Hyponatremia : Causes, Symptoms, Diagnosis & Treatment

Last Updated: 22/07/2022

Hyponatremia is a decrease in plasma sodium (Na) levels below 135 mmol/L (mEq/L). This condition has a wide range of causes - from excessive water intake with a salt-free diet, uncontrolled use of drugs to severe kidney disease and malignant tumors. Clinical symptoms are represented mainly by neuropsychiatric disorders. The diagnosis is made on the basis of determining the level of sodium and the osmolarity of blood serum, urine. Treatment should include control of the cause, administration of isotonic or hypertonic saline solutions, and maintenance of euvolemia.

Hyponatremia is the most common electrolyte disorder encountered in clinical practice. Sodium is a vital macronutrient that performs many functions in the human body (providing the resting and action potential, maintaining plasma osmotic pressure, acid-base balance). Decrease in Na concentration primarily adversely affects the functioning of neurons. The bulk of cases of this condition occur in patients in intensive care units (about 15-20%). There are no more precise statistical data on the incidence of hyponatremia.

Causes of hyponatremia

Sometimes hyponatremia develops for conditionally physiological reasons. For example, blood sodium levels decrease in people on a salt-free diet. Hyponatremia can occur with prolonged increased sweating (this is often observed in professional athletes, in people working outdoors in hot countries). The pathological causes of the condition are as follows:

  • Excess loss of sodium . Increased excretion of sodium from the body occurs in chronic diarrhea, profuse vomiting. So-called salt-losing nephropathies are considered to be a characteristic cause of hyponatremia, i.e. kidney diseases in which Na reabsorption in the nephron tubules is impaired (tubulointerstitial nephritis, polycystic kidney disease, congenital tubular dysfunction).
  • Fluid retention in the body . A common cause of hyponatremia are pathologies characterized by a violation of fluid excretion from the body (acute or chronic renal failure, CHF, cirrhosis of the liver with ascites. Electrolyte imbalance can occur with the syndrome of inappropriate secretion of antidiuretic hormone (SIADH), which develops against the background of various diseases (endocrine, pulmonary , oncology).
  • Pathological hemodilution (breeding) . An increase in the water content in the vascular bed can also cause hyponatremia. This occurs with excessive drinking of ordinary water (non-mineral) with diabetes or diabetes insipidus, psychogenic polydipsia. Parenteral administration of large amounts of low- or salt-free solutions as detoxification therapy is the cause of iatrogenic hyponatremia.
  • Endocrine Disorders . Deficiency of mineralocorticoid hormones, which is observed in primary and secondary adrenal insufficiency, a salt-losing form of congenital adrenal cortex dysfunction, disrupts the absorption of sodium ions in the renal tubules. The cause of hyponatremia can be hypothyroidism, severe hyperglycemia in decompensated diabetes mellitus.
  • Taking medications . The use of diuretics (especially thiazide and osmotic), such as hydrochlorothiazide, mannitol, to relieve emergency conditions in patients in the intensive care unit often causes a drop in sodium concentration. The condition can develop while taking medications such as hypoglycemic, psychotropic drugs.
  • Other reasons . Hyponatremia occurs in pancreatitis, peritonitis, massive burns. This condition is observed in some surgical operations, especially transurethral resection of the prostate (TUR syndrome).



Sodium is one of the most important cations that ensures the normal functioning of many cells, especially nerve and muscle cells. With a decrease in its content, the excitability of neurons and myocytes decreases as a result of a change in their membrane potential. Due to this, the formation and conduction of a wave of excitation in the nervous system is inhibited, the tone of skeletal muscles, blood vessels and myocardium decreases, which causes clinical symptoms.

Hyponatremia leads to plasma hypoosmolarity, water rushes along the concentration gradient from the intercellular space into the cells. As a result, there is swelling and swelling of the cells, which disrupts their normal functioning. The volume of circulating blood (volemia) can be different. This is determined by the cause that caused hyponatremia. With hypovolemia, the BCC decreases, the secretion of ADH increases compensatory, which further exacerbates the pathology.


There are several types of hyponatremia:

  • Pseudohyponatremia . It is caused by a decrease in the proportion of the water part of the blood due to the large amount of proteins and lipids.
  • Hypertensive hyponatremia . It develops as a result of the movement of water from cells into the interstitial space due to the presence of highly osmotic substances (glucose, mannitol) in the blood.
  • Hypotonic hyponatremia . Depending on the BCC is divided into:
  • Hypovolemic. It is characterized by a deficiency of Na, water, a decrease in BCC. It is observed with salt-losing nephropathies, mineralocorticoid deficiency, vomiting and diarrhea.
  • Isovolemic. This form occurs in SIADH, in which water retention and increased natriuresis occur.
  • Hypervolemic. In this form, the intravascular volume decreases due to the diffusion of fluid into various body cavities (abdominal, thoracic), which leads to an increase in the secretion of ADH and "dilution" of sodium. This is typical for CHF and cirrhosis of the liver.

According to the severity of hyponatremia is divided into:

  • Light - from 13 to 134 mmol / l.
  • Moderate - from 125 to 129 mmol / l.
  • Heavy - less than 125 mmol / l.

According to the rate of development, hyponatremia is:

  • Acute - lasting up to 48 hours.
  • Chronic - lasting more than 48 hours.

Symptoms of hyponatremia

Clinical symptoms depend on the rate of onset and severity of hyponatremia. With a slight and slowly developing decrease in the level of Na, there are no serious symptoms of CNS damage. There may be slight drowsiness, imbalance. In a severe degree, there is a pronounced doubtfulness, a soporous state. A person begins to react badly to external stimuli. Characterized by epileptiform seizures.

