Facts about Hyponatremia : Definition

This is a condition characterized by lower-than-normal levels of sodium in the blood. The normal serum sodium concentration in the body is between 135 and 145mEq/L.

If the sodium serum levels go below 135mEq/L, you are considered to be suffering from hyponatremia. The condition is said to be severe if the serum levels go below 125mEq/L. The body uses sodium as an electrolyte to regulate water in and around the cells of the tissues. [1, 2, 3,5,8,9, 10, 13,]


  • The sodium concentration in the serum is regulated by secretion of the ADH (antidiuretic hormone), variations of the renal handling of filtered sodium, and the renin-angiotensin-aldosterone system.
  • If the serum osmolality increases over the normal range of 280-300mOsm/kg, there is the stimulation of hypothalamic osmoreceptors that cause an increase in thirst and circulate the ADH.
  • The ADH increase the reabsorption of the water from urine that results in low volume urine with high osmolality but returns the serum osmolality to normal.
  • Aldosterone is also released to curb hypovolemia through renin-angiotensin-aldosterone feedback system. The hormone causes the sodium absorption in the kidney’s renal tubule.
  • The sodium retention causes the retention of water that in turn corrects the hypovolemic problem. The kidney is able to balance the sodium-independent of these two hormones.
  • In a hypovolemic, state such as dehydration or hemorrhage increases the absorption of sodium in the proximal tubule of the kidney.
  • When the vascular volume is increased, tubular sodium reabsorption is suppressed and helps to restore the normal vascular volume. Thus, the sodium balance disorders can be traced to aldosterone, renal sodium transport, ADH, or disturbance water or thirst acquisition.
  • Hyponatremia is said to be physiologically significant when there is the indication of extracellular hyposmolality and the tendency of free water shifting from vascular space to the intracellular space.
  • The body tolerates cellular edema to a greater extent by not at the bony calvarium. Thus, hyponatremia clinical manifestations are primarily on cerebral edema. The rate at which the condition develops is critical to its treatment. [4,6,10, 11]

Hypovolemic Vs Hypervolemic hyponatremia

A condition is considered hypovolemic hyponatremia if there is a decrease in total body water with the decrease in the total amounts of the body sodium.

Hypervolemic hyponatremia occurs when there is an increase in total body sodium along with the increase in total body water.
If the normal body sodium levels do not change with the increase in total body water, the condition is said to be euvolemic hyponatremia. [1, 2, 3,5,8,9]

There are cases where water shifts from the intercellular to extracellularly environment and results in dilution of sodium. The total body water is not changed in this case.

This condition occurs if one is suffering from hyperglycemia and is referred to as redistributive hyponatremia. [1, 2, 3, 5, 8, 9]

Acute hyponatremia and chronic hyponatremia

Acute hyponatremia is the case where sodium levels fall rapidly in less than 48 hours.

The condition is more dangerous than hyponatremia that occurs over several day or weeks, commonly known as chronic hyponatremia. In the latter, the brain cells adjust to the condition, and there is minimal swelling. [1, 2, 3, 5, 8, 9]

Signs and symptoms of hyponatremia

  • Vomiting
  • Short memory loss
  • Lethargy
  • Fatigue
  • Irritability
  • Loss of appetite
  • Nausea
  • Confusion and muscle weakness.
  • Some patients also complain muscle cramps, seizures, and decreased consciousness.
  • In severe cases, one may fall into a coma.

Neurological symptoms only occur when sodium levels get very low at quantities below 115mEq/L. At this level of sodium, water enters the brain causing the brain to swell.

The condition later causes pressure in the skull, a condition called hyponatremic encephalopathy. If the condition is not checked at the point, there is squeezing of the brain across the structures of the skull. The physical symptoms of this condition are:

  • Confusion
  • Respiratory arrest
  • Non-cardiogenic fluid accumulation in lungs
  • Brain stem compression.

The condition is fatal if not treated at once.

