An abnormally low plasma sodium level is best considered in conjunction with the person's plasma osmolality and extracellular fluid volume status. Indeed, correct ascertainment of volume status, as well as determination of the presence or absence of edema, are both critical in establishing the cause of hyponatremia. As described above, a state of volume depletion leads to increased blood levels of ADH and thus water retention.
The greater the amount of water that is retained, the more the blood sodium will become diluted to cause worsening degrees of hyponatremia. The presence of edema indicates that blood volume has been lost insofar as fluid from the blood has shifted out into the peripheral tissues to cause the edema. In other words, edema is usually reflecting a state of blood volume depletion. As a result, edematous states are also associated with increased blood levels of ADH, water retention, and hyponatremia. In all cases of volume depletion-associated hyponatremia, it is important to note that retention of water ''per se'', such as that promoted by ADH, does not correct the volume depleted state.
In addition to volume depletion, there are other causes of increased ADH levels (and ultimately, therefore, of hyponatremia). These include nausea, pain, and opiate drugs such as codeine and morphine. Such factors often play a role in the hyponatremia that is frequently seen in hospitalized patients.
| Type | Serum osmolality (mOsm/kg) | Description |
|---|
| Hypotonic hyponatremia | < 280 | When the plasma osmolality is low, the extracellular fluid volume status may be in one of three states: low volume, normal volume, or high volume. |
| Isotonic hyponatremia | between 280 and 295 | 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. |
| Hypertonic hyponatremia | > 295 | Hypertonic hyponatremia can be associated with shifts of fluid due to osmotic pressure. |
Further Reading
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