Hyponatremia is a low sodium concentration in the blood. 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. Symptoms can be absent, mild or severe. Mild symptoms include a decreased ability to think, headaches, nausea, and poor balance. Severe symptoms include confusion, seizures, and coma.

The causes of hyponatremia are typically classified by a person's body fluid status into a low volume, normal volume, or high volume. Low volume hyponatremia can occur from diarrhoea, 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 blood 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 should generally be gradual in those in whom the low levels have been present for more than two days.

Hyponatremia is the most common type of electrolyte imbalance. 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.

Signs and symptoms

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. 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 (for example, increased falls, altered posture and gait, reduced attention, impaired cognition, and possibly higher rates of death).

Neurological symptoms typically occur with very low levels of plasma sodium (usually <115 mmol/L). When sodium levels in the blood become very low, water enters the brain cells and causes them to swell (cerebral oedema). 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. This is usually fatal if not immediately treated.

Symptom severity depends on how fast and how severe the drop in blood sodium level. 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 diseases such as a seizure disorder or non-neurological metabolic abnormalities also affects the severity of neurologic symptoms.

Chronic hyponatremia can lead to such complications as neurological impairments. These neurological impairments most often affect gait (walking) and attention and can lead to increased reaction time and falls. Hyponatremia, by interfering with bone metabolism, has been linked with a doubled risk of osteoporosis and an increased risk of bone fracture.

Causes

The specific causes of hyponatremia are generally divided into those with low tonicity (lower than normal concentration of solutes), without low tonicity, and falsely low sodiums. Those with low tonicity are then grouped by whether the person has high fluid volume, normal fluid volume, or low fluid volume. Too little sodium in the diet alone is very rarely the cause of hyponatremia.

High volume

Both sodium and water content increase: Increase in sodium content leads to hypervolemia and water content to hyponatremia.

        ·         cirrhosis of the liver;

        ·         congestive heart failure;

        ·         nephrotic syndrome in the kidneys;

        ·         Excessive drinking of fluids;

Normal volume

There is volume expansion in the body, no oedema, but hyponatremia occurs

        ·         SIADH (and its many causes);

        ·         Hypothyroidism;

        ·         Not enough ACTH;

        ·         Beer potomania;

        ·         Normal physiologic change of pregnancy;

Low volume

Hypovolemia (extracellular volume loss) is due to total body sodium loss. Hyponatremia is caused by a relatively smaller loss in total body water.

        ·         any cause of hypovolemia such as prolonged vomiting, decreased oral intake, severe diarrhoea diuretic use (due to the diuretic causing a volume-depleted state and thence ADH release, and not a direct result of diuretic-induced urine sodium loss).

        ·         Addison's disease and congenital adrenal hyperplasia in which the adrenal glands do not produce enough steroid hormones (combined glucocorticoid and mineralocorticoid deficiency)

        ·         pancreatitis.

        ·         Prolonged exercise and sweating, combined with drinking water without electrolytes is the cause of exercise-associated hyponatremia (EAH). It is common in marathon runners and participants of other endurance events.

        ·         The use of MDMA (ecstasy) can result in hyponatremia.

Other causes

Miscellaneous causes that are not included under the above classification scheme include the following:

        ·         False hyponatremia (due to massive increases in blood triglyceride levels or extreme elevation of immunoglobulins as may occur in multiple myeloma).

        ·         Hyponatremia with elevated tonicity can occur with high blood sugar.

Hypernatremia also spelt hypernatraemia, is a high concentration of sodium in the blood. Early symptoms may include a strong feeling of thirst, weakness, nausea, and loss of appetite. Severe symptoms include confusion, muscle twitching, and bleeding in or around the brain. Normal serum sodium levels are 135–145 mmol/L (135–145 mEq/L). Hypernatremia is generally defined as a serum sodium level of more than 145 mmol/L. Severe symptoms typically only occur when levels are above 160 mmol/L.

Hypernatremia is typically classified by a person's fluid status into a low volume, normal volume, and high volume. Low volume hypernatremia can occur from sweating, vomiting, diarrhoea, diuretic medication, or kidney disease. Normal volume hypernatremia can be due to fever, inappropriately decreased thirst, prolonged increased breath rate, diabetes insipidus, and from lithium among other causes. High volume hypernatremia can be due to hyperaldosteronism, excessive administration of intravenous 3% normal saline or sodium bicarbonate, or rarely from eating too much salt. Low blood protein levels can result in a falsely high sodium measurement. The cause can usually be determined by the history of events. Testing the urine can help if the cause is unclear. The underlying mechanism typically involves too little free water in the body.