Tag: SIADH

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More questions from the minds of the M2s at OUWB

The minds of OUWB continue to provide thoughtful questions.
My roommate and I have encountered a question regarding the content on Sodium/Water Balance and also its application to SIADH. We have been using some outside resources to supplement the learning in class, and I feel that they have been somewhat contradictory in these 2 scenarios. The following are the scenarios that I am trying to think through
1) Patient eats a high salt meal, increasing total body Na+, resulting in an increase in ADH release (via increased plasma osmolarity) and eventually reaching baseline Na+ concentration and osmolarity at a higher ECV. Now, the increase in ECV would result in a down regulation of Sympathetic NS and RAAS; however, what I am hearing is that this down regulation would just return the kidney to Na+ in = Na+ out and would not actually return the individual to the original ECV. So, my question is how does this person get back to original ECV? What I am reading is that the person will continue to operate at this higher ECV until sodium restriction takes place. However, I am wondering how decreased RAAS (decrease aldosterone – decrease Na+ reabsorption – increase sodium excretion) wouldn’t do this, and also if pressure natriuresis wouldn’t do this also? Basically, why don’t these mechanisms do the work automatically, and why do you have to sodium restrict?
You have it right. That is the currently accepted understanding of sodium metabolism. It is not quite complete, because, though some subjects increase their blood pressure with increased sodium intake, not all patients increase their blood pressure. As to why the renin-angiotensin aldosterone system does not down regulate itself sufficiently to fully correct the volume overload situation, it is not well understood. The sodium regulating systems in the body strive to match sodium absorption with sodium excretion. With an increase in sodium intake there will be a modest expansion of the extracellular compartment until the sodium excretion is upregulated to match sodium intake. We can see evidence of the increase total body sodium with an increase in body weight associated with increased sodium intake.
2) In SIADH – high levels of ADH cause increased water reabsorption but euvolemic hyponatremia. Fitting in with my previous questions in the earlier scenario, how does the patient maintain euvolemic status? If increased water reabsorption occurs and the ECV is increased, the same down regulation of Sympathetic NS and RAAS would occur. Now, the outside resources in this case state that a decreased RAAS would actually cause increased sodium excretion that would allow for increased water excretion that would maintain euvolemic status. This makes sense because then the hyponatremia that results is not only an effect of the dilution from increased water reabsorption, but also from the increased excretion of Na+. But, this goes directly against the whole logic of needing to sodium restrict in the earlier case (i.e. RAAS can’t do the work to return the individual in scenario 1 back to a normal ECV).
So again you are well versed in what is happening in SIADH. SIADH is largely euvolemic and largely is a situation where patients are in sodium balance, i.e. sodium = sodium out. However if you do meticulous metabolic balance studies you will find that patients do gain weight during SIADH. There is excess water and this does serve to expand the patient’s extracellular volume. This also will suppress the renin-angiotensin-aldosterone-system so that patients will get a modest increase in urine sodium excretion. But I don’t quite understand how you think this is any different than the first scenario. There is a modest increase in sodium excretion but in the presence of continued unremitting ADH activity the patient continues to deal with the modest increase in volume. So like the first scenario, the modulation of the RAAS is unable to fully restore euvolemia.
For more on SIADH and volume status see this post.
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Adrenal insufficiency and hyponatremia

Every intern knows that the evaluation of hyponatremia includes a TSH and a cortisol level to rule out hypothyroidism and adrenal insufficiency as occult causes of euvolemic hyponatremia.

The mechanism of adrenal insufficiency is a bit confusing with some sources stating that these patients are volume depleted while others are euvolemic.

In some patients without aldosterone, the patients develop severe salt wasting, become hypotensive and get a non-osmotic release of ADH resulting in hyponatremia. These patients will respond to saline. Treat the hypovolemia and the sodium will go up.

Hyponatremia is a common manifestation of adrenal insufficiency even in cases without adrenal crisis. Giving saline to these patients is not effective at correcting the hyponatremia. Giving cortisol, however, results in a brisk water diuresis and rapid correction of the serum sodium (Oelkers, NEJM, 1989).

In addition to being resistant to saline, ADH antagonists (think tolvaptan) protect against this type of adrenal insufficiency-induced hyponatremia.

This means cortisol corrects an abnormality that is due to excess ADH. 

