Approach to hyponatremia: Clinical sciences

Hyponatremia is a common electrolyte disturbance in which the serum sodium concentration is less than 135 milliequivalents per liter. Several mechanisms can contribute to hyponatremia, including increased serum levels of antidiuretic hormone, or ADH, increased renal sensitivity to ADH, excessive free water intake, and low solute intake. Now, based on the underlying cause, hyponatremia can be categorized as hypovolemic, euvolemic, and hypervolemic.

Now, if your patient presents with hyponatremia, you should first perform an ABCDE assessment to determine if they are unstable or stable.

If unstable, stabilize the airway, breathing, and circulation; obtain IV access and consider giving your patient IV fluids. You can also consider central venous access. Next, put your patient on continuous vital sign monitoring, including blood pressure, heart rate, and pulse oximetry, and don’t forget to monitor your patient’s urine output! Finally, if needed, provide supplemental oxygen.

Now, here’s a high-yield fact to keep in mind! Unstable individuals with hyponatremia often have a sodium level below 125 milliequivalents per liter, which can result in cerebral edema and severe clinical manifestations, like seizures or even respiratory arrest!

Now, let’s go back to the ABCDE assessment and look at stable patients. In this case, obtain a focused history and physical examination and order a basic metabolic panel or BMP. Your patient may report symptoms like headache, nausea, vomiting, or confusion; with lab results revealing a sodium level lower than 135 milliequivalents per liter. If these findings are present, your first step is to assess for true hyponatremia, so order a plasma osmolality.

Let’s first look at a plasma osmolality that’s greater than 280 milliosmoles per kilogram. This could be either normal if it’s between 280 and 295, or hypertonic when it’s higher than 295. If the plasma osmolality is between 280 and 295, consider pseudohyponatremia.

Next, order serum triglycerides and serum protein, and if either is elevated, diagnose pseudohyponatremia. Pseudohyponatremia is a lab artifact caused by high serum levels of triglycerides or protein, both of which can displace free water in the plasma, causing inaccurate measurement of sodium levels despite normal serum osmolality.

On the other hand, if the measured plasma osmolality is above 295 milliosmoles per liter, consider hyperosmolality. This indicates the presence of other osmotically active solutes, most commonly glucose or mannitol. Plasma hypertonicity, or hyperosmolality, causes an osmotic shift of water from the intracellular space to the extracellular fluid, resulting in dilutional hyponatremia. When you see an elevated plasma osmolality in the setting of hyponatremia, always make sure to check the serum glucose concentration! For every 100 milligrams per deciliter increase in serum glucose, serum sodium falls by about 1.6 milliequivalents per liter. If hyperglycemia is present, you can diagnose hyperosmolality.

If the plasma osmolality is below 280 milliosmoles per kilogram, you can confirm the diagnosis of hyponatremia. Next, assess the patient’s volume status, which can help you determine the underlying cause.

First, let’s discuss hypovolemic individuals. Physical exam findings often include an elevated heart rate and an orthostatic drop in blood pressure. You may also notice sunken eyes, dry mucous membranes, and decreased skin turgor. These findings are highly suggestive of hypovolemic hyponatremia, so your next step is to order a urine sodium level.

Let’s take a look at the urine sodium levels. If urine sodium levels are less than 20 millimoles, or milliequivalents, per liter, consider extrarenal causes.

First, if your patient has a recent history of vomiting, diarrhea, or nasogastric suction, the underlying cause of hyponatremia is probably gastrointestinal losses.

On the other hand, if your patient has had extensive burns or if they have been sweating excessively, the likely underlying cause includes insensible losses. Individuals with cystic fibrosis in particular are susceptible to hyponatremia.

Finally, if your patient has a condition like pancreatitis, small bowel obstruction or SBO, or a severe allergic reaction, then the most likely cause of your patient’s hyponatremia is third spacing. Third spacing refers to the accumulation of extracellular fluid within extravascular spaces, such as the bowel lumen or peritoneum.

Alright, now that we’ve discussed extrarenal causes, let’s go back and discuss patients whose urine sodium is greater than 20 millimoles per liter.

In these patients, you should consider renal causes. First, if your patient has a recent history of thiazide, furosemide, or other diuretic use, then your patient’s hyponatremia is likely the result of their diuretics.

Next, if physical examination reveals low blood pressure, and lab results are significant for hyperkalemia, the underlying cause of hyponatremia is likely mineralocorticoid deficiency. This is commonly seen in individuals with primary adrenal insufficiency or hypoaldosteronism.

Now, if your patient has a history of renal tubular acidosis, or if labs demonstrate an elevated urine bicarbonate level, as well as metabolic alkalosis, then your patient’s hyponatremia is likely the result of bicarbonaturia.

On the other hand, a condition affecting the central nervous system, like meningitis, head trauma, or recent brain surgery, in combination with an acute increase in urine output is highly suggestive of cerebral salt wasting.

Finally, if your patient reports abdominal or flank pain with a diminished urinary stream, and the physical exam reveals a palpable distended urinary bladder, it’s likely that obstructive uropathy is the underlying cause.

Okay, let’s go back and look at the patient’s volume status, but this time let's consider euvolemic individuals.

Physical exam usually reveals a normal heart rate with no orthostatic drop in blood pressure. In this case, mucous membranes are moist, the skin turgor is normal, and there are no signs of hypervolemia, such as ascites or subcutaneous edema. This spectrum of findings is highly suggestive of euvolemia, so your next step is to order urine sodium and random urine osmolality.

If urine sodium is greater than 20 millimoles per liter, and random urine osmolality is greater than 100, order labs.