Principles
Principle #1: Disorders of serum sodium are generally caused by changes in total body water, not total body sodium.
The serum/plasma sodium concentration is regulated by changes in water intake and water excretion, not by changes in total body sodium. That is, the total amount of sodium in the body remains the same. The only thing that changes is water coming in and going out.
- Hyponatremia is primarily due to the intake of water that has not been excreted,
- Hypernatremia is primarily due to the loss of water that has not been replaced,
- Hypovolemia represents the loss of both sodium and water, and
- Edema is primarily due to sodium and water retention.
Principle #2: Rapid changes in plasma osmolality (from high to low or low to high) lead to rapid water shifts that cause the volumes of brain cells to change. This can cause altered mental status, seizures, osmotic demyelinating syndrome.
Principle #3: Antidiuretic hormone and Aldosterone are the two key hormones that are involved in the sodium and water homeostasis. However, when it comes to hyponatremia and hypernatremia, ADH is the hormone to consider because ADH is the primary hormone that regulates sodium concentration. Aldosterone is the primary hormone that regulates total body sodium, which remains fairly constant in both hyponatremia and hypernatremia.
Pathophysiology of Hyponatremia
Hyponatremia is caused by an excess of water relative to sodium. This is almost always due to increased ADH. The increased ADH may be appropriate (as when there is hypovolemia or hypervolemia with decrease EAV) or the increased ADH may be inappropriate (SIADH).
Note that ADH activity is also increased in endocrine disorders like glucocorticoid deficiency (b/c of co-secretion of ADH and CRH) and severe hypothyroidism/myxedema coma (because of a decrease in cardiac output and a decrease in GFR).
In very rare circumstances, ADH is appropriately suppressed, instead of increased. However, the kidneys are not able to maintain a normal serum sodium concentration. Examples include 1) Primary Polydipsia, 2) “Tea and toast” and “beer potomania”
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