Hyponatremia made easy—a simple diagnostic algorithm

Learn how to easily diagnose the etiology of hyponatremia. This video was taken from our CME accredited Fluids & Electrolytes Masterclass.

Joel Topf, MD
Joel Topf, MD
26th Sep 2017 • 4m read

When your patient has hyponatremia, the immediate question you must answer is, what's causing the low sodium? In this video from our Fluids and Electrolytes Masterclass, you’ll learn our simple algorithm for diagnosing the etiology of hyponatremia (and treating each case) in just a couple of steps.

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Video Transcript

[00:00:00] Hyponatremia diagnosis. So, the diagnosis of hyponatremia is really trivial. All you need to do is find a blood test with a sodium less than 135 and then you have your diagnosis of hyponatremia. But usually what people are talking about when they say what's the diagnosis is what is the etiology? What is driving that low sodium? What's the diagnosis behind the low sodium? And so, we can use an algorithmic approach to get that answer.

[00:00:30] And so, if once you have a sodium of 135, you have a diagnosis of hyponatremia then you want to check the serum osmolality, okay. So, that serum osmolality is low then you have true hyponatremia. If it's normal, you have pseudohyponatremia. This is the lab error, due to high fats or high proteins in the blood. And if you have a high osmolality, we have what we were calling a factitious hyponatremia where the sodium is truly low but it doesn't have the same implications as the

[00:01:00] true hyponatremia. Water is actually moving out of the cells rather than into the cells, then this will be due to elevated glucose or maybe Mannitol or maybe a glycine infusion during the urologic procedure. Both pseudohyponatremia and factitious hyponatremia make up false hyponatremia, indicating that these patients don't have water movement into the cells nor do they show any of the symptoms that are typical for hyponatremia.

[00:01:30] Driving down into true hyponatremia, the next step is to check the urine osmolality or the urine specific gravity. And what we're trying to do is we're trying to determine if there is much ADH activity going on. You can think of urine osmolality as being an ADH dipstick. If there is low urine osmolality, the lower the more accurate this is, you're going to have ADH independent disease, okay. And so, that will be tea and toast syndrome or beer drinkers potomania.

[00:02:00] These are due to low solute diets and they really respond quite briskly to IV fluids. And you want to treat them with a high solute or a high protein diet. Psychogenic polydipsia, these are the patients with compulsive water drinking. These patients can be tremendously ill. It's kind of a silly name. It seems like a silly disease but these are the patients that die of hyponatremia and need acute therapy. They respond to fluid restriction. Though, if these patients are symptomatic,

[00:02:30] they need to be treated with hypertonic saline. And then the renal failure patients, these are the patients that are on dialysis or very, very low urine outputs. They will have the urine osmolality as close to 300. These are kidneys that are so sick, they're not able to concentrate or dilute urine. And you're going to get urine that's very similar to the serum osmolality, and this responds to fluid restriction and dialysis treatment. Moving to the other side of the fence, we have the ADH

[00:03:00] dependent hyponatremias. These patients will have high osmolalities and again, the higher the osmolality that you measure in the urine, the more accurate this diagnosis is going to be. And the step here is you want to check the volume status because that's how we're going to divide this up. We have hypervolemic patients with heart failure, cirrhosis, and nephrotic syndrome. We have hypovolemic patients with GI losses, renal losses, and other losses. And we have euvolemic patients, hypothyroidism, adrenal insufficiency,

[00:03:30] and SIADH, syndrome of inappropriate antidiuretic hormone. But this is kind of a fiction because even experts are unable to make an accurate clinical assessment of volume status in more than 50% of cases. Essentially, it's just a coin toss whether you can accurately determine the volume status. And so, we're really going to rely on a more biochemical assessment here. And so, you now want to measure the urine sodium and the serum uric acid,

[00:04:00] okay. And so, the hypervolemic patients, they're going to have a urine sodium less than 20 and a high uric acid. Now, it may not be frankly high but it's going to be towards the upper range of normal. The hypovolemic patients will also have a urine sodium less than 20 and a high uric acid or a high normal uric acid. And finally, the euvolemic patients will have a urine sodium greater than 20 and a low uric acid. One of the things to be aware of

[00:04:30] is that when we say that experts are unable to differentiate volume status, we're not saying they can't differentiate hypervolemic from hypovolemic. We are really saying, they have a hard time differentiating euvolemic from hypovolemic or euvolemic from hypervolemic. I don't think people are going to have too much trouble, along with clinical background, to differentiate between decompensated heart failure and diarrhea. But the situations with SIADH is part of the differential, do become a little bit more tricky.

[00:05:00] Some other things to caution you about, the urine sodium is not going to be accurate in patients that have recently received diuretics, which is probably 100% of patients in heart failure. They could have an artificially elevated urine sodium, due to the diuretic effect. Secondly, patients with euvolemic hyponatremia who are on a very low sodium diet, maybe even NPO because they're in the hospital. Their urine sodium may not be greater than 20, they could have an

[00:05:30] artificially low urinary sodium, that can be a tricky part. Patients on treatment for gout may have an artificially low uric acid, due to treatments with drugs such as allopurinol. Thanks.

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