Importance  Hyponatremia is the most common electrolyte disorder and it affects approximately 5% of adults and 35% of hospitalized patients. Hyponatremia is defined by a serum sodium level of less than 135 mEq/L and most commonly results from water retention. Even mild hyponatremia is associated with increased hospital stay and mortality.

Observations  Symptoms and signs of hyponatremia range from mild and nonspecific (such as weakness or nausea) to severe and life-threatening (such as seizures or coma). Symptom severity depends on the rapidity of development, duration, and severity of hyponatremia. Mild chronic hyponatremia is associated with cognitive impairment, gait disturbances, and increased rates of falls and fractures. In a prospective study, patients with hyponatremia more frequently reported a history of falling compared with people with normal serum sodium levels (23.8% vs 16.4%, respectively; P < .01) and had a higher rate of new fractures over a mean follow-up of 7.4 years (23.3% vs 17.3%; P < .004). Hyponatremia is a secondary cause of osteoporosis. When evaluating patients, clinicians should categorize them according to their fluid volume status (hypovolemic hyponatremia, euvolemic hyponatremia, or hypervolemic hyponatremia). For most patients, the approach to managing hyponatremia should consist of treating the underlying cause. Urea and vaptans can be effective treatments for the syndrome of inappropriate antidiuresis and hyponatremia in patients with heart failure, but have adverse effects (eg, poor palatability and gastric intolerance with urea; and overly rapid correction of hyponatremia and increased thirst with vaptans). Severely symptomatic hyponatremia (with signs of somnolence, obtundation, coma, seizures, or cardiorespiratory distress) is a medical emergency. US and European guidelines recommend treating severely symptomatic hyponatremia with bolus hypertonic saline to reverse hyponatremic encephalopathy by increasing the serum sodium level by 4 mEq/L to 6 mEq/L within 1 to 2 hours but by no more than 10 mEq/L (correction limit) within the first 24 hours. This treatment approach exceeds the correction limit in about 4.5% to 28% of people. Overly rapid correction of chronic hyponatremia may cause osmotic demyelination, a rare but severe neurological condition, which can result in parkinsonism, quadriparesis, or even death.

Conclusions and Relevance  Hyponatremia affects approximately 5% of adults and 35% of patients who are hospitalized. Most patients should be managed by treating their underlying disease and according to whether they have hypovolemic, euvolemic, or hypervolemic hyponatremia. Urea and vaptans can be effective in managing the syndrome of inappropriate antidiuresis and hyponatremia in patients with heart failure; hypertonic saline is reserved for patients with severely symptomatic hyponatremia.