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S2D: The Symptom to Diagnosis Podcast - Episode 23: Hyponatremia
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S2D: The Symptom to Diagnosis Podcast - Episode 23: Hyponatremia
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Segment:0 .
[upbeat intro music] [upbeat intro music] [upbeat intro music]
DR. CIFU: I'm Adam Cifu.
DR. STERN: And I'm Scott Stern.
DR. CIFU: And we're here with another episode of S2D, the Symptom to Diagnosis podcast. This podcast teaches evidence-based strategies for diagnosing common medical symptoms. We begin each episode with a case unknown to one of us. We then discuss five high yield features that help to accurately diagnose the cause of the symptom at hand. We then return to our case, before finishing up with a discussion of fingerprints, common misconceptions, pet peeves, and other random pearls of knowledge pertaining to the week's symptom.
DR. CIFU: The cases that we discuss are drawn from our clinical experience but because protecting patient privacy is part of our oath, we never discuss actual patients, and most cases are composites. So what are we talking about today, Scott?
DR. STERN: Today is a fascinating day, today is hyponatremia day.
DR. CIFU: Sounds like a holiday, doesn't it? Hyponatremia day-
DR. STERN: Exactly, it does sound like a holiday.
DR. CIFU: Like 4th of July. Okay, hit me with it.
DR. STERN: All right, so our case. This is a 68-year-old man who presents with a seizure and he's found to be hyponatremic down to a sodium of 121, which is not normal. He had previously been in good health. And the day before, he was taking his lovely colonoscopy prep which all of our patients thoroughly enjoy, and he was exceptionally compliant with his prep. And the night before the colonoscopy after he'd had his prep, he seized, and was brought to the hospital.
DR. STERN: On physical exam, he was lethargic initially, but improved over the next 30 minutes to his baseline. His vital signs were normal, his blood pressure was 135/95, I guess a little high, pulse 92, temperature 37, respiratory rate of 16. He was not orthostatic, his lungs were clear, his cardiac exam was unremarkable without a jugular venous distention or gallop, and he had no ascites or edema.
DR. CIFU: Hmm. Interesting.
DR. STERN: It is interesting.
DR. CIFU: Okay, I'm going to make a couple of assumptions because you didn't tell me that much, but I'll ask further. I'm going to assume the guy was healthy, he's having a colonoscopy, he's probably in pretty good shape.
DR. STERN: Right.
DR. CIFU: And I'm going to assume he's not on any meds going into this?
DR. STERN: Right.
DR. STERN: He might have been on amlodipine for hypertension. But I think that was all.
DR. CIFU: Okay. And you didn't tell me much other than his sodium. So for point of argument, I guess I'll assume that his potassium, glucose, renal function are all okay, at this point?
DR. STERN: That is correct.
DR. CIFU: Okay. So other than the things that you seem to have withheld from me, you gave me a lot of important information. It sounds like he went from normal or at least like, normal asymptomatic to symptomatic hyponatremia quickly in the setting of a bowel prep. And 121 is low enough to make almost anyone symptomatic, but if I'm assuming a rapid fall, you know, given that this was associated with the prep, that would clearly be a reason for symptoms, right?
DR. CIFU: Because it's usually people who fall quickly are more symptomatic than those who kind of drift down.
DR. STERN: Exactly, and as a matter of fact, seizures at 121 if it was chronic would be a little bit surprising.
DR. CIFU: True, true, good point. From his vital signs and the exam you gave me he sounds neither hypovolemic, kind of dry, or hypervolemic, I didn't hear about edema, I didn't hear about an S3, I didn't hear about rales, anything like that. And the history - which I'll actually talk about later - the history is not really suggestive of a guy who should be dry or not dry. I often use a urine sodium at this point in the evaluation just to sort of make sure the guy's not dry with me missing it, but I think at this point, it might actually confuse me, so I'm not even going to ask for that.
