed medicines yourself; speak to your clinician or pharmacist. Remember this is general education, not personal medical advice.