Confused about Euvolemic hyponatremia

[pnlitt]

As Goljan defines Euvolemic hyponatremia is no change in total sodium and increase in total body water as water is mainly reabsorbed, not Na

But FA says for SIADH there is an increase in urinary Na secretion and normal extracellular fluid volume due to decrease of aldosterone, which also makes sense.

So which one is right, and what is the official definition of Euvolemic hyponatremia? Why does it named euvolemic if the hyponatremia is caused by increase in total body water.

I have tried so search the internet to no extend and still is confused!!!!

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[jqueb29]

As Goljan defines Euvolemic hyponatremia is no change in total sodium and increase in total body water as water is mainly reabsorbed, not Na

But FA says for SIADH there is an increase in urinary Na secretion and normal extracellular fluid volume due to decrease of aldosterone, which also makes sense.

So which one is right, and what is the official definition of Euvolemic hyponatremia? Why does it named euvolemic if the hyponatremia is caused by increase in total body water.

I have tried so search the internet to no extend and still is confused!!!!

There is initially increased total body water, but the sodium excretion corrects this, resulting in euvolemia and hyponatremia.

 

[pnlitt]

But isn't by definition, Euvolemic hyponatremia is no change in total sodium and increase in total body water. As euvolumic hypernatremia caused by Diabetes insipidus is defined as no change in total sodium and decrease in total body water
I'm very confused with the euvolemic name but there are changes in body water in both conditions.

 

[deep582]

Ok, confusion about the interplay of volume status and sodium concentration is VERY common among everyone from med students all the way up to attendings (barring the renal guys)
Here is what you HAVE to know to understand these concepts:

Sodium concentration (i.e. hyponatremia / hypernatremia) is a function of the amount of FREE WATER in the body
Hyponatremia by definition means TOO MUCH FREE WATER, just as hypernatremia indicates insufficient free water
Sodium concentration is theoretically mediated by ADH secretion which regulates the amount of free water retention in the kidneys via aquaporin channel insertion in the collecting ducts

This is (theoretically) INDEPENDENT of volume status (i.e. hypovolemia/euvolemia/hypervolemia) which is a function of TOTAL AMOUNT OF SODIUM in the body
Euvolemia by definition means NORMAL TOTAL BODY SODIUM, just as hypovolemia indicates decreased sodium and hypervolemia indicates increased sodium retention
Volume status is theoretically mediated by aldosterone which regulates Na/K transporters in the distal tubulues

Things get a little confusing because there is crossover between aldosterone and ADH for regulation of these parameters. But as far as exams you should keep these concepts separate.

As far as your specific case:

Euvolemic hyponatremia means you have too much FREE WATER (i.e. hyponatremic) because of free water retention mediated by excessive ADH but have normal VOLUME STATUS (NORMAL TOTAL SODIUM which is clinically determined by no increased JVD/lower extremity edema/pulmonary edema/normal IVC diameter, etc.) as ADH does not regulate sodium transport.

Below is a diagram i have found very helpful for distinguishing hyponatremia from volume status. If you can understand this chart, you will have a pretty good grasp of sodium/volume status
Hope this helps

 

[zhopv10]

Awesome explanation deep!


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[zeevee]

Below is a diagram i have found very helpful for distinguishing hyponatremia from volume status. If you can understand this chart, you will have a pretty good grasp of sodium/volume status

dont get the chart but do get what u said about the 2 hormones , what they regulate and what euvolemia means .What is the right hand side of the chart saying ?

 

[deep582]

Just start working at the part that says 'ECF volume' (understanding the rest is bonus). You will see that hyponatremia can be seen with decreased, normal, or increased volume states. Your job is to not only know the differential for each of these states but to understand what is going on with aldosterone and ADH at each point.

Hypervolemic hyponatremia is caused by increased aldosterone AND increased ADH. These cases are characterized by decreased "effective circulating volume" (e.g. CHF) that results in unregulated RAAS activity -> aldosterone -> increased sodium retention/hypervolemia. The body starts increasing ADH secretion as well to try to increase plasma volume which causes the hyponatremia (remember i said there is crossover? turns out the body also tries to use ADH to increase volume status at the expense of maintining normal osmolality ).

Euvolemic hyponatremia is due to increased ADH (e.g. SIADH) and ~normal aldosterone. I don't recall what the exact mechanisms of hypothyroid and hypocortisolism are but you should remember these are part of the differential.

Hypovolemic hyponatremia is due to decreased sodium with relatively less decrease in free water. These can either be high aldo or low aldo states depending on the cause.
Urine Na+ can be used to distinguish be renal (high urine Na+) from extrarenal (low urine Na+) sodium loss
I actually think 'vomiting' in this chart is incorrectly placed under the high urine Na category; it is actually a low urine Na+ state.

Not sure if that answers what you were asking

 

[sonicbender]

This was the best explanation I found, with ANP playing a role in the resulting euvolemic hyponatremia (= natriuresis):

The continued presence of ADH as in SIADH with water intake causes retention of ingested water. While a large fraction of this water is intracellular, the extracellular fraction causes volume expansion. Volume receptors are activated and peptides (eg, atrial natriuretic peptide from heart ) are secreted, which causes natriuresis with some degree of accompanying kaliuresis and diuresis.As a general rule ANP tends to antagonize the effect of ADH and AT-II. Thus, these patients are euvolemic or are slightly volume-expanded.

The plasma sodium concentration is the primary osmotic determinant of ADH release. However, in persons with SIADH, a non-physiologic secretion of ADH results in enhanced water reabsorption, leading to dilutional hyponatremia. Sodium excretion is intact, and the amount of sodium excreted in the urine varies with diet. Ingestion of water is an essential prerequisite to the development of dilutional hyponatremia; regardless of cause
(pitutary ,pulmonary)...... hyponatremia does not occur if water is restricted.