Hyponatraemia
Hyponatraemia is defined as a serum sodium concentration below 135 mEq/L.
It is the most common electrolyte disorder in clinical practice. Around 20–30% of hospitalised patients have some alteration in their plasma sodium levels, which may be associated with increased morbidity and mortality, clinical deterioration and prolonged hospital stays.
It is classified according to different parameters, such as natraemia (mild–moderate–severe), the speed of onset (acute or chronic), the severity of symptoms, serum osmolality and the patient’s volemic status.
Clinically, the main issue is cellular oedema, caused by the movement of water from the extracellular space to the intracellular space in an attempt to increase the sodium concentration in the extracellular fluid. At the cerebral level, this may lead to symptoms such as headache, nausea, vomiting, seizures, delirium, stupor and even cerebral herniation due to increased intracranial pressure, as well as coma.
The speed at which hyponatraemia is corrected through treatment is related to neurological complications, such as central pontine myelinolysis or osmotic demyelination syndrome, an extremely serious condition with severe residual lesions that occurs when hyponatraemia is corrected too rapidly.
(12 mEq/L in the first 24 hours)
The most frequent cause of euvolaemic hyponatraemia is the Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH), a condition characterised by abnormally high concentrations of antidiuretic hormone (ADH).
The aetiology of syndrome of inappropriate antidiuretic hormone secretion of ADH mainly includes neoplasms, such as small cell lung cancer, pulmonary diseases, or almost any condition that affects the central nervous system.
In a situation of normal sodium and water intake, this excess of ADH causes most of the water to be reabsorbed in the collecting tubule into the interstitium, resulting in highly concentrated urine.
For the management of hyponatraemia, international guidelines recommend an individualised approach according to the level and subtype of hyponatraemia. For hyponatraemia caused by SIADH, international guidelines recommend fluid restriction as the first-line treatment, and urea or hypertonic saline solution combined with diuretics as second-line treatment.