Hyponatremia
Hyponatremia or hyponatraemia is a low concentration of sodium in the blood.[4] It is generally defined as a sodium concentration of less than 135 mmol/L (135 mEq/L), with severe hyponatremia being below 120 mEq/L.[3][8] Symptoms can be absent, mild or severe.[2][9] Mild symptoms include a decreased ability to think, headaches, nausea, and poor balance.[1][3] Severe symptoms include confusion, seizures, and coma;[1][2][9] death can ensue.[10]
Hyponatremia
Hyponatraemia, low blood sodium, hyponatræmia
Low volume, normal volume, high volume[4]
Serum sodium < 135 mmol/L[3]
Ethanol intoxication or withdrawal, high protein levels, high blood fat levels, high blood sugar[5][6]
Based on underlying cause[4]
The causes of hyponatremia are typically classified by a person's body fluid status into low volume, normal volume, or high volume.[4] Low volume hyponatremia can occur from diarrhea, vomiting, diuretics, and sweating.[4] Normal volume hyponatremia is divided into cases with dilute urine and concentrated urine.[4] Cases in which the urine is dilute include adrenal insufficiency, hypothyroidism, and drinking too much water or too much beer.[4] Cases in which the urine is concentrated include syndrome of inappropriate antidiuretic hormone secretion (SIADH).[4] High volume hyponatremia can occur from heart failure, liver failure, and kidney failure.[4] Conditions that can lead to falsely low sodium measurements include high blood protein levels such as in multiple myeloma, high blood fat levels, and high blood sugar.[5][6]
Treatment is based on the underlying cause.[4] Correcting hyponatremia too quickly can lead to complications.[5] 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.[4][6] Low volume hyponatremia is typically treated with intravenous normal saline.[4] SIADH is typically treated by correcting the underlying cause and with fluid restriction while high volume hyponatremia is typically treated with both fluid restriction and a diet low in salt.[1][4] Correction should generally be gradual in those in whom the low levels have been present for more than two days.[4]
Hyponatremia is the most common type of electrolyte imbalance, and is often found in older adults.[11][12] It occurs in about 20% of those admitted to hospital and 10% of people during or after an endurance sporting event.[3][5] Among those in hospital, hyponatremia is associated with an increased risk of death.[5] The economic costs of hyponatremia are estimated at $2.6 billion per annum in the United States.[13]
Signs and symptoms[edit]
Signs and symptoms of hyponatremia include nausea and vomiting, headache, short-term memory loss, confusion, lethargy, fatigue, loss of appetite, irritability, muscle weakness, spasms or cramps, seizures, and decreased consciousness or coma.[1] Lower levels of plasma sodium are associated with more severe symptoms. However, mild hyponatremia (plasma sodium levels at 131–135 mmol/L) may be associated with complications and subtle symptoms[14] (for example, increased falls, altered posture and gait, reduced attention, impaired cognition, and possibly higher rates of death).[15][16]
Neurological symptoms typically occur with very low levels of plasma sodium (usually <115 mmol/L).[1] When sodium levels in the blood become very low, water enters the brain cells and causes them to swell (cerebral edema). This results in increased pressure in the skull and causes hyponatremic encephalopathy. As pressure increases in the skull, herniation of the brain can occur, which is a squeezing of the brain across the internal structures of the skull. This can lead to headache, nausea, vomiting, confusion, seizures, brain stem compression and respiratory arrest, and non-cardiogenic accumulation of fluid in the lungs.[17] This is usually fatal if not immediately treated.
Symptom severity depends on how fast and how severe the drop in blood sodium level is. A gradual drop, even to very low levels, may be tolerated well if it occurs over several days or weeks, because of neuronal adaptation. The presence of underlying neurological disease such as a seizure disorder or non-neurological metabolic abnormalities, also affects the severity of neurologic symptoms.
Hyponatremia, by interfering with bone metabolism, has been linked with a doubled risk of osteoporosis and an increased risk of bone fracture.[18]
Epidemiology[edit]
Hyponatremia is the most commonly seen water–electrolyte imbalance.[12] The disorder is more frequent in females, the elderly, and in people who are hospitalized. The number of cases of hyponatremia depends largely on the population. In hospital it affects about 15–20% of people; however, only 3–5% of people who are hospitalized have a sodium level less than 130 mmol/L. Hyponatremia has been reported in up to 30% of the elderly in nursing homes and is also present in approximately 30% of people who are depressed on selective serotonin reuptake inhibitors.[14]
People who have hyponatremia who require hospitalisation have a longer length of stay (with associated increased costs) and also have a higher likelihood of requiring readmission. This is particularly the case in men and in the elderly.[44]