Hyponatremia
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Synonyms and keywords: hyponatraemia
Overview
The electrolyte disturbance hyponatremia exists in humans when the sodium (Natrium in Latin) concentration in the plasma falls below 130 mmol/L. At lower levels water intoxication may result, an urgently dangerous condition. Hyponatremia is an abnormality that can occur in isolation or, as most often is the case, as a complication of other medical illnesses.
Classification
The etiology of hyponatremia can be categorized pathophysiologically in three primary ways, based on the patient's plasma osmolality.
- Hypertonic hyponatremia, caused by resorption of water drawn by osmols such as glucose (hyperglycemia or diabetes) or mannitol (hypertonic infusion).
- Isotonic hyponatremia, more commonly called "pseudohyponatremia," is caused by lab error due to hypertriglyceridemia (most common) or hyperparaproteinemia.
- Hypotonic hyponatremia is by far the most common type, and is often used interchangeably with "hyponatremia." Hypotonic hyponatremia is categorized in 3 ways based on the patient's blood volume status. Each category represents a different underlying reason for the increase in ADH that led to the water retention and thence hyponatremia:
- Hypervolemic hyponatremia, wherein there is decreased effective circulating volume even though total body volume is increased (by the presence of edema). The decreased effective circulating volume stimulates the release of ADH, which in turn leads to water retention. Hypervolemic hyponatremia is most commonly the result of congestive heart failure, liver failure (cirrhosis), or kidney disease (nephrotic syndrome).
- Euvolemic hyponatremia, wherein the increase in ADH is secondary to either physiologic but excessive ADH release (as occurs with nausea or severe pain) or inappropriate and non-physiologic secretion of ADH, i.e. syndrome of inappropriate antidiuretic hormone hypersecretion (SIADH). Often categorized under euvolemic is hyponatremia due to inadequate urine solute as occurs in beer potomania or "tea and toast" hyponatremia, hyponatremia due to hypothyroidism or adrenal insufficiency, and those rare instances of hyponatremia that are truly secondary to excess water intake (i.e., extreme psychogenic polydipsia)
- Hypovolemic hyponatremia, wherein ADH secretion is stimulated by volume depletion.
The volemic classification fails to include spurious and/or artifactual hyponatremia, which is addressed in the osmolar classification. This includes hyponatremia that occurs in the presence of massive hypertriglyceridemia, severe hyperglycemia, and extreme elevation of immunoglobulin levels.
In chronic hyponatremia, sodium levels drop gradually over several days or weeks and symptoms and complications are typically moderate. Chronic hyponatremia is often called asymptomatic hyponatremia in clinical settings because it is thought to have no symptoms; however, emerging data suggests that "asymptomatic" hyponatremia is not actually asymptomatic.[1]
In acute hyponatremia sodium levels drop rapidly, resulting in potentially dangerous effects, such as rapid brain swelling, which can result in coma and death.
Causes
An abnormally low plasma sodium level is best considered in conjunction with the person's plasma osmolarity and extracellular fluid volume status.
Most cases of hyponatremia are associated with reduced plasma osmolarity. In fact, the vast majority of adult cases are due to increased vasopressin, i.e., anti-diuretic hormone (ADH). Vasopressin is a hormone that causes retention of water, but not salt. Hence, the patient with hyponatremia can be viewed as the patient with increased ADH activity. It is the physician's task to identify the cause of the increased ADH activity in each case.
In patients who are volume depleted, i.e., their blood volume is too low, ADH secretion is increased, since volume depletion is a potent stimulus for ADH secretion. As a result, the kidneys of such patients hold on to water and produce a very concentrated urine. Treatment is simple (if not without risk) — simply restore the patient's blood volume, thereby turning off the stimulus for ongoing ADH release and water retention.
Some patients with hyponatremia have normal blood volume. In those patients, the increased ADH activity and subsequent water retention may be due to "physiologic" causes of ADH release such as pain or nausea. Alternatively, they may have the Syndrome of Inappropriate ADH (SIADH). SIADH represents the sustained, non-physiologic release of ADH and most often occurs as a side effect of certain medicines, lung problems such as pneumonia or abscess, brain disease, or certain cancers (most often small cell lung carcinoma).
A third group of patients with hyponatremia are often said to be "hypervolemic". They are identified by the presence of peripheral edema. In fact, the term "hypervolemic" is misleading since their blood volume is actually low. The edema underscores the fact that fluid has left the circulation, i.e., the edema represents fluid that has exited the circulation and settled in dependent areas. Since such patients do, in fact, have reduced blood volume, and since reduced blood volume is a potent stimulus for ADH release, it is easy to see why they have retained water and become hyponatremic. Treatment of these patients involves treating the underlying disease that caused the fluid to leak out of the circulation in the first place. In many cases, this is easier said than done when one recognizes that the responsible underlying conditions are diseases such as liver cirrhosis or heart failure — conditions that are notoriously difficult to manage, let alone cure.
Hyponatremia can result from dysfunctions of the mineralocorticoid aldosterone (i.e. hypoaldosteronism) due to adrenal insufficiency, congenital adrenal hyperplasia, and some medications.
