What is hyponatraemia?

Hyponatraemia means abnormally low levels of sodium in the bloodstream or a deficiency of sodium in the body. The amount of sodium in the blood mainly depends on the balance between salt and water levels in the body.

What causes hyponatraemia?

Hyponatraemia is when there is too little sodium in the blood. The type of hyponatraemia that an individual has can be categorised according to the water volume levels ('hydration status') in the body:

1. Dehydration (hypovolaemic hyponatraemia) – this means that a person will be in a dehydrated condition, with low water levels in the body. It is caused by two mechanisms:
• Loss of sodium and water (in excess) through the kidneys as a result of:
• an underactive adrenal gland (such as Addison’s disease)
• kidney problems
• the use of diuretics.
   
• Loss of sodium and water through other sources such as:
• diarrhoea and vomiting
• pancreatitis (inflammation of pancreas)
• trauma
• burns
• fistulas.
   
2. Fluid overload (hypervolaemic hyponatraemia) – in this case, the patient will be swollen due to too much water in the body. The causes can include:
• cardiac failure
• cirrhosis of the liver
• nephrotic syndrome.
   
3. Normal hydration status (euvolaemic hyponatraemia) – this means water levels in the body are normal so the condition can be caused by:
• syndrome of inappropriate secretion of anti-diuretic hormone (SIADH). This is due to the secretion of too much anti-diuretic hormone (ADH), the hormone which is responsible for managing the excretion of fluid and salt from the body. This can be caused by medications taken to treat other conditions. It can also be the result of chest infection (pneumonia) or brain disorders such as meningitis
• water overload, which can be caused by drinking excessive amounts of water
• severely underactive thyroid gland (hypothyroidism).

What are the signs and symptoms of hyponatraemia?

The symptoms of hyponatraemia depend on the severity and rate of development of the condition. In mild cases, there are usually no symptoms but, as the severity increases, patients may experience headache, nausea, vomiting, lack of energy and anorexia. More serious hyponatraemia can lead to confusion, muscle cramps and weakness, problems with gait and personality changes.

In most severe cases, the main symptoms are seizures and/or drowsiness. However, there are other signs that could lead to a diagnosis of hyponatraemia. These are generally:

• neurological symptoms, including reduced level of consciousness, fits, difficulty in breathing and coma
• dehydration-related symptoms, including dry mucous membranes and reduced elasticity of the skin
• over-hydration-related symptoms, such as swelling of the arms, legs, swelling of the abdomen and breathlessness.

How common is hyponatraemia?

Hyponatraemia is the most common electrolyte condition and is most often seen in elderly, females and hospitalised patients. Estimates of how common it is do vary. It is thought that approximately 3–5% of all hospitalised patients, and 30% of elderly patients have some degree of hyponatraemia. Hyponatraemia is common in patients taking certain medications including thiazide diuretics, anti-epileptics and some antidepressants.

Is hyponatraemia inherited?

Hyponatraemia is not itself inherited. However, some conditions with a genetic component can be associated with hyponatraemia.

How is hyponatraemia diagnosed?

Hyponatraemia is usually picked up by a simple blood test. Once identified, there is a need for other tests to clarify the cause. These would be glucose, a cholesterol profile, cortisol and thyroid function tests. A very high glucose or cholesterol can falsely make sodium levels appear low. Other specialised tests to establish the cause of low sodium may also be carried out, such as a serum and urine test for osmolality (amount of chemicals dissolved in the liquid) and to establish the amount of sodium being passed in the urine. Chest x-rays should be taken for patients showing symptoms of a cough and loss of weight to exclude lung cancer, as this is a common cause of SIADH.

How is hyponatraemia treated?

The treatment given will depend on the underlying cause. In cases of mild hyponatraemia, where the patient has no symptoms, or very minimal symptoms, treatment can be managed as an outpatient. This can include:

1. Stopping any medication that may be responsible for the condition such as diuretics, antidepressants or anti-epileptics. Sodium levels are then rechecked regularly and, if levels do not rise, further investigations need to be started.
2. Starting specific treatment if blood glucose or cholesterol levels are high.
3. Restricting daily fluid intake to 1.0–1.5 litres if necessary, if the diagnosis is SIADH. Usually the sodium will correct when the underlying cause is treated.
4. Salt (sodium chloride) tablets if sodium levels do not improve after the above action has been taken, although this drug is not commonly used.
5. If levels of sodium still low, then a referral for more specialist advice should be sought.

Where hyponatraemia is more serious, patients will usually be admitted to hospital. Treatment will include:

• Initially, stopping any medication that may be responsible for the condition.
• Assessing the amount of fluid a patient has:

• If fluid overload is diagnosed, the fluid and salt intake may be restricted. Patients are often given diuretics to offload the excess fluid, and this requires close monitoring of sodium levels. The relevant specialist may be consulted depending on whether overload is due to heart, kidney or liver problems.
• if dehydration is diagnosed, controlled fluid replacement is administered.

• A new class of drugs called ‘Vaptans’ can be used if SIADH is secondary to malignancy. However, it needs to be prescribed and monitored under specialist care.

In the most severe cases of hyponatraemia, the majority of patients will show neurological symptoms and will require admission to a hospital high dependency unit. Correction of sodium levels will be carefully controlled. 

Are there any side-effects to the treatment?

The majority of patients with hyponatraemia do not experience side-effects to treatment. However, a small minority may experience:

1. A rare adverse reaction in the brain causing reduced conscious level and a ‘locked-in’ syndrome. This is called osmotic demyelination syndrome (ODS), previously called central pontine myelinolysis (CPM).This occurs if hyponatraemia is corrected too rapidly.
2. Drug-specific side effects – The vaptans may cause a dry mouth and thirst and/or increased urinary frequency, and may also give rise to over-rapid correction of sodium, so require close monitoring of blood sodium levels.

If the patient has any concerns about these side-effects, they should speak to their doctor.

What are the longer-term implications of hyponatraemia?

The longer-term implications of hyponatraemia depend on the underlying cause of the condition and how severe the hyponatraemia is.

In cases of chronic or long-term hyponatraemia, neurological impairment such as confusion or altered attention can result. It can also affect how the patient walks, which can lead to trips and falls. Some studies suggest that chronic hyponatraemia may be linked to osteoporosis.