The Curbsiders' 'Hot Takes'

Simplifying Hyponatremia With the Salt Whisperer

The Curbsiders


October 30, 2020

This transcript has been edited for clarity.

Matthew F. Watto, MD: This is The Curbsiders. I'm going to throw it over to Dr Paul Nelson Williams.

Paul N. Williams, MD: This video is a recap of our excellent podcast with the great Dr Joel Topf (@Kidney_Boy), who talked us through hyponatremia, which I know many internists find very vexing. But the way he breaks it down for us we found incredibly helpful. We'll share some of the pearls from that episode now. I'll start with Stuart. You had a pearl about a way to differentiate ADH-dependent and ADH-independent hyponatremia.

Stuart K. Brigham, MD: Dr Topf gave us a wonderful picture algorithm that provides a better framework to approach hyponatremia.

Hyponatremia (part 1). Algorithm by Joel Topf, MD. Redesigned by Edison Jyang. Courtesy of The Curbsiders.

It's always seemed like this weird Pandora's box when you see someone with hyponatremia. How do you approach it? This provides an easy-to-understand framework. Assuming that the serum osmolality is low (which suggests true hyponatremia vs pseudohyponatremia), the only thing you have to get is a urine osmolality level. This tells you whether the hyponatremia is ADH independent (meaning that ADH is turned off) or ADH dependent.

Hyponatremia (part 2). Algorithm by Joel Topf, MD. Redesigned by Edison Jyang. Courtesy of The Curbsiders.

For ADH-independent hyponatremia (low urine osmolality), the differential diagnosis is actually pretty small. The problem is that if the hyponatremia is ADH dependent, the next thing you are supposed to do is assess volume status, which is really difficult, from what I hear. Paul?

Williams: Seamless segue — you can tell we are becoming professionals. One of the things I picked up from this episode was that Joel thought that the history was the most useful diagnostic modality you have here. If you have an older female patient, you might think tea and toast syndrome. If you have a patient with alcohol use disorder, you might think beer potomania. With a student who has been choking down water in some sort of hazing ritual, you might think polydipsia. If someone comes in with a creatinine level of 8, then you might think of renal failure.

The history seems to be much more useful, for the most part, than a volume status examination, unfortunately. He cited a study by Chung et al (1987) that basically showed that even if you take seasoned expert clinicians (people far smarter than us) and ask them to assess volume status using physical examination, it's a coin flip. About 50% of the time they're right, and 50% of the time they're not.

Brigham: In a book we both love, Evidence-Based Physical Diagnosis, the only thing that was statistically significant (as far as sensitivity is concerned) for the diagnosis of hypovolemia was the lack of axillary sweat. Of course, the gold standard that they used was urine sodium. But I thought that was interesting. Who checks for the lack of axillary sweat? How do you get that?

Williams: We won't get into it, but some of my personal heroes are big axillary sweat fans. I have yet to use it on a routine basis.

Watto: I wanted to point out that Dr Topf called ADH the "add hydration to the body hormone" to help you remember what it does. It helps the body hold on to free water which dilutes sodium. We are really talking about a too-much-water problem, not a too-little-sodium problem, for the most part. Paul named all of the ADH-independent causes of hyponatremia: tea and toast syndrome, beer potomania, primary polydipsia, and severe kidney failure. You rely on the history to figure out which one is the cause. A patient having 20 episodes of diarrhea a day who hasn't been able to eat for a week is probably hypovolemic, and that will lead you down that pathway.

Brigham: In ADH-independent hyponatremia, assessing for volume status isn't very helpful. It's helpful for ADH-dependent hyponatremia.

With euvolemic hyponatremia, that's where you get into SIADH (syndrome of inappropriate antidiuretic hormone secretion), adrenal insufficiency, or hypothyroidism. The history is exceptionally helpful for hypervolemia — heart failure, liver failure, and nephrotic syndrome — and there are a lot of things that can point to that. In hypovolemic hyponatremia, obviously the body is losing solute and fluid through gastrointestinal, renal, or other losses. That would be like a marathon runner.

Watto: I want to bring up one pearl about SIADH, which is largely a diagnosis of exclusion. Dr Topf said it can be helpful to order a uric acid level because it will often be elevated in patients with volume depletion or those with decreased effective circulating volume who aren't perfusing their kidneys. He talked about SIADH being transient, and that was something I hadn't known. I think maybe I've seen this before, but I didn't really understand it. I thought SIADH was something that happened in someone with a tumor, for example. But someone who is in severe pain or has an injury can develop transient SIADH. Once you make the diagnosis, Dr Topf gave us some equations. But what it really boils down to in treating SIADH with these equations is that there are two things you can adjust: You can either restrict the fluid intake (have them drink less water) or boost the solute intake.

He talked about tea and toast syndrome, which causes hyponatremia because toast (a carbohydrate) is metabolized to water and CO2. You will breathe off the CO2, but if you already have a water problem with hyponatremia, now you are just adding more water. The same thing happens with beer, which is just carbohydrate and water. He made a joke about the bartenders being amateur nephrologists. They have salted peanuts on the bar. People who drink the beer aren't getting solute from the beer, but they might get solute — protein and salt — from the peanuts.

We did about 90 minutes on hyponatremia with the great Dr Joel Topf. It's our most downloaded episode of all time. Dr Topf is just a delight, and Stuart and Paul have some nice jokes on that one, too. Click to hear the full episode of REBOOT #48 Hyponatremia Deconstructed or find The Curbsiders' podcasts on iTunes.

You can read our show notes on that episode and join our mailing list.

Thank you for watching.

The Curbsiders are a national network of students, residents, and clinician educators from across the country representing 15 different institutions. They "curbside" experts to deconstruct various topics in the world of medicine to provide listeners with clinical pearls, practice-changing knowledge, and bad puns. Learn more about their contributors and follow them on Twitter.

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