DR. CIFU: So most hyponatremia I think about due to high ADH levels in hypovolemic hyponatremia, it's because the body's need for volume is sort of overwhelmed the need to regulate osmolality, right?
DR. STERN: I'm sorry, so let's just emphasize that for a second. So what you said is very important. The primary regulator for ADH or the primary thing that affects ADH secretion is osmolality. And only when the body thinks, oh boy, I'm in tremendous trouble, will volume precipitate a release in ADH independent of the osmolality.
DR. CIFU: Right, right, for the most part, you're saying I got to keep my sodium normal. But if you're going to die from hypovolemia, you're going to say, screw the sodium. I just got to keep my blood pressure.
DR. STERN: Exactly, that's right.
DR. CIFU: Okay, in hypervolemic hypernatremia, I kind of think of the body responding to decreased intravascular volume. So the body is sort of saying, I need fluid, even though I don't really need fluid.
DR. STERN: Well, I'd say it slightly differently. It's decreased perfusion. So for instance, in severe, severe heart failure, intravascular volume is high but effective circulating volume is low. So it's still perceived as inadequate per circulating volume although strictly speaking, the intravascular volume is high.
DR. CIFU: Perfect. So with euvolemic hyponatremia, which I'm saying this is, we're usually, and I'm going to say that cautiously, dealing with SIADH syndrome, inappropriate secretion, ADH levels, whatever high- ADH levels that are higher than they should be. Other possibilities are rare, things like secondary adrenal insufficiency, profound hypothyroidism.
DR. CIFU: I don't think I've ever really seen that, excessive free water intake certainly happens. And I'll bet in this case, we're looking at excessive free water ingestion. You made some comment about this guy being really good about his colonoscopy prep. So maybe he was a little bit too good, did a couple of days of clear liquids, took in very little sodium, and then maybe overdid it with the water in his prep.
DR. CIFU: But I'm a little cautious here, one, because I think that would be too easy for you to give me and you always try to torture me. And I honestly think, I mean, I have prepped hundreds of patients in my career for colonoscopy, and I've never had anybody end up in the hospital seizing from hyponatremia. So I wonder if maybe this is a guy who's got a little bit of SIADH, and then, you know, got a lot of free water and therefore tipped over the edge.
DR. CIFU: So I would think whatever else you're going to tell me, I probably want urine osms in this guy. If it was really just because of free water overload, those should be low, you know, I think of 50 being the lowest you can go, so maybe 50 to 150. On the other hand, if it's much higher than that, then I got to think further, I got to think about SIADH, given the seizure, this guy is symptomatic, I think I would say he needs some hypertonic saline now to get him up a little bit into a safe range, and given this occurred quickly, his risk of complications from correcting his sodium is probably pretty low because he got there quickly, so we can reverse him more quickly.
DR. CIFU: I don't know.
DR. STERN: Well, that's really right on target. I mean, all of that is great, we'll work through it as we go. But I think that your points are all right on target, and the treatment, which we'll touch on at the end is also, I think what you said is exactly right and we'll try to emphasize that again.
DR. CIFU: Okay. Do you want to give me more about the case, or do you want to go right into your five points?
DR. STERN: I think let's go into the five points, because I think that will help people to use the data better as we go.
DR. CIFU: Sounds great.
DR. STERN: So the first point is actually where we differ a little bit in the book from what's traditionally taught. So the traditional approach to hyponatremia emphasizes volume, and what nobody talks about is that what the experts automatically do is look at that basic metabolic panel that they get, and certain clinical pieces of information to see if there's clues. So let's start first with the clinical clues that would automatically send you on a diagnostic path.
DR. STERN: So one is thiazides. So thiazides are probably the most common or the second most common cause of hyponatremia. So if you get a patient who's on a thiazide, most of the time people are going to say you're done, stop the thiazide and only if it doesn't get better, would you look further.
