Approach to hyponatremia (pediatrics): 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.

Okay, if a pediatric patient presents with a chief concern suggestive of 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. 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! If these findings are present, emergent administration of hypertonic saline might be necessary.

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. Lab results will reveal a sodium level lower than 135 milliequivalents per liter. With these findings, consider hyponatremia and order a plasma osmolality to help you determine the underlying cause.

Let’s take a look when the plasma osmolality is above 295 milliosmoles per kilogram. In this case, the plasma is considered hypertonic and indicates the presence of other osmotically active solutes, most commonly glucose. In this case, consider hyperglycemia, so review the blood glucose from the BMP. If glucose is elevated, diagnose hyperosmolality due to hyperglycemia.

Here’s a clinical pearl to keep in mind! 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 milligram per deciliter increase in serum glucose, serum sodium falls by about 1.6 milliequivalents per liter.

On the other hand, if the plasma osmolality is between 280 and 295 milliosmoles per kilogram, consider pseudohyponatremia. Next, order serum triglycerides and serum protein. If either of these are elevated, diagnose pseudohyponatremia, which 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 plasma osmolality is below 280 milliosmoles per kilogram, the plasma is hypotonic, and you can confirm the diagnosis of hyponatremia. Next, assess the patient’s volume status.

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 to help you determine the underlying cause.

If urine sodium level is less than or equal to 20 millimoles or milliequivalents per liter, consider extrarenal causes of sodium loss. First, if your patient has a recent history of vomiting, diarrhea, or nasogastric suction, the underlying cause of hyponatremia is probably gastrointestinal loss of sodium.

On the other hand, if your patient has extensive burns or has been sweating excessively, they likely have insensible fluid losses. Moreover, children with cystic fibrosis are susceptible to hyponatremia due to sodium loss through sweat.

Finally, if your patient has a condition like pancreatitis, small bowel obstruction, or a severe allergic reaction, 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 of sodium loss, 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 of sodium loss.

First, let’s discuss preterm or very-low-birthweight infants or VLBW. In this case, hyponatremia is likely due to prematurity, since the kidneys of preterm infants are underdeveloped and have a limited ability to reabsorb sodium.

On the other hand, if your patient has recently taken thiazide, loop, or other diuretic, then your patient’s hyponatremia is likely the result of diuretic use.

Next, your patient might present with a known history of congenital adrenal hyperplasia, low blood pressure, and hyperkalemia. In this case, the underlying cause of hyponatremia is likely mineralocorticoid deficiency related to the salt-wasting forms of congenital adrenal hyperplasia, such as 21-hydroxylase deficiency.