Serum sodium concentration and serum osmolarity normally are maintained under precise control by homeostatic mechanisms involving stimulation of thirst, secretion of antidiuretic hormone (ADH), and renal handling of filtered sodium. Clinically significant hyponatremia is relatively uncommon and is nonspecific in its presentation; therefore, the physician must consider the diagnosis in patients presenting with vague constitutional symptoms or with altered level of consciousness. Irreparable harm can befall the patient when abnormal serum sodium levels are corrected too quickly or too slowly. The physician must have a thorough understanding of the pathophysiology of hyponatremia to initiate safe and effective corrective therapy. The patient's fluid status must be accurately assessed upon presentation, as it guides the approach to correction. 
Total body water (TBW) decreases; total body sodium (Na+) decreases to a greater extent. The extracellular fluid (ECF) volume is decreased.
TBW increases while total sodium remains normal. The ECF volume is increased minimally to moderately but without the presence of edema.
Total body sodium increases, and TBW increases to a greater extent. The ECF is increased markedly, with the presence of edema.
Water shifts from the intracellular to the extracellular compartment, with a resultant dilution of sodium. The TBW and total body sodium are unchanged. This condition occurs with hyperglycemia or administration of mannitol.
The aqueous phase is diluted by excessive proteins or lipids. The TBW and total body sodium are unchanged. This condition is seen with hypertriglyceridemia and multiple myeloma.
Serum sodium concentration is regulated by stimulation of thirst, secretion of ADH, feedback mechanisms of the renin-angiotensin-aldosterone system, and variations in renal handling of filtered sodium. Increases in serum osmolarity above the normal range (280-300 mOsm/kg) stimulate hypothalamic osmoreceptors, which, in turn, cause an increase in thirst and in circulating levels of ADH. ADH increases free water reabsorption from the urine, yielding urine of low volume and relatively high osmolarity and, as a result, returning serum osmolarity to normal. ADH is also secreted in response to hypovolemia, pain, fear, nausea, and hypoxia.
Aldosterone, synthesized by the adrenal cortex, is regulated primarily by serum potassium but also is released in response to hypovolemia through the renin-angiotensin-aldosterone axis. Aldosterone causes absorption of sodium at the distal renal tubule. Sodium retention obligates free water retention, helping to correct the hypovolemic state. The healthy kidney regulates sodium balance independently of ADH or aldosterone by varying the degree of sodium absorption at the distal tubule. Hypovolemic states, such as hemorrhage or dehydration, prompt increases in sodium absorption in the proximal tubule. Increases in vascular volume suppress tubular sodium reabsorption, resulting in natriuresis and helping to restore normal vascular volume. Generally, disorders of sodium balance can be traced to a disturbance in thirst or water acquisition, ADH, aldosterone, or renal sodium transport.
Hyponatremia is physiologically significant when it indicates a state of extracellular hyposmolarity and a tendency for free water to shift from the vascular space to the intracellular space. Although cellular edema is well tolerated by most tissues, it is not well tolerated within the rigid confines of the bony calvarium. Therefore, clinical manifestations of hyponatremia are related primarily to cerebral edema. The rate of development of hyponatremia plays a critical role in its pathophysiology and subsequent treatment. When serum sodium concentration falls slowly, over a period of several days or weeks, the brain is capable of compensating by extrusion of solutes and fluid to the extracellular space. Compensatory extrusion of solutes reduces the flow of free water into the intracellular space, and symptoms are much milder for a given degree of hyponatremia.
When serum sodium concentration falls rapidly, over a period of 24-48 hours, this compensatory mechanism is overwhelmed and severe cerebral edema may ensue, resulting in brainstem herniation and death.
Hyponatremia is the most common electrolyte disorder, with a marked increase among hospitalized and nursing home patients. A 1985 prospective study of inpatients in a US acute care hospital found an overall incidence of approximately 1% and a prevalence of approximately 2.5%. On the surgical ward, approximately 4.4% of postoperative patients developed hyponatremia within 1 week of surgery. Hyponatremia has also been observed in approximately 30% of patients treated in the intensive care unit. 
Though clearly not indicative of the overall prevalence internationally, hyponatremia has been observed in as high as 42.6% of patients in a large acute care hospital in Singapore and in 30% of patients hospitalized in an acute care setting in Rotterdam. [3, 4]
Pathophysiologic differences between patients with acute and chronic hyponatremia engender important differences in their morbidity and mortality.
Patients with acute hyponatremia (developing over 48 h or less) are subject to more severe degrees of cerebral edema for a given serum sodium level. The primary cause of morbidity and death is brainstem herniation and mechanical compression of vital midbrain structures. Rapid identification and correction of serum sodium level is necessary in patients with severe acute hyponatremia to avert brainstem herniation and death.
Patients with chronic hyponatremia (developing over more than 48 h) experience milder degrees of cerebral edema for a given serum sodium level. Brainstem herniation has not been observed in patients with chronic hyponatremia. The principal direct causes of morbidity and death are status epilepticus (when chronic hyponatremia reaches levels of 110 mEq/L or less) and cerebral pontine myelinolysis (an unusual demyelination syndrome that occurs in association with chronic hyponatremia and its rapid correction).
The distinction between acute hyponatremia and chronic hyponatremia has critical implications in terms of morbidity and mortality and in terms of proper corrective therapy.
A 2009 study of 98,411 hospitalized patients found that even mild degrees of hyponatremia were associated with increased in-hospital, 1-year and 5-year mortality rates. Mortality was particularly increased in those with cardiovascular disease, metastatic cancer, and those undergoing orthopedic procedures. 
A 2009 study in Copenhagen concluded that hyponatremia in the range of 130-137 mEq/L is also associated with increased mortality rates in the general population. 
Overall incidence of hyponatremia is approximately equal in males and females, though postoperative hyponatremia appears to be more common in menstruant females.
Hyponatremia is most common in the extremes of age; these groups are less able to experience and express thirst and less able to regulate fluid intake autonomously. Specific settings that have been known to pose particular risk include the following:
Infants fed tap water in an effort to treat symptoms of gastroenteritis
Infants fed dilute formula in attempt to ration
Prognosis is dependent on the underlying condition and the severity of disease.
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