DR. CIFU: Great. I like this approach, because sort of what you're doing is, you're separating the type one thinking from the type two thinking, right? This is kind of the pattern recognition that I'm getting the history, and if there's something really obvious which just says, this is the cause of hyponatremia, I'm done. If I get through these, then you're into the sort of hypothetical deductive type two reasoning.
DR. CIFU: Let me work through algorithms and labs and stuff and figure it out.
DR. STERN: Exactly. So what we're trying to do is just articulate what that pattern recognition is that the experts do automatically, and then they don't tell you, right? They just say go to the volume status.
DR. CIFU: It just looks like magic.
DR. STERN: Some of the other clinical clues that will automatically suggest a diagnosis is a marathon runner. So another relatively common cause, less so now, because people have learned about this, but it's exercise associated hyponatremia. So it turns out, there's often been a lot of advice to drink excessive amounts of water during these really long runs, and people have managed to drink more water than they can secrete and seized during marathons, which is pretty horrible.
DR. STERN: So you know, if you have a patient who comes in, just having completed a marathon or 100 mile bike ride, and they are hyponatremic, you probably better ask them how much water they've been drinking.
DR. CIFU: I was reading the Symptom to Diagnosis chapter to prepare for this and there's a hilarious case in there, where a guy comes in, he's delirious with hyponatremia, and he's wearing running clothes, and so the doctors assume that oh, this must be from running. But then in fact, later after he's better they learn that he just likes to wear running clothes, he wasn't actually running. [chuckles]
DR. STERN: That wasn't very nice. I wrote that chapter too, so maybe I was trying to be confusing.
DR. CIFU: It's funny, you usually don't have much of a sense of humor. [chuckles]
DR. STERN: Oh, my goodness. Okay, so I'm going to skip right over that comment. And the other clinical clue that you should take note of is if somebody comes in on a Friday, Saturday night having been partying, you really need to think about ecstasy and MDMA both because of its direct effects, and because people are advised to drink a lot of water- "Advised" to drink a lot of water, is another common cause of life-threatening actual hyponatremia.
DR. CIFU: Right, so people are told, you know, we're using Molly, so we're going to drink a lot to prepare for it, but then it also has some direct effect on increasing ADH levels, correct?
DR. STERN: Yeah, some mechanism, I'm not exactly sure what that is. So the first point is those clinical clues, the second point that people should review right away before they delve into the volume status is that basic metabolic panel often can give you incredibly useful clues that would suggest the cause of hyponatremia. So let's just run through those quickly. One is, marked hyperglycemia.
DR. STERN: So glucose has an osmotic effect. If people's sugar is way out of control, it draws water from the intracellular compartment into the plasma and extracellular compartment and dilutes the sodium. And you can get a guess of how much that is by estimating that for every 100 mg/dL that the sugar's up, that will actually drop the sodium by about 2.4 mEq/L.
DR. CIFU: Right, and so for me, people in the hospital, glucose over 250, a little hyponatremic, I ignore it. People with higher sugars and more severe hyponatremia, then I pull out my phone, I do the calculation, and I make sure that their sodium cracks into the mid 130s, which it almost always does.
DR. STERN: Right, exactly, and then you fix their glucose, draw all the water back into the cells, and it'll be fine. One that's less obvious and comes up- That actually happens a lot because we see so many out of control diabetics.
DR. CIFU: Sure, all the time.
DR. STERN: The rest of these are less common, but you should notice them if they happen. So one is low sugar. So most hypoglycemia is easy to explain and is not occluded to hyponatremia. So most of it is iatrogenic, you took too much insulin, you took too much sulfonylureas and you're hypoglycemic, doesn't mean anything. But if you had a patient who is hyponatremic and hypoglycemic and it was unclear why, you should think of adrenal insufficiency.
DR. CIFU: Right, right.
DR. STERN: It turns out that adrenal insufficiency, the lack of cortisol causes ADH to go up. And ADH causes, of course, hyponatremia, but the hypocortisolism is a cause of hypoglycemia.