Due to a decrease in the excitability of myocytes, vascular tone and contractile function of the myocardium, muscle weakness, symptoms of arterial hypotension (increased heart rate, dizziness, loss of consciousness) appear. Skin, mucous membranes become dry, turgor and elasticity of the skin decrease. Sometimes there is a decrease in diuresis, symptoms from the gastrointestinal tract - loss of appetite, nausea.


This syndrome is characterized by a large number of complications. The vast majority of adverse effects are associated with damage to the central nervous system. These include coma, edema, and herniation of the brain. Pulmonary edema, infarcts of the hypothalamus and posterior pituitary gland are sometimes observed. Lethal outcome at the level of Na 125 meq/l occurs in 25%, at levels below 115 meq/l - in 50% of cases.

A dangerous complication of improper treatment of hyponatremia is osmotic demyelinating syndrome (pontine and extrapontine myelinolysis), which develops as a result of dehydration and shrinkage of brain cells due to a sharp increase in plasma osmolarity with too rapid administration of saline solutions. Symptoms include dysphagia, bulbar disturbances, tetraplegia. The mortality rate in this syndrome reaches more than 50%.


Almost all patients with hyponatremia, especially severe, should be under the joint supervision of a resuscitator and a specialized specialist (endocrinologist, nephrologist). To determine the cause of this syndrome, anamnestic data are important - previous diarrhea, vomiting, medication. To establish a specific type of pathology helps to identify signs indicating dehydration - dry skin, hypotension, decreased diuresis.

Information about the patient's comorbidities is also required. On examination, attention is drawn to symptoms such as swelling of the face, lower extremities, an increase and tension in the abdomen, dilation of the saphenous veins on the anterior abdominal wall. An additional examination is prescribed, aimed at establishing the type of hyponatremia and finding out its cause:

  • Laboratory Research . The osmolarity of blood serum and the level of other electrolytes (potassium, calcium, magnesium) are determined. In a biochemical blood test, the content of glucose, liver enzymes (ALT, AST), kidney function indicators (urea, creatinine) is measured. The level of thyroid hormones, adrenal glands (TSH, St. T4, cortisol) is being studied. The quantity, osmolarity, specific gravity of urine, the concentration of Na, glucose in it, the presence of ketone bodies are checked.
  • Instrumental Research . The most important clinical value is the measurement of central venous pressure (CVP). This is the most accurate way to determine the BCC, which allows you to specify the type of hyponatremia (hypovolemic, hypervolemic or euvolemic). If pulmonary edema is suspected, a chest X-ray is performed, if there are symptoms of cerebral edema, a CT scan of the brain is performed.

This condition must be differentiated from hypernatremia, since both of these pathologies have almost completely identical clinical symptoms. Cerebral edema in hyponatremia should be distinguished from cerebral edema of another etiology (hypertensive crisis, stroke, traumatic brain injury). It is much more important to differentiate the varieties of this syndrome (hyper-, hypo-, euvolemic, hyper- and hypotonic).


Treatment of hyponatremia

Most often, patients with hyponatremia are subject to hospitalization in the intensive care unit and intensive care. First you need to stop taking drugs that can cause hyponatremia, and stop the introduction of hypotonic solutions. You can additionally prescribe the intake of ordinary table salt inside. Sometimes with a mild degree of hyponatremia, this is enough. For moderate to severe, the following treatment is carried out:

  • Fluid restriction . This is the main condition for the treatment of the hypervolemic form, as well as SIADH. Fluid intake, both orally and in the form of solutions, should not exceed 100 ml / day.
  • The introduction of saline solutions . Infusion therapy with a 0.9% NaCl solution is necessary both to eliminate sodium deficiency and to maintain BCC in the hypovolemic form. In parallel, you need to make up for the deficiency of other electrolytes. If there are bright neurological symptoms, hypertensive (3%) NaCl is administered. For the prevention of osmotic demyelinating syndrome, monitoring of the rate of increase in the serum sodium level is mandatory, it should be less than 0.8 mmol / day.
  • Diuretic drugs . They are used to remove excess fluid in hypervolemic form. For this purpose, loop diuretics (furosemide) are used. Thiazide diuretics are strictly contraindicated as they exacerbate hyponatremia.
  • Blockade of ADH . Since increased secretion of ADH (vasopressin) often occurs in hyponatremia, measures to suppress its effect are important. Demeclocycline, ADH receptor antagonists (konivaptan, tolvaptan) have an inhibitory effect on ADH. However, these drugs should be avoided in patients with kidney disease.

Since hyponatremia itself is a very dangerous condition that can lead to death in a short time, the sodium level is corrected first. Therefore, only after the symptoms of threatening cerebral edema have been eliminated, they begin to treat the disease that caused hyponatremia:

  • CHF : ACE inhibitors, beta-blockers, loop diuretics.
  • Liver cirrhosis : albumin administration, fresh frozen plasma transfusion, alcohol withdrawal.
  • Endocrine disorders : hormone replacement therapy with fludrocortisone, hydrocortisone (for adrenal insufficiency), levothyroxine (for hypothyroidism).
  • CRF : hemodialysis.

Forecast and prevention

Hyponatremia is a dangerous life-threatening condition with a fairly high mortality rate (from 25 to 50% of patients die in various forms of this pathology). According to some authors, the death rate reaches 65%. The cause of death is mainly cerebral edema, coma. However, with timely competent treatment, serious complications do not occur.

An unfavorable outcome may be such neurological disorders as dysphagia, dysarthria and tetraplegia due to generalized demyelination of nerve fibers due to inadequately rapid correction of serum Na. Prevention of this condition is reduced to the timely treatment of those diseases that can potentially cause hyponatremia, regular monitoring of plasma sodium levels.

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