Aetiology

CAUSES
An abnormally low plasma sodium level is best considered in conjunction with the person's plasma osmolarity and extracellular fluid volume status.
It is worth considering separately, the hyponatremia that occurs in the setting of diuretic use. Patients taking diuretic medications such as furosemide , 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.


Reduced blood volume
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 recover water and produce a fairly 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.

Exercise-associated hyponatremia
Exercise associated hyponatremia (EAH) is predominantly the occurrence of dilutional hyponatremia during or up to 24 hours after prolonged physical activity, caused by an increase in total body water relative to the amount of total body exchangeable sodium. This means consumption of fluids in excess of total body fluid losses and/or impaired renal water clearance: maximal urinary excretory rate is about 1 L/h in normal adults under resting conditions.
Inappropriate secretion of the hormone ADH, is also a contributory factor to the development of EAH. This excess hormone secretion prevents the kidneys from excreting the excess water in the urine. The primary means of avoiding EAH is to avoid excess fluid retention (weight gain during or after exercise). This can be accomplished by drinking only according to thirst and monitoring body weight before and during exercise - it is best to lose around two percent of body weight and never gain weight during exercise.
Normal blood volume
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 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).

Increased blood volume
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.
Recent deaths from hyponatremia have been attributed to overintake of water while under the influence of MDMA. This may also be related to inappropriate release of ADH that is stimulated by the drug.

Hypoosmolar hyponatremia
When the plasma osmolarity is low, the extracellular fluid volume status may be in one of three states: low volume, normal volume, or high volume.
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; salt is also retained but to a lesser extent. 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.