It is worth considering separately, the hyponatremia that occurs in the setting of diuretic use. Patients taking diuretic medications such as furosemide (Lasix), hydrochlorothiazide, chlorthalidone, etc., become volume depleted. That is to say that their diuretic medicine, by design, has caused their kidneys to produce more urine than they would otherwise make. This extra urine represents blood volume that is no longer there, that has been lost from the body. As a result, their blood volume is reduced. As mentioned above, lack of adequate blood volume is a potent stimulus for ADH secretion and thence water retention.
A recent surge in death from hyponatremia has been attributed to overintake of water while under the influence of MDMA. Also, Almond et al.[2] found hyponatremia in as many as 13% of runners in a recent Boston Marathon, with life-threatening hyponatremia (serum Na below 120 mmol/L) in 0.6%. The runners at greatest risk of serious water intoxication had moderate weight gain during the race due to excessive water consumption (see reference). Siegel et al [3] recently found that in addition to over-zealous drinking, the cause of exercise-associated hyponatremia (EAH) is from an inappropriate secretion of the hormone arginine vasopressin, or antidiuretic hormone. This excess hormone secretion prevents the kidneys from excreting the excess water in the urine.
Common Causes
Causes by Organ System
Causes in Alphabetical Order
Pseudohyponatremia
Certain conditions that interfere with laboratory tests of serum sodium concentration (such as extraordinarily high blood levels of lipid or protein) may lead to an erroneously low measurement of sodium. This is called pseudohyponatremia, and can occur when laboratories use the flame-photometric and indirect (but not direct) ion-selective electrode assays.[4][5] This is distinct from a true dilutional hyponatremia that can be caused by an osmotic shift of water from cells to the bloodstream after large infusions on mannitol or intravenous immunoglobulin.
Hypoosmolar hyponatremia
When the plasma osmolarity is low, the extracellular fluid volume status may be in one of three states:
- Low volume. Loss of water is accompanied by loss of sodium.
- Excessive sweating
- Burns
- Vomiting
- Diarrhea
- Urinary loss
- Diuretic drugs (especially thiazides)
- Addison's disease
- Cerebral salt-wasting syndrome
- Other salt-wasting kidney diseases
Treat underlying cause and give IV isotonic saline. It is important to note that sudden restoration of blood volume to normal will turn off the stimulus for continued ADH secretion. Hence, a prompt water diuresis will occur. This can cause a sudden and dramatic increase the serum sodium concentration and place the patient at risk for so-called "central pontine myelinolysis" (CPM). That disorder is characterized by major neurologic damage, often of a permanent nature.
Because of the risk of CPM, patients with low volume hyponatremia may eventually require water infusion as well as volume replacement. Doing so lessens the chance of a too rapid increase of the serum sodium level as blood volume rises and ADH levels fall.
- Normal volume.
- SIADH (syndrome of inappropriate antidiuretic hormone)
- Some cases of psychogenic polydipsia
The cornerstone of therapy for SIADH is reduction of water intake. If hyponatremia persists, then demeclocycline (an antibiotic with the side effect of inhibiting ADH) can be used. SIADH can also be treated with specific antagonists of the ADH receptors, such as conivaptan or tolvaptan.
- High volume. There is retention of water.
Placing the patient on water restriction can also help in these cases.
Severe hyponatremia may result from a few hours of heavy exercise in high temperature conditions, such as hiking in desert areas, or from endurance athletic events when electrolytes are not supplied. (Such an incident notably happened to long-distance athlete Craig Barrett in 1998).
Epidemiology and Demographics
Hyponatremia is the most common electrolyte disorder. Its frequency is higher in females, the elderly, and in patients who are hospitalized. The incidence of hyponatremia depends largely on the patient population. A hospital incidence of 15–20% is common, while only 3–5% of patients who are hospitalized have a serum sodium level of less than 130 mEq/L. Hyponatremia has been reported in up to 30% of elderly patients in nursing homes and is also present in approximately 30% of depressed patients on selective serotonin reuptake inhibitors.[1]
Diagnosis
Symptoms
Most patients with chronic water intoxication are asymptomatic, but may have symptoms related to the underlying cause.
Severe hyponatremia may cause osmotic shift of water from the plasma into the brain cells. Typical symptoms include nausea, vomiting, headache and malaise. As the hyponatremia worsens, confusion, diminished reflexes, convulsions, stupor or coma may occur. Since nausea is, itself, a stimulus for the release of ADH, which promotes the retention of water, a positive feedback loop may be created and the potential for a vicious circle of hyponatremia and its symptoms exists.
Laboratory Findings
- Serum osmolality
- Blood urea nitrogen (BUN)/creatinine
- Calcium
- Magnesium
- Urine sodium
- Thyroid stimulating hormone (TSH)
- Serum glucose
Treatment
- Hyponatremia is due to an excess of free water in the body, not due to a deficiency of sodium.
- The treatment of hyponatremia is therefore free water restriction.
- In patients who are receiving intravenous therapy, you should make sure that D5W is not infusing, and that drug infusions are mixed in normal saline, and not D5W.