DR. CIFU: Right, and that is rare. I mean, the time that you see an adrenal crisis presenting to the hospital, I think, when we see it the most these days are probably the people on chronic steroids who are then in a time of stress. It's a fairly obvious diagnosis at that time. To think about it, you know, we used to see a reasonable amount of primary adrenal insufficiency during the worst of the AIDS epidemic.
DR. CIFU: But I feel like other than kind of iatrogenic adrenal insufficiency, I haven't seen much lately.
DR. STERN: Not much. I mean, every now and then you have someone with a polyglandular autoimmune disorder, or pituitary tumors and secondary adrenal insufficiency. They're rare. So that's it, sugar's high or low. Potassium also comes on your basic metabolic panel and if somebody had unexplained hyperkalemia, that's not from ARBs and ACEs and potassium and renal failure, which it is 98% of the time, you might think about primary adrenal insufficiency.
DR. STERN: In that case, the loss of aldosteronism actually prevents you from secreting potassium and you can get hyperkalemic, but because the adrenals are wiped out, you've again lost cortisol which causes the hyponatremia. Contrary to what people often assume, it's not the loss of aldosterone that causes the hyponatremia, it's the loss of cortisol that causes the increased ADH secretion.
DR. CIFU: Right. And that's why when you have secondary hypoadrenalism, is that are we call it?
DR. STERN: Secondary
DR. STERN: adrenal insufficiency.
DR. CIFU: Secondary adrenal insufficiency, thank you, that doesn't happen because your aldosterone is maintained.
DR. STERN: That's right. That's exactly right.
DR. CIFU: Got it.
DR. STERN: So we've talked now about potassium and glucose, the other ones are kind of obvious, but boy, just in case you didn't think of it, if the BUN and creatinine are way high, and the person's not on dialysis and they have renal failure, it's easy to get hyponatremia because you're drinking too much water you can't excrete. So that wouldn't be very complicated.
DR. CIFU: Good, good. Okay, so you're up to- There have been a lot of sub points here, but I think we're moving on to point three, right?
DR. STERN: Right, so point one was clinical features, and point two was that basic metabolic panel.
DR. CIFU: And so those are things it's important to know that that's really before you even start thinking about the details of hyponatremia. That's a regular history and physical, and that's like a regular BMP. And you should be- You probably have diagnosed about half the people at this point.
DR. STERN: Right. So it's just noticing the data you've already been given, frankly. But now we need to talk about two measures that you don't get routinely and that's the serum osmolality and the urine osmolality. So, let's talk about the serum osmolality. This is one of the cases where you'll measure it. Now the major osmol in the blood is sodium. So if your sodium is low, your serum osmolality should be low.
DR. STERN: And if they're both low and they're kind of in concert, that's fine. If it's not low, if the serum osmolality is normal, you got to think, hey, that's screwy, right? And that suggested that serum sodium measurement is an error and that's where you get the pseudo hyponatremias. That can happen with marked, marked hyperlipidemia, you know, triglycerides in the thousand, or marked hyperproteinemia like in multiple myeloma.
DR. CIFU: Yeah, I think it's worth underlining that, although that's all true, you don't really get fooled by that much, right?
DR. STERN: No, this is really rare. I mean, I say it because we should be complete. But I have to say, I think I've seen one person with triglycerides high enough to have thrown off the measurement of their sodium, and that's in 30 years.
DR. CIFU: Right and often, I feel like these days with people getting regular health care and getting too many labs, if someone's got that kind of hypertriglyceridemia, or hyperproteinemia, you probably know that they've got the hypertriglyceridemia or that they've got myeloma, and so you're not surprised and you just sit back, scratch your chin and say, interesting and-
DR. STERN: Yes, yes. Now the next one is important. It does come up more often than people think. So let's explain urine osmolality. So almost all the causes of hyponatremia because ADH is too high. So in SIADH, syndrome of inappropriate ADH, people are secreting ADH for no reason that we know of or there's an underlying cause, like a pulmonary or central nervous system cause but they're releasing too much ADH.
