What causes hyponatraemia?
Hyponatraemia is very common - especially mild hyponatraemia. It occurs equally in men and women. Some patients are more at risk of hyponatraemia - such as:
- Children and the elderly - less likely to express thirst.
- Infants with diarrhoea given tap water - they need electrolyte replacement fluid (for example, Dioralyte®).
- Infants who are on watered-down milk formula who then become unwell.
Levels of sodium in the blood are tightly controlled. This involves a number of factors including:
- Kidneys - which can choose to remove or retain sodium in the blood.
- Antidiuretic hormone (also known as vasopressin) - a hormone which works to retain body fluid.
- Renin angiotensin and aldosterone system - regulates sodium excretion by the kidneys and levels of fluid in the body, and maintains blood pressure.
Sodium levels are measured in the blood and an example of the normal range of blood sodium is 136-142 mmol/L. Mild hyponatraemia occurs when the level is less than 136 mmol/L. Severe hyponatraemia is a level less than 120 mmol/L.
Hyponatraemia usually occurs with changes in the body fluid levels and can be divided into three groups:
- Euvolaemic - body fluid levels are normal.
- Hypovolaemia - reduced body fluid levels, the most common cause of which is dehydration.
- Hypervolaemic - increased body fluid levels, an example of which is heart failure leading to fluid retention.
The causes of hyponatraemia can be classified in various ways but most commonly are based on the body fluid levels. The levels of sodium in the urine are used to help determine the underlying cause.
Low body fluid (hypovolaemia)
- Being sick (vomiting).
- Addison's disease: adrenal glands fail to produce the necessary steroid hormones.
- Excessive sweating.
- Medications such as diuretics - for example, thiazide diuretics such as bendroflumethiazide or loop diuretics.
Increase in body fluid (hypervolaemia)
- Cardiac failure.
- Cirrhosis - liver fibrosis.
- Kidney failure.
No abnormalities of body fluid (euvolaemia) - resulting in excess antidiuretic hormone
- Neurological: brain tumour, brain trauma, multiple sclerosis, systemic lupus erythematosus, brain haemorrhage.
- Lung disorders: lung cancer, pneumonia, asthma, tuberculosis.
- Other cancers: stomach cancer, pancreatic cancer, leukaemia and lymphoma.
- Medications: selective serotonin reuptake inhibitors (SSRIs), proton pump inhibitors, angiotensin-converting enzyme (ACE) inhibitors.
- Other causes: pain, following surgery, stress, endurance exercise and marathon running.
Falsely low sodium levels
- Very high cholesterol levels.
- Very high blood sugar levels.
In these cases the result for the blood sodium will be low but that is due to interference from the high cholesterol or sugar levels. The actual blood sodium level is normal. Once the high cholesterol or blood sugar levels are corrected, the sodium will also become normal.
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