Here is the explanation from the late 90’s from The Fluid, Electrolyte and Acid-Base Companion. 

Bartter and Schwartz original definition of SIADH required a normal cortisol specifically to exclude patients with hypopituitism and primary adrenal insufficiency. In primary adrenal insufficiency, in addition to loss of cortisol there is an aldosterone deficiency which can result in sodium wasting, volume depletion and a non-osmotic (decreased perfusion in this case) stimulates for the release of ADH. In this scenario, the patient should appear salt/volume depleted and would not be considered euvolemic.

A nice review of secondary adrenal insufficiency and hyponatremia was done by Sven Diederich.

Remember:

  • Primary adrenal insufficiency: destruction of the adrenal glands leads to loss of endogenous cortisol. These patients typically also have aldosterone deficiency, so they will be salt wasters, resulting in hypovolemia. They will also have hyperkalemia.
  • Secondary adrenal insufficiency: decreased ACTH (from pituitary or hypothalamic disease, or from pharmacologic steroids) prevents secretion of cortisol

In Diederich’s review they pulled 139 cases of hyponatremia that were referred to endocrinology. (Clearly this study suffers from profound selection bias. Here is a a cleaner study on the epidemiology of hyponatremia by Schrier, from back in the day.) They found 28 cases of hypopituitism leading to hyponatremia. Patients tended to be older (average age 68) and more female (75%). In 25 cases the hypopituitism had not previous been diagnosed and 12 patients had previously been admitted (between 1 and 4 times) for severe hyponatremia without an adequate diagnosis.

Basal cortisol levels were as follows:

  • Below 100 nmol/L (3.6 mcg/dL) in 7
  • Below 200 nmol/L (7.2 mcg/dL) in 21
  • The mean level was 157 nmol/L (5.7 mcg/dL)
  • The highest basal level was 439 nmol/L (16 mcg/dL)

Imaging results:

  • Twelve patients had an ‘empty sella’
  • Six patients had pituitary tumors

  • One had secondary adrenal insufficiency due to chronic treatment with prednisolone because of ankylosing spondylitis

Don’t be the doctor that corrects the hyponatremia but fils to diagnose adrenal insufficiency and discharges and sends the patient home only to redevelop hyponatremia another day. Turn over every stone, especially in patients with SIADH of undetermined etiology.

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OUWB Euvolemic Hyponatremia question

Hello Dr. Topf,

I hope you are enjoying your weekend. I had a question in regards to one of your lectures. I was wondering why there is a low level of Uric acid in euvolemic hyponatremia but not in hypervolemic or hypovolemic hyponatremia. Also, how is it that Na taken in equals Na excreted in euvolemic hyponatremia?

All the best,

 
So why is there a low level of uric acid with euvolemic hyponatremia? Let’s first look at what happens to uric acid in the other causes of hyponatremia, namely hypovolemic and hypervolemic. In both of these situations the kidney is experiencing decreased perfusion, either from absolute volume depletion (diuretics, diarrhea) or perceived volume depletion from pump failure (CHF) or fluid maldistribution (cirrhosis and nephrotic syndrome).
In these volume depleted states there is an increase in the filtration fraction, i.e. more of the plasma that enters the glomerulus is actually filtered. This is how the kidney compensates for a decrease in renal plasma flow while maintaining GFR, it increases the fraction of fluid that is filtered.
A consequence of this, is that the oncotic pressure in the blood leaving the glomerulus is higher because more of the fluid (but none of the protein) has gone down the glomerular drain leaving the plasma in the efferent arterioles with a higher oncotic pressure.
This plasma then enters the vasa recta where it surrounds on the proximal tubule. Here the increased oncotic pressure pulls more fluid back.
This is an ideal situation. The increased filtration fraction maintains GFR in the face of decreased renal plasma flow, and the increased filtration fraction results in enhanced reabsorption of fluid in the proximal tubule limiting fluid loss in situations where patients have decreased perfusion.
Uric acid handling is complex and not fully worked out.
It appears that there is both uric acid secretion and reabsorption in the proximal tubule.
Functionally, uric acid clearance tracks with renal perfusion:
  • Decreased uric acid clearance with decreased renal perfusion
  • Increased uric acid clearance with increased perfusion of the kidney

This is similar to what we see with urea. The following description of urea handling gives a model that will work for uric acid, though the truth of uric handling is much more complex.