DR. STERN: In almost all the other causes, it's still that the body is triggering ADH release, that your volume depleted and you're releasing ADH, or the effective circulating volume is so poor, you're releasing ADH. And what does the ADH do? Well, it puts aquaporins into the tubules, right? So your reabsorb water from the tubules, and you get hyponatremic, makes total sense.
DR. STERN: But think about the urine now. So if you've inserted aquaporins into the tubules, water is leaving the tubular lumen and going into the kidney. So what's left behind in the urine is concentrated, the water has come out of the tubules, so the expectation in 98% of the patients who have hyponatremia is that that urine osmolality is high and higher than plasma.
DR. STERN: If it's low, it suggests it's not ADH mediated, and it's water intoxication.
DR. CIFU: Right, and that brings me back to my point about the case-
DR. STERN: Absolutely.
DR. CIFU: -that in that case, it's going to be really key, and it's going to really change both how we manage the person, and if there's a next stage in the evaluation of that guy.
DR. STERN: And every time I've seen water intoxication that's been missed in people have assumed they have SIADH because people don't pay enough attention to that urine osmolality. So that's really key. So now, let's say that none of those are true, now we're into the traditional kind of teaching, which is look at the volume of the patient.
DR. CIFU: This is point four, underlying four.
DR. STERN: Point four. And so the volume status is assessed, it's a clinical determination. You look for hypervolemia, such as might be seen with edema and ascites and jugular venous distention.
DR. CIFU: Oh, you're going to talk about orthostatic vital signs?
DR. STERN: I am, I'm so excited. I haven't mentioned those in multiple podcasts. But you also look for hypovolemia, and that's looking for hypotension, tachycardia, orthostatic hypotension, and if you like, skin tenting which I think is ridiculous.
DR. CIFU: You only say that because you're like an old sun damaged guy. [chuckles]
DR. STERN: That's true. And if you pull my skin out, it will stay out until tomorrow when I wash my face again. Anyway. And if patients don't look either hypervolemic or hypovolemic, then we call them euvolemic. Now, I do want to mention something you said. You said something that's absolutely spot on, which is to check a urine sodium in this situation. So if you have a patient who looks euvolemic, they can have subtle hypovolemia that you miss.
DR. STERN: And the way you would know that is get a urine sodium. So if someone's hypovolemic, you expect them to reabsorb more sodium and the urine sodium is low. So you would reclassify such a patient as being hypovolemic if their urine sodium was law.
DR. CIFU: And I think about urine sodium and urine osms completely separately because I get confused and urine osms completely separately because I get confused when I start thinking about them together.
DR. STERN: Yes, I think we've got to do just the way we said it, you look at urine osms to see if they're water intoxicated, right? So the final fifth point is if you really have someone who has euvolemic hyponatremia, it is important before you diagnose SIADH to rule out adrenal insufficiency and hypothyroidism. Check a cortisol, check a TSH, you may need to do a corticotropin stim test, but it's imperative that you do that because if you miss adrenal insufficiency, it can be life-threatening.
DR. CIFU: Good, good. Okay, so let's go back to our case.
DR. STERN: All right, so we got an SMA-6, you know, basic metabolic panel and as you mentioned, the potassium was normal, the glucose was normal, the BUN and creatinine were unremarkable, and then you asked me for his urine osmolality, and it was 110.
DR. CIFU: Ha! So that is low, and that does suggest that water intoxication is at least a large part of this, which makes sense given his history. I think I would ask him more questions at this point about his colonoscopy prep, did he do something weird?
DR. STERN: He did. As a matter of fact, he was trying to be very compliant, and he thought the gallon of GoLytely might not be enough to wash him out. So he also had a gallon of water with it.
DR. CIFU: Oh.
DR. STERN: And the other interesting thing was we looked at his prior sodium. So would you like to guess what his prior sodium was?
