Na
Sodium (Na⁺) is the main ion outside cells and the main determinant of blood osmolality. The serum sodium level mostly reflects the balance of water, not the total amount of salt in the body: too much water relative to sodium lowers it (hyponatraemia), too little water raises it (hypernatraemia). Because water moves into or out of brain cells when sodium changes, both disorders mainly affect the brain.
Low sodium — hyponatraemia
Hyponatraemia is the most common electrolyte disorder in clinical practice. Causes are grouped by the person's fluid status:
| Fluid status | Typical causes |
|---|---|
| Low (dehydration) | Vomiting, diarrhoea, diuretics (especially thiazides), adrenal insufficiency |
| Normal | Syndrome of inappropriate antidiuresis (SIAD) due to lung disease, brain disorders, cancer or medicines (antidepressants, antiepileptics, antipsychotics); hypothyroidism; drinking very large volumes of water or beer with little food |
| High (oedema) | Heart failure, liver cirrhosis, nephrotic syndrome, advanced kidney disease |
Mild chronic hyponatraemia is associated with falls, attention problems and fractures in older people. Acute or severe hyponatraemia causes headache, nausea, vomiting, confusion, seizures and coma. Correction must be gradual: raising sodium too fast can cause osmotic demyelination, a severe brain injury.
High sodium — hypernatraemia
Hypernatraemia almost always means water deficit: not enough drinking (older people with reduced thirst or limited access to water, infants, unconscious patients), water losses through fever, sweating, diarrhoea or large urine volumes (diabetes insipidus, osmotic diuresis with high glucose), or rarely salt overload. Symptoms are thirst, lethargy, irritability, confusion, muscle twitching, seizures and coma. Hypernatraemia in hospitalised patients is associated with increased mortality.
if sodium is below 125 or above 155 mmol/L, or if an abnormal level comes with confusion, severe headache, vomiting, drowsiness or seizures.
Mild abnormalities need repeat testing, a medication review and assessment of fluid intake, kidney, thyroid and adrenal function. Do not drastically restrict or increase water or salt without medical advice.
References
- Spasovski G, Vanholder R, Allolio B, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Eur J Endocrinol. 2014;170(3):G1–G47. doi:10.1530/EJE-13-1020
- Adrogué HJ, Madias NE. Hypernatremia. N Engl J Med. 2000;342(20):1493–1499. doi:10.1056/NEJM200005183422006
This article is for general information and does not replace a consultation with a doctor. Laboratory results must be interpreted by a physician together with symptoms and other tests.
Published on 6 July 2019