- The patients access to water and juice should be restricted.
- In some refractory cases, the water to the room must be turned off.
The treatment of hyponatremia will depend on the underlying cause and whether the patient's volume status is hypervolemic, euvolemic, or hypovolemic. In the setting of hypovolemia, intravenous administration of normal saline may be effective, but caution must be exercised not to raise the serum sodium level too quickly (see below). Euvolemic hyponatremia is usually managed by fluid restriction and treatment to abolish any stimuli for ADH secretion such as nausea. Likewise, drugs causing SIADH should be discontinued if possible. Patients with euvolemic hyponatremia that persists despite those measures may be candidates for a so-called vaptan drug as discussed below. Hypervolemic hyponatremia should be treated by treating the underlying cause (e.g. heart failure, cirrhosis). In practice, it may not be possible to do so, in which case the treatment of the hyponatremia becomes the same as that for euvolemic hyponatremia (i.e. fluid restriction and/or use of a vaptan drug).
Hyponatremia must be corrected slowly in order to lessen the chance of the development of central pontine myelinolysis (CPM), a severe neurological disease. In fact, overly rapid correction of hyponatremia is the most common cause of that potentially devastating disorder.[6] During treatment of hyponatremia, the serum sodium should not be allowed to rise by more than 8 mmol/l over 24 hours (i.e. 0.33 mmol/l/h rate of rise). In practice, too rapid correction of hyponatremia and thence CPM is most likely to occur during the treatment of hypovolemic hyponatremia. In particular, once the hypovolemic state has been corrected, the signal for ADH release disappears. At that point, there will be an abrupt water diuresis (since there is no longer any ADH acting to retain the water). A rapid and profound rise in serum sodium can then occur. Should the rate of rise of serum sodium exceed 0.33 mmol/l/h over several hours, vasopressin may be administered to prevent ongoing rapid water diuresis.[7]
Pharmaceutically, vasopressin receptor antagonists can be used in the treatment of hyponatremia, especially in patients with SIADH, congestive heart failure or liver cirrhosis. A vasopressin receptor antagonist is an agent that interferes with the action at the vasopressin receptors. A new class of medication, the "vaptan" drugs has been specifically developed to inhibit the action of vasopressin on its receptors (V1A, V1B, and V2). These receptors have a variety of functions, with the V1A and V2 receptors are expressed peripherally and involved in the modulation of blood pressure and kidney function respectively, while the V1A and V1B receptors are expressed in the central nervous system. V1A is expressed in many regions of the brain, and has been linked to a variety of social behaviors in humans and animals.
Vaptan drugs
The “vaptan” class of drugs contains a number of compounds with varying selectivity, several of which are either already in clinical use or in clinical trials as of 2010.
Unselective (mixed V1A, V2)
V1A selective
- Relcovaptan
V1B selective
- Nelivaptan
V2 selective
- Mozavaptan
- Satavaptan
The V2-receptor antagonists tolvaptan and conivaptan allow excretion of electrolyte free water and are effective in increasing serum sodium in euvolemic and hypervolemic hyponatremia.[8]
Rate of Na Correction
The rate of correction of hyponatremia should be 0.5-1.0meq/L/hr, with not more than a 12 meq/l correction in 24 hrs. If the patient has ongoing seizures (or [Na+]<115 meq/li), correction can be attempted at up to 2 meq/L/hr, but only while seizure activity lasts and the [Na+] exceeds 125-130 meq/Li.
See also
References
- ↑ 1.0 1.1 Schrier, Robert W. "Does 'asymptomatic hyponatremia' exist?" Nature Reviews Nephrology. Vol 6, Apr 2010; p 185.
- ↑ Almond CS et al. (2005) Hyponatremia among runners in the Boston Marathon. N Engl J Med, 352(15):1550-6. PMID 15829535
- ↑ Siegel AJ et al. (2007) Am J Med, 120(5):461.e11-7. PMID 17466660
- ↑ Weisberg LS. (1989) Pseudohyponatremia: a reappraisal. Am J Med, 86(3):315-8. PMID 2645773
- ↑ Nguyen MK et al. (2007) A new method for determining plasma water content: application in pseudohyponatremia. Am J Phys - Renal, 292(5):F1652-6. PMID 17299138
- ↑ Bernsen HJ, Prick MJ (1999). "Improvement of central pontine myelinolysis as demonstrated by repeated magnetic resonance imaging in a patient without evidence of hyponatremia". Acta Neurol Belg. 99 (3): 189–93. PMID 10544728. Unknown parameter
|month=ignored (help) - ↑ Horacio J. Adrogué, M.D. and Nicolaos E. Madias, M.D (2000-05-25). "Hyponatremia". N Engl J Med 2000; 342:1581-1589. The New England Journal of Medicine.
- ↑ Robert D. Zenenberg,Do, et. al (2010-04-27). "Hyponatremia: Evaluation and Management". Hospital Practice. 38 (1): 89–96. doi:10.3810/hp.2010.02.283. PMID 20469629. Unknown parameter
|month=ignored (help)
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