The key with urea is that it’s handling in the proximal tubule tracks with total fluid reabsorption in the proximal tubule.
With volume depletion, increased filtration fraction causes increased oncotic pressure in the vasa-recta increasing urea reabsorption in the proximal tubule.
In volume overload, decreased angiotensin 2 decreases sodium reabsorption resulting in less fluid reabsorption and less passive reabsorption of urea  so increased urea loss in the urine and lower serum urea.
Now what happens in euvolemic hyponatremia.
Sodium in equals sodium out. This means that these patients do not have a primary volume abnormality as we see in the hypovolemic and hypervolemic patients. Because of this their sodium regulation volume regulation system is not stressed, they are at homeostasis with regards to body sodium. When you are in homeostasis, in order to stay in homeostasis you need to excrete all the sodium that comes in. In other words sodium in equals sodium out.
However these patients are not in water balance. they have a disease that forces their ADH to 11. They have a fixed ADH secretion and it is set at full blast. This minimizes urinary water excretion, but they are able to stay in sodium balance. So the net of this is they make only a little bit of urine but that small amount of urine carries all of their ingested sodium (sodium in = sodium out) so the sodium is excreted in a small volume at a high concentration.
Now the obvious problem here is that they are holding on to an excess of water. And that will increase their total body volume. This is subtle and doesn’t cause edema, or heart failure, or fluid overload in the lungs, but it is there. This fluid overload suppresses angiotensin 2 and decrease sodium resorption in the proximal tubule and hence decreases urea (and uric acid in our model) reabsorption.
And yes this does mean it is not exactly sodium in = sodium out, there will be a slight excess of sodium excretion.
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Two more OUWB questions

Hi Dr. Topf,

I had a question regarding the Macula Densa.  When reviewing your powerpoint on volume control, you have a slide that said there is only one osmoreceptor (Hypothalamus) because osmolarity across the body is the same at all times.  I’ve had some confusion regarding the Macula Densa, but from what I understand it is also an osmoreceptor (sensing Na+ in the tubule), which would make sense because the tubules are the only part of the body where osmolarity is different.

I thought that the Macula Densa would affect GFR and stimulate the release of renin from the Juxtaglomerular cells, but that would seem to affect volume (RAAS System maintains volume), so my question is why does the macula densa (which senses Na+) controlling volume and not Osmolarity?

Good question.

So the macula densa is a major part of a process called tubulo-glomerular feedback

As the name implies this is important for balancing GFR with tubular reabsorption.

If you had excess GFR and limited tubular reabsorption, people could literally pee them selves to death in minutes.

Think about the math, you have 3 liters of plasma and filter 125 ml of it every minute. so it would only take 24 minutes to completely filter all of the plasma. if you are not constantly reabsorbing 99% of the filtered fluid you could very rapidly become volume depleted and suffer from cardiovascular collapse.

Tubular glomerular feedback prevents that. At the end of the thick ascending limb of the loop of henle, there are chloride receptors as part of the juxtaglomerular apparatus. If there is too much filtration and not enough reabsorption, the excess chloride will bind these receptors and cause a release of intra-renal signals that decrease GFR by adjusting the dilation of the afferent and efferent arterioles.

So yes there are receptors that bind chloride and you can think of them responding to the various concentrations of chloride (like an osmoreceptor) but they are not involved in volume regulation or osmoregulation, but rather the safe running of the kidney to prevent a person from accidentally peeing themselves to death.

Hope this helps

Hello Dr. Topf,

I hope you are doing well. I had a few questions in regard to your last lecture at OUWB. I was wondering if you could explain the pathophysiology behind euvolemic hyponatremia caused by hypothyroidism and adrenal insufficiency. Also, in the case of SIADH, whay wouldn’t the person have hypervolemia if there is a constant reabsorption of water? Is there a pathophysiologic explaination for this as well? I could not find any answers online.


So the key here is to remember that volume is determined by total body sodium and that SIADH is generally Na in = Na out. So they are in sodium balance and will not be volume overloaded.

You are right that these patients will have excess water, but much of this water disappears into the intracellular compartment and the excess volume can not be picked up clinically (by exam or by conventional blood and radiology tests). Yes there is excess water.