The severity of the symptoms is dependent on the severity of the sodium drop and show fast the condition happens. The body may tolerate gradual drop even to levels that are very low. As the body has a neurotic adaptation capability. However, the presence of neurological diseases, seizure disorders, and other non-neurological metabolic abnormalities influence the severity of the condition. [1, 2, 3,5,8,9, 13]

Causes of hyponatremia

Sodium in the body fluid is used to maintain electrolyte balance, blood pressure and for the working of muscles and nerves. If the level of sodium in the fluids outside the cells goes down, the fluids enter the cells causing the cells to swell. Here are the main causes of hyponatremia:

  • Diarrhea
  • The intake of diuretic medicines that increase the urine output
  • Burns that cover large areas of the body
  • Heart failure
  • Vomiting
  • Sweating
  • Vomiting
  • Heart failure
  • Kidney diseases
  • Liver cirrhosis
  • The Syndrome of Inappropriate antidiuretic hormone secretion (SIADH)
  • Hypothyroidism, a condition where there is underperformance of the pituitary glands
  • Deficiency of glucocorticoid, a steroid
  • Congenital adrenal hyperplasia that makes the adrenal glands unable to produce enough steroid


Exercise-associated hyponatremia from prolonged period of exercise while taking water alone
Certain medications such as Lasix for treating blood pressure and antidiuretics [1, 2, 3,5,8,9, 13]

A clinical diagnosis chart used to determine the cause of hyponatremia image

Workup : A clinical diagnosis chart used to determine the cause of hyponatremia

Legionnaire’s disease workup

Cases of pneumonia are caused by several pathogens that share similar laboratory findings. Hyponatremia that is secondary to SIADH is common in Legionnaires’ disease that that which is caused by pathogens. However, the condition is not specific to Legionnaires’ disease. [4, 5]


Hyponatremia is also classified according to effective osmolality. It can be said to be:

  • Hypotonic hyponatremia
  • Hypertonic hyponatremia
  • Isotonic hyponatremia.

During the diagnosis, the patients undergo three tests that when combined with physical examination and history, the doctor is able to establish the etiological mechanism as urinary sodium concentration, urine osmolality, serum osmolality. [1, 2, 5, 6, 9, 13]

Urine osmolality

This osmolality test is used to differentiate primary polydipsia from free-water excretion. Osmolality that is greater than 100mOsm/kg shows that the kidneys are unable to dilute the urine. . [1,2,5,6,9,13]

Serum osmolality

Serum osmolality is used for differentiating between pseudo hyponatremias and true hyponatremia. The earlier is secondary to hyperproteinemia or hyperlipidemia or could be hypertonic hyponatremia that is linked to elevated mannitol, glucose, and glycine maltose or sucrose. Symptoms do not appear until the plasma levels drop below 120 mmol per L . [1,2,5,6,9]

The urinary sodium concentration

This test is used to differentiate between the syndrome of inappropriate antidiuretic hormone secretion (SIADH) and hyponatremia that is secondary to hypovolemia. The condition is considered SIADH hyponatremia when the urine sodium is greater than 20-40mEq/L. The typical measurements of urine sodium for hypovolemia patients are usually less than 25mEq/L. However, an SIADH patient taking low sodium will have the values falling below 25mEq/L. . [1,2,5,6,9,11,13]

Hyponatremia treatment

  • The treatment of the hyponatremia condition is dependent on the underlying cause. Such a condition should be treated first to correct the condition.
  • Treatment starts with the examination of the condition to determine if one has euvolemic, hypervolemic or hypovolemic condition
  • If the patient is suffering from hypovolemia, the condition is corrected by an intravenous administration of normal salt in a saline solution.
  • Euvolemic hyponatremia is treated by restriction of fluid and abolishment of the stimuli that causes the secretion of the antidiuretic hormone such as nausea.
  • Any drug that the patient is taking that could be causing SADH is also discontinued.
  • Hypervolemic hyponatremia is treated by treating the disease that could be causing the condition. In most cases, the cause is usually liver or heart failure. If this is not resolved, the patient receives the same treatment as that of the euvolemic hypervolemic hyponatremia condition.

