View source for Hyponatremia

{{DiseaseDisorder infobox |
  Name        = Hyponatremia |
  Image       = Na-TableImage.png  |
  Caption     = [[Sodium]] |
  ICD10       = {{ICD10|E|87|1|e|70}} |
  ICD9        = {{ICD9|276.1}} |
}}
{{SI}}

'''For patient information click [[Hyponatremia (patient information)|here]]'''

{{CMG}}

==Overview==
The [[electrolyte disturbance]] '''hyponatremia''' (British ''hyponatraemia'') exists in humans when the [[sodium]] (''Natrium'' in [[Latin]]) concentration in the [[blood plasma|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.

== Causes ==
[[Image:Hyponatraemia Causes.png|left|thumb|400px|Causes of hyponatraemia]] 
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) &nbsp;&mdash; 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&nbsp;&mdash; 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.''<ref>Almond CS et al. (2005) Hyponatremia among runners in the Boston Marathon. N Engl J Med, 352(15):1550-6.  PMID 15829535</ref> 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'' <ref> Siegel AJ et al. (2007) Am J Med, 120(5):461.e11-7. 
PMID 17466660</ref> 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.

=== Causes of Hyponatremia in Alphabetical Order===
* Acute or chronic kidney disease
* [[Addison's Disease]]
* After pituitary surgery
* After surgery
* [[Ascites]] puncture
* [[Beer Potomania]]
* [[Burns]]
* [[Cirrhosis]]
* [[Congestive Heart Failure]] (CHF)
* [[Diabetic coma]]
* [[Diabetes]] with [[glucosuria]]
* [[Diarrhea]]
* [[Diuretics]]
* [[Drugs]]
* Excessive [[hydration]]
* [[Fistula]]
* Gastric drainage
* [[Glucocorticoid deficiency]]
* [[Heart failure]]
* [[Hyperglycemia]] without [[dehydration]]
* [[Hyperlipidemia]]
* [[Hyperproteinemia]]
* [[Hypoalbuminemia]]
* [[Hypothyroidism]]
* [[Hypotonic]] infusions
* [[Iatrogenic]] hypotonic infusions
* [[Ileus]]
* [[Ketonuria]]
* [[Metabolic acidosis]]
* [[Mineralacorticoid deficiency]]
* [[Nephrotic syndrome]]
* Nonsteriodal anti-inflammatory drugs ([[NSAID]]s) 
* [[Pancreatitis]]
* Patients with urea [[diuresis]]
* [[Peritonitis]]
* Pleuracentesis
* [[Pregnancy]]
* Pseudohyponatremia
* [[Psychogenic polydipsia]]
* [[Psychosis]]
* [[Renal Failure]]
* Renal losses
* [[Renal Tubular Acidosis]]
* [[Subarachnoid hemorrhage]]
* [[Syndrome of inappropriate antidiuretic hormone]] ([[SIADH]])
* [[Tubulointerstitial kidney disease]]
* [[Vomiting]]

=== 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.<ref>Weisberg LS. (1989) Pseudohyponatremia: a reappraisal. Am J Med, 86(3):315-8.  PMID 2645773 </ref><ref>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</ref>  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 [[IVIG|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 [[sweat]]ing
**[[Burn (injury)|Burns]]
**[[Vomit]]ing
**[[Diarrhea]]
**[[Urine|Urinary]] loss
***[[Diuretic]] drugs (especially [[thiazide]]s)
***[[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 [[antidiuretic hormone|ADH]] receptors, such as [[conivaptan]] or [[tolvaptan]].

*'''High volume'''. There is retention of water.
**[[Congestive heart failure]]
**[[Hypothyroidism]] and [[hypocortisolism]]
**[[Liver]] [[cirrhosis]]
**[[Nephrotic syndrome]]
**[[Psychogenic polydipsia]]

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).

==Diagnosis==
=== Symptoms ===
Most patients with chronic water intoxication are asymptomatic, but may have [[symptom]]s related to the underlying cause.

Severe hyponatremia may cause [[osmosis|osmotic]] shift of water from the plasma into the [[brain]] [[cell (biology)|cells]]. Typical symptoms include [[nausea]], [[vomit]]ing, [[headache]] and [[malaise]]. As the hyponatremia worsens, confusion, diminished [[reflex]]es, [[convulsion]]s, [[stupor]] or [[coma]] may occur. Since nausea is, itself, a stimulus for the release of [[Vasopressin|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 D<sub>5</sub>W is not infusing, and that drug infusions are mixed in normal saline, and not D<sub>5</sub>W. The patients access to water and juice should be restricted.  In some refractory cases, the water to the room must be turned off.

===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<sup>+</sup>]<115 meq/li), correction can be attempted at up to 2 meq/L/hr, but only while [[seizure activity]] lasts and the [Na<sup>+</sup>] exceeds 125-130 meq/Li.

== See also ==
*[[Water intoxication]]
*[[Hypernatremia]]
*[[Edible salt]]
*[[Osmotic demyelination syndrome]]

==References==
{{Reflist|2}}

{{Endocrine, nutritional and metabolic pathology}}

[[Category:Electrolyte disturbance]]
[[Category:Nephrology]]
[[Category:Blood tests]]
[[Category:Emergency medicine]]
[[Category:Intensive care medicine]]

[[cs:Hyponatremie]]
[[de:Hyponatriämie]]
[[es:Hiponatremia]]
[[fr:Hyponatrémie]]
[[ja:低ナトリウム血症]]
[[pl:Hiponatremia]]
[[ru:Гипонатремия]]
[[sv:Hyponatremi]]

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