DR. CIFU: I'm going to guess his prior sodium was a little bit low, I'm going to guess 134.
DR. STERN: Well, it was normal. Just proving it was acute that he had, you know-- Totally fit with him, just washed himself out.
DR. CIFU: Okay, so I would say at this point, I said I would give him hypertonic saline. I'd be a little bit gentle with that because my understanding is these people tend to fix themselves pretty quickly. And so like all people with hyponatremia, I'm going to monitor this guy very closely. But he's probably not going to need much hands on management by us.
DR. STERN: That's right, with water intoxication he'll correct quickly on his own, because the ADH will be suppressed, but he's at low risk for the osmotic demyelinisation syndrome, which we can talk about later, because an acute hyponatremia over-rapid correction won't cause that.
DR. CIFU: You've got it. Okay, I think we're good. So let's move on to our favorite, the fingerprints, common misconceptions, pet peeves and other random pearls of knowledge. Scott, why don't you start us off?
DR. STERN: All right, so for fingerprints. Ascites has a good likelihood ratio for it being cirrhotic, cirrhosis as a cause of this hyponatremia of 6.6, although severe heart failure can also cause ascites and it can also cause hyponatremia, so you need to be a little careful on that one. The well-talked about but rarely seen caput medusae has a very high likelihood ratio for cirrhotic hyponatremia 9.5, but I think I've seen that once. How many times have you seen that?
DR. CIFU: I've seen it 20 or 30 times.
DR. STERN: You have not.
DR. CIFU: [chuckles] No, I have not.
DR. STERN: All right, do you have fingerprints for us?
DR. CIFU: You know, I don't have any fingerprints and I think you're kind of cheating, because you're kind of giving fingerprints for underlying diseases that can cause hyponatremia. I mean, I could throw out the S3 as a diagnosis of heart failure, which, as you say, could cause hyponatremia. So I think you're cheating.
DR. STERN: No, well I don't understand why that's cheating, so we're looking for physical exam findings that would be highly suggestive of a particular diagnosis causing hyponatremia. So if a caput or ascites have high likelihood ratios for cirrhosis, why is that cheating?
DR. CIFU: I guess so. The likelihood ratios you're giving me I assume are for cirrhosis, and not for hyponatremia.
DR. STERN: Correct, no, that's right. It's the underlying disorder, which is actually more helpful.
DR. CIFU: Right, and it's true. I mean, we're only thinking about it, because it's a lab finding, you're not examining someone and saying, we don't need labs on this person because I'm making the diagnosis of hyponatremia without labs.
DR. STERN: Well, the other interesting thing I just have to say is, the volume overload states of cirrhosis and heart failure when they cause hyponatremia, are usually very severe. It's not mild heart failure that does this, it's not mild liver disease that does this. The data says it's very advanced, so actually, those findings are somewhat more frequent in hyponatremia associated with those disorders.
DR. CIFU: And I remember, not to date myself, though I've done that a lot-
DR. STERN: Oh, you've dated me even more often, I think.
DR. CIFU: Ah, yes! I was going to mention that aquaporins didn't exist when I went to medical school.
DR. STERN: Right, that's true.
DR. CIFU: But the other thing that didn't exist when I went to medical school, and at the very start of my residency were ACE inhibitors, and at that point, if you had heart failure with hyponatremia the mortality was in the metastatic cancer rate.
DR. STERN: Totally, it was so severe and our treatments for heart failure were so bad prior to the ACE inhibitors, yeah, exactly.
DR. CIFU: Okay, fingerprints, so we're up to common misconceptions.
DR. STERN: Okay. So one I've already mentioned, but I'll just give a little bit more detail, drinking excessive water during exercise is not good. It's a common misconception that it is good. In the Boston Marathon in 2002, they looked at the sodium levels of a bunch of the athletes who volunteered and 13% of them were mildly hyponatremic at the end, 0.6% were severely hyponatremic, and all the athletes who were hyponatremic gained weight, two kilos during running 26 miles.