We try to reserve terms like hypervolemia for excess total body sodium, and this is not found in SIADH.

Hope that is helpful.

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I get some great letters, here is one of the best from a woman with SIADH

An e-mail I received last month:

I love your blog.  I have had SIADH for a zillion years.  I only found out what I had when I went with 5 girlfriends to a fancy spa hotel  in Tucson for a mini-vacation/ 4th of July Weekend in 1997 where the heat increased to an uncomfortable 117 degrees.

Healthcare workers in the hotel kept handing out bottles of water at each hotel exercise location with orders to “hydrate, hydrate, hydrate” and I stupidly followed their directions.  I drank myself into a 6 day coma.

The only time that sentence has been used for water, not alcohol.

Very non-traumatic for me. Very traumatic for my family. I woke up on day 6 saying, “I am STARVING!  Will someone go get me a taco?” which was very anxiety-relieving for all of them; they’d been sure I’d wake up cognitively impaired.  I wasn’t.  This “taco” sentence sounded JUST like me.  And I have continued to be not cognitively impaired despite interesting lab numbers.

My dad (who is a physician too) has SIADH as well, though his was diagnosed after mine.  I was mis-diagnosed for 9 years prior to my coma as having a “seizure disorder.”  The excellent care I received when my mental status went to heck in a handbasket was truly life-saving.  I remain a very grateful nephrology patient.  And I really do love your blog.

Thought you should know this.
She wrote back a few days later giving me permission to post her letter:
I am dying (well not literally dying) to start an SIADH group on Facebook.  We are so not connected to one another, and each of our nephrologists only have a handful of patients and of course the doctors can’t introduce us to eachother because of HIPAA. 
  • For those of us that have the Syndrome without lung cancer and so on and have to live our lives thirsty
  • and our summers avoiding the sun through our sunroofs (Demeclocycline)
  • and have to, if we’re female, find inventive ways to paint our nails to avoid Demeclocycline making our nailbeds ugly colors
  • and have to fear that Otsuka Pharmaceutical will convince the ONE manufacturer who makes Demeclocycline to stop making it and force us into buying Tolvaptan even though they never tested it in 3rd stage human trials on people that weren’t already cognitively impaired (I know because I volunteered for every single US trial), 
Well, we NEED each other.  We need tips on nail polish, tips on drinking our fluids out of 1 ounce shot glasses, tips on rolling ice cubes around our mouths during the days our sodium is tanking, and your website is a GREAT place for us to meet up!  
You have my non-dying thanks and permission to reprint/repost any or all of my statements!
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It’s summer, make sure to warn all of your SIADH patients about sun sensitivity

This came into my office on Friday.

Demeclocycline induced sun-sensitivity
Demeclocycline is minimally effective for SIADH and has a bad side-effect profile to boot. This patient has heart failure in addition to idiopathic SIADH. So salt tablets are poorly tolerated, and he needs chronic loop diuretics to stay out of the hospital. This makes managing his water metabolism pretty tricky. The only reason I use demeclocycline as opposed to the highly-effective and safer tolvaptan is cost. Tolvaptan is $300/day wholesale, and not one of my patients has been able to get it covered by insurance.
From the University of Utah New Drug Bulletin
Way to price that drug Otsuka, such that even well insured patients can’t use it. I’ll never understand drug pricing.
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Patient list

Yesterday:

  • My first patient had SIADH and a sodium of 125
  • My last patient had nephrogenic diabetes insipidus and a sodium of 150
The statistician in me, looked at the patient list and concluded, normal sodium.
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Fellow talk on sodium

I was scheduled to give a talk on disorders of sodium and water to the fellows yesterday. We have a particularly clever cohort of fellows this year and I really couldn’t give them a warmed over version of my resident and student sodium lecture so I put together this talk which looks five different issues with hyponatremia and some data regarding them:

  1. mannitol induced pseudohyponatremia
  2. TURP syndrome
  3. uremia and propensity for myelinolysis
  4. exercise induced hyponatemia
  5. differentiation of salt delpetion from SIADH with FENa, FEUrea and FE Uric acid with a couple of slides on treating SIADH with saline

Remember, downloading the native Keynote file will give you animations and a better  looking experience.