Question from OUWB M2 on SIADH

Another question from the e-mail
I am trying to understand why SIADH does not cause edema. I understand that in SIADH, there is an increase in Total Body Water, as the increased ADH causes increased water reabsorption. However, there is no change in total body sodium. This implies that the issue is a euvolemic hyponatremia. I would imagine that with total body water increase, there is increased ECF and therefore increased capillary hydrostatic pressure. How come this doesn't result in edema?
This is a question I get every year.

The question comes from a student with clear thinking about SIADH. And it is true that careful and precise measurements of total body water will show that people with SIADH have excess total body water, so they are not truly "euvolemic." But we use the term euvolemia here because they are in sodium balance. Their sodium intake equals their sodium excretion:

This is very different than patients with hypervolemic hyponatremia (heart failure and liver failure) where sodium intake is much greater than sodium excretion. With positive sodium balance (total body sodium increases everyday) heart failure patients develop progressive and clinically evident edema.

The other way to look at the increased water that patients have with SIADH is to quantify it. If a patient with SIADH drops their sodium from 140 to 120 they have dropped there sodium by 14%. This comes from an increase in total body water of 14%, so in a 70 kg young man (42 liters total body water), this represents a increase in total body water of 5.88 liters. Two thirds of this water would be intracellular, so only 2 liters would be extracellular. In heart failure, dogma states people gain 5 kilograms of body weight before they develop clinically evident edema. Since the edema is from excess sodium all of this fluid gain is extracellular. So the amount of water that needs to be retained to lower the sodium 20 points, is less than half the amount that is needed to cause clinically evident edema.