DR. STERN: I mean, think about that. You've just run 26 miles and you've gained four pounds. It's got to be water, right? So don't overdo it, it's dangerous.
DR. CIFU: I did while I was reading this chapter, I circled a thing where I guess you mentioned that a good way to avoid this is to weigh yourself before and after exercise. I was like, come on, nobody does that.
DR. STERN: Well, actually, I have done that. [both chuckle]
DR. STERN: So as you know, I'm an avid cyclist, and on very long rides, you know, where I'm out for hours and hours, and I wonder if I'm hydrating adequately, I've measured myself before and after it.
DR. CIFU: [chuckles] You're not someone to base anything on.
DR. STERN: [chuckles] Well, I'm just saying it can be done.
DR. CIFU: Okay, obviously, we've talked a lot about things because your common misconception is something we've mentioned earlier, mine is too, mine is that it's all about the labs. So often people get so caught up in the urine sodium, and the urine osms, and the plasma osms, when in fact, the diagnosis is staring you in the face, and if you had just taken a good history and done a physical examination, you'd know exactly what was going on, and very often what you need to do without all that.
DR. CIFU: So those things are very important, but probably not the most important thing. The one thing I was going to clarify as part of this is that one place that the labs really do help is that urine sodium to figure out if someone's volume deplete, and just the numbers, I really use extremes of that. So I say that if a urine sodium is less than 20, that person's dry, and if it's greater than 30, that person's probably not, and in between, I'm a little confused, but whatever.
DR. STERN: I will say that's only useful in the euvolemic patients.
DR. CIFU: Right.
DR. STERN: Because if you have bad heart failure, and you're not on a diuretic, you reabsorb sodium, and it would be confusing, so to your point initially, first, look at the person.
DR. CIFU: Absolutely.
DR. STERN: All right, pet peeves, so my first pet peeve is the common assumption that everybody who is hyponatremic has SIADH because my last two patients who were diagnosed in the hospital as having SIADH in fact, didn't, one had a pituitary tumor with secondary adrenal insufficiency, the other one was another patient with water intoxication. So it is common that people often default to SIADH, you really need to go through the process.
DR. CIFU: And maybe I'll follow that up with a pet peeve that just sort of takes that all a step further. So suppose you've done what you just did, you do a very good job, you actually make a diagnosis of SIADH, that would be you have a euvolemic person, you've got urine osms greater than serum osms, maybe you fluid restrict this person to get their ADH more in line with their volume status, and their sodium improves and you're like, ha! This person has SIADH.
DR. CIFU: You don't stop there, right? Why does this person have SIADH? You got to figure it out. And sometimes it's obvious, sometimes this person has pneumonia and that's it, but maybe there's something bad hiding there. And they've got, you know, SIADH as a paraneoplastic syndrome.
DR. STERN: Yeah, that's a really great point. I'm glad you mentioned that, thanks.
DR. CIFU: Clinical pearls.
DR. STERN: All right, so let's see. The first one is, interestingly enough, it's somewhat shocking but the physiology is interesting, but since we're talking too much, I won't go into it. The thiazide diuretics can obviously cause hyponatremia, interestingly enough, the loop diuretics won't.
DR. CIFU: And it's little old ladies, right?
DR. STERN: Often little old ladies.
DR. CIFU: Okay, LOL. For me, and I don't know if this will be helpful to anybody or everybody or whatever, it helps me. ADH for some reason, antidiuretic hormone has always sort of bugged me, I find it confusing, it's like a double negative or something, I don't know. I do better with thinking in terms of function, and you sort of mentioned this. So what I remember is that when osmolality serum osmolality or sodium rises, then ADH rises, aquaporins also rise, you put more aquaporins in, that brings in more water and so your osms fall.
