Hyponatremia, also spelled hyponatraemia, is a low sodium level in the blood. Symptoms can vary from none to severe. Mild symptoms include a decreased ability to think, headaches, nausea, and poor balance. Severe symptoms include confusion, seizures, and coma. Normal serum sodium levels are 135–145 mmol/L (135–145 mEq/L). Hyponatremia is generally defined as a serum sodium level of less than 135 mmol/L and is considered severe when the level is below 120 mmol/L.
The cause of hyponatremia is typically classified by a person's fluid status into low volume, normal volume, and high volume. Low volume hyponatremia can occur from diarrhea, vomiting, diuretics, and sweating. Normal volume hyponatremia is divided into cases with dilute urine and concentrated urine. Cases in which the urine is dilute include adrenal insufficiency, hypothyroidism, and drinking too much water or too much beer. Cases in which the urine is concentrated include syndrome of inappropriate antidiuretic hormone secretion (SIADH). High volume hyponatremia can occur from heart failure, liver failure, and kidney failure. Conditions that can lead to falsely low sodium measurements include high protein levels such as in multiple myeloma, high blood fat levels, and high blood sugar.
Treatment is based on the underlying cause. Correcting hyponatremia too quickly can lead to complications. Rapid partial correction with 3% normal saline is only recommended in those with significant symptoms and occasionally those in whom the condition was of rapid onset. Low volume hyponatremia is typically treated with intravenous normal saline. SIADH is typically treated with fluid restriction while high volume hyponatremia is typically treated with both fluid restriction and a diet low in salt. Correction, in those in whom the low levels have been present for more than two days should generally be gradual.
Hyponatremia is one of the most commonly seen water–electrolyte imbalances. It occurs in about 20% of those admitted to hospital and 10% of people during or after an endurance sporting event. Among those in hospital hyponatremia is associated with an increased risk of death.