There is a risk of the patient developing severe neurological disorder called Central Pontine Myolysis that breaks down the sheaths covering parts of the nerve cells if hyponatremia is corrected rapidly. As a precautionary measure the salt level in blood, or called sodium serum, should not rise beyond 0.33mmol/l/h during the application of the saline solution. [1,2,4,5,6,8,9,13]

Hyponatremia correction calculator

Hyponatremia calculator is used for calculating the amount and the intensity of the saline solution that is needed to correct the serum hyponatremia. The formula for calculation of the infusateRate is;

Infusate Rate = (1000 * Serum Na Change Per Hr * ((Water Fract * Weight) + 1)) / (IVNa + IVK – SerumNa)
Serum Na Change Per Liter = (IVNa + IVK – SerumNa) / ((Water Fract * Weight) + 1) [12]

ICD-9-CM Diagnosis Code 276.1

ICD-9-CM Diagnosis Code 276.1 is a billable medical code used to indicate that hyposmolality and hyponatremia were diagnosed for reimbursements. However, the code can only be used for claims for services rendered before October 1, 2015. For claims after the date, the code ICD-10-CM code is used. [7]


  1. Merck Manuals Professional Edition. Hyponatremia – Endocrine and Metabolic Disorders [Internet]. 2015 [cited 22 December 2015]. Available from: http://www.merckmanuals.com/professional/endocrine-and-metabolic-disorders/electrolyte-disorders/hyponatremia#sec12-ch156-ch156d-714
  2. Melissa Conrad Stöppler M. Hyponatremia: Get the Facts on Symptoms and Treatment [Internet]. MedicineNet. 2015 [cited 24 December 2015]. Available from: http://www.medicinenet.com/hyponatremia/article.htm
  3. Goh K. Management of Hyponatremia – American Family Physician [Internet]. Aafp.org. 2015 [cited 24 December 2015]. Available from: http://www.aafp.org/afp/2004/0515/p2387.html
  4. Emedicine.medscape.com. Legionnaires Disease Workup: Approach Considerations, Histologic Findings, Laboratory Studies [Internet]. 2015 [cited 24 December 2015]. Available from: http://emedicine.medscape.com/article/220163-workup
  5. Updated by: David C. Dugdale a. Hyponatremia: MedlinePlus Medical Encyclopedia [Internet]. Nlm.nih.gov. 2015 [cited 24 December 2015]. Available from: https://www.nlm.nih.gov/medlineplus/ency/article/000394.htm
  6. Haralampos J. Milionis M. The hyponatremic patient: a systematic approach to laboratory diagnosis. CMAJ: Canadian Medical Association Journal [Internet]. 2002 [cited 24 December 2015];166(8):1056. Available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC100882/
  7. Icd9data.com. 2012 ICD-9-CM Diagnosis Code 276.1 : Hyposmolality and/or hyponatremia [Internet]. 2015 [cited 24 December 2015]. Available from: http://www.icd9data.com/2012/Volume1/240-279/270-279/276/276.1.htm
  8. Mayoclinic.org. Hyponatremia Symptoms – Mayo Clinic [Internet]. 2015 [cited 24 December 2015]. Available from: http://www.mayoclinic.org/diseases-conditions/hyponatremia/basics/symptoms/con-20031445
  9. Healthline. Low Blood Sodium (Hyponatremia) [Internet]. 2015 [cited 24 December 2015]. Available from: http://www.healthline.com/health/hyponatremia
  10. Moritz M, Ayus J. The pathophysiology and treatment of hyponatraemic encephalopathy: an update. Nephrology Dialysis Transplantation. 2003;18(12):2486-2491.
  11. Encyclopedia Britannica. syndrome of inappropriate antidiuretic hormone (SIADH) | pathology [Internet]. 2014 [cited 24 December 2015]. Available from: http://www.britannica.com/science/syndrome-of-inappropriate-antidiuretic-hormone
  12. Reference.medscape.com. Hyponatremia Correction Infusate Rate [Internet]. 2015 [cited 24 December 2015]. Available from: http://reference.medscape.com/calculator/hyponatremia-correction-infusate-rate
  13. Updated by: David C. Dugdale a. Hyponatremia: MedlinePlus Medical Encyclopedia [Internet]. Nlm.nih.gov. 2015 [cited 24 December 2015]. Available from: https://www.nlm.nih.gov/medlineplus/ency/article/000394.htm
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Published on by under Diseases and Conditions.
Article was last reviewed on September 26th, 2017.

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