DR. CIFU: On the other hand, everything goes in reverse when osms are low, ADH levels fall and aquaporins are withdrawn and so then you absorb less water. I like to remind myself that the other name for ADH is vasopressin, right? Vasopressin. That reminds me that ADH release is also stimulated by hypotension as we talked about before, you know, low blood pressure, you need something to increase blood pressure, vasopressin.
DR. STERN: Right. No, I think that's spot on.
DR. CIFU: You got another?
DR. STERN: I do have another. So we don't normally talk about treatment in our podcasts here, but I think in hyponatremia we would be doing a disservice not to mention treatment. And the key about treatment, if you remember nothing else from this podcast is, you can actually do much more harm treating hyponatremia than in most of the other disorders we talk about. Over-rapid correction of hyponatremia can cause myelin in the brain to lyse, and where a lot of that myelin is, is in the brainstem and you can actually make someone lyse their myelin and then be completely quadriplegic.
DR. STERN: The syndrome is called osmotic demyelinisation syndrome. So unless you're an expert in treating hyponatremia, you should get experts involved, and the aim is to go slow because if you go fast and you might think you're fixing the lab, you can actually really permanently and irrevocably harm a patient.
DR. CIFU: Now, you can go backwards, right? Is that true?
DR. STERN: It is true, you don't want to try it, but there is data to- When you say go backwards, let's explain what that means. So if you overly correct someone, there is data, especially if they start getting symptomatic, that you can give them D5W and reverse it. But nonetheless, you don't want to make this mistake. I'll say one more thing about that, where you're likely to make this mistake is the patient who is hypovolemic and hyponatremic.
DR. STERN: So the person who's had a lot of diarrhea who's been drinking water and the water is getting reabsorbed because their ADH is high, right? Now you give them a liter of normal saline and you fix their volume status. It's not that that liter of normal saline brings up their sodium that much, it's that you suppress their ADH. Now they diurese all that water and they get central pontine myelinolysis or osmotic demyelinisation syndrome.
DR. CIFU: They've changed the name, huh?
DR. STERN: They have changed the name, they have.
DR. CIFU: Kind of drives me crazy, too. I'm going to throw out kind of on maybe the same theme about treatment, that another common therapeutic error is to fail to appreciate that normal saline can cause sodium levels to drop in patients with SIADH, right? This is because their ADH is elevated, that's the SIADH, and that causes them to hold on to the water in their normal saline, but they still excrete the salt.
DR. CIFU: So don't assume that normal saline makes everybody with hyponatremia better, because often - and sometimes it helps with the diagnosis - in someone who's got SIADH, you give them saline, their sodium will drop, which kind of makes no sense because you're like, you know, I'm giving them something with 150 mEq/L of saline, why the heck is their sodium going down? And it's because, you know, they're not just a bucket, right?
DR. CIFU: They're a human who's making too much ADH-
DR. STERN: Right, they reabsorb that water and secrete the salt, totally. Well, I think that's it for our clinical pearls. It's fun talking about hyponatremia. And we certainly hope we didn't confuse you. [chuckles]
DR. CIFU: It might be fun for you to talk about hyponatremia.
DR. STERN: I like hyponatremia.
DR. CIFU: I know, you're obsessed with it. So we hope you found this episode of S2D, the Symptom to Diagnosis podcast useful and a bit enjoyable. As a reminder, our textbook, Symptom to Diagnosis: An Evidence-Based Guide which I think we shamelessly self-promoted during this episode [chuckles] the book takes a much deeper dive into how to think about and reason through the diagnosis of medical presentations.
DR. CIFU: The book is available in print through all the usual places, on your mobile device and also available and fully searchable via the Access Medicine website, available worldwide from McGraw Hill. I actually looked something up on it just last night.
DR. STERN: You did?
DR. CIFU: I did.
DR. CIFU: It's very useful.
DR. STERN: Well, that's good.
DR. CIFU: And as a reminder, as always, the music for the S2D podcast is courtesy of Dr. Maylyn Martinez.
DR. STERN: Thank you. [upbeat outro music] [upbeat outro music fading]