Hyponatremia is an important
electrolyte abnormality with the potential for significant morbidity and mortality. Common causes include medications and the syndrome of inappropriate
antidiuretic hormone (
SIADH) secretion.
Hyponatremia can be classified according to the volume status of the patient as
hypovolemic, hypervolemic, or euvolemic. Hypervolemic
hyponatremia may be caused by
congestive heart failure,
liver cirrhosis, and renal disease. Differentiating between euvolemia and
hypovolemia can be clinically difficult, but a useful investigative aid is measurement of plasma osmolality.
Hyponatremia with a high plasma osmolality is caused by
hyperglycemia, while a normal plasma osmolality indicates pseudohyponatremia or the post-transurethral prostatic resection syndrome. The urinary
sodium concentration helps in diagnosing patients with low plasma osmolality. High urinary
sodium concentration in the presence of low plasma osmolality can be caused by renal disorders, endocrine deficiencies, reset osmostat syndrome,
SIADH, and medications. Low urinary
sodium concentration is caused by severe
burns, gastrointestinal losses, and acute water overload. Management includes instituting immediate treatment in patients with acute severe
hyponatremia because of the risk of
cerebral edema and hyponatremic
encephalopathy. In patients with chronic
hyponatremia, fluid restriction is the mainstay of treatment, with
demeclocycline therapy reserved for use in persistent cases. Rapid correction should be avoided to reduce the risk of
central pontine myelinolysis.
Loop diuretics are useful in managing edematous hyponatremic states and chronic
SIADH. In all instances, identifying the cause of
hyponatremia remains an integral part of the treatment plan.