Renowned pediatric nephrologist, Michelle Starr, returns to the podcast to help unravel and simplify the approach to pediatric hyponatremia, providing valuable insights into diagnosis and treatment strategies. Tune in to elevate your salt game to the next level!
- Hyponatremia is all about water balance.
- Before working up hyponatremia, one should first confirm a patient is truly hyponatremic by sending a serum sodium and serum osm.
- To narrow the differential diagnosis, look at the urine osm to determine whether ADH is active or inactive. Urine osm < 200 points towards an ADH inactive state, while > 250 is consistent with active ADH.
- The most common cause of euvolemic hyponatremia is SIADH.
- It is recommended to correct sodium by no more than ~ 8-10 mEq per day.
Sodium, Osmolality, and Tonicity
- We use serum sodium as a surrogate marker for osmolality, which refers to the concentration of all particles dissolved in the body’s serum. Remember, the equation for osmolality includes 2x the serum sodium concentration.
- Tonicity is closely related to osmolality, but specifically refers to only those particles in the serum that are osmotically active (i.e. sodium and glucose).
- Sodium, osmolality, and tonicity are important because they impact the movement of water into tissues.
- Remember, serum sodium is actually a reflection of water balance!
Key Historical Items
- Is the patient’s hyponatremia acute or chronic? If acute, the likelihood that a child will present with symptoms is much higher.
- Review past medical history. Ask about predisposing conditions that may contribute to development of hyponatremia (congenital adrenal hyperplasia, CNS anomalies).
- Thorough dietary history (water intake and carbohydrate vs. protein consumption).
- Review of symptoms for volume status. Ask about vomiting, diarrhea, PO intake, number of wet diapers to determine whether the patient might be dehydrated.
- Are parents using formula to feed their child? If so, ask about how they’re mixing it!
- Is the patient symptomatic? Think about irritability/neurologic changes (i.e. seizures).
Determining the Type of Hyponatremia
- In order to do this, you must first consider whether you’re dealing with a true hyponatremia or pseudohyponatremia. In order to differentiate between these two entities, it’s helpful to re-order a serum sodium along with a serum osm.
- 3 buckets of hyponatremia based on serum osm: 1. Normal serum osm (280-290), 2. Elevated serum osm (> 290 ), and 3. Low serum osm (< 280).
- If normal osm with low serum sodium, this is not a true hyponatremia. Rather, this is a lab error as the machine measures sodium per plasma volume, but reports the value as sodium per plasma water (i.e. the machine assumes plasma water and plasma volume are in an equilibrium, which isn’t always true). This is sometimes referred to as “pseudohyponatremia.” Examples include hyponatremia in the context of hypertriglyceridemia or paraproteinemia.
- In patients with low sodium and elevated serum osm (i.e. hypertonic hyponatremia), there are separate osmotically active particles such as glucose that cause water to shift out of cells and into the plasma. This is a true hyponatremia and should be treated as such.
Approach to Hypotonic Hyponatremia
- Patients with low serum sodium and low serum osm (< 280) are said to have hypotonic hyponatremia. Within this category, there are many causes of hyponatremia, which we can differentiate from one another based on the patient’s ADH and volume status.
- Hypotonic hyponatremia may be caused by both ADH-independent as well as ADH-dependent states (see below).
- Lab workup: serum sodium, serum osm, urine osm, urinalysis for urine specific gravity, +/- urine sodium.
- Use patient weight and vital signs to assist in determining volume status. Remember, a thorough history (see above) can often give you clues about a patient’s volume status.
Antidiuretic Hormone (ADH) Status
- Determine whether the hyponatremia is ADH-dependent or ADH-independent.
- A low urine osm points towards an ADH-independent state and a high urine osm reflects an ADH-dependent state. This is because ADH under normal physiologic conditions leads to free water retention, and therefore a higher urine osm.
- Urine specific gravity can be a helpful surrogate for urine concentrating ability in lieu of urine osm.
- For ADH-independent (ADH “off”) causes of hyponatremia, expected urine osm is usually < 200 with specific gravity < 1.010. In this case, your kidneys are appropriately trying to excrete excess free water.
- For ADH-dependent (ADH “on”) causes of hyponatremia, expected urine osm is usually > 250 with specific gravity > 1.020.
- A urine sodium may help confirm a patient’s volume status, but is unlikely to change management and may confound the picture if discordant with the urine osm.
- Consider sending a TSH and/or cortisol level if labs are consistent with an ADH-dependent cause of hyponatremia and history is suggestive of possible thyroid or adrenal pathology.
- Hyponatremia is related to water balance and tells us that there is too much water in the body.
- This is usually secondary to 1 of 3 reasons: 1. Too much water in, 2. Not enough water out, or 3. Normal water balance with dysregulated ADH.
- ADH-independent causes include: Primary polydipsia, tea and toast diet, renal failure.
- ADH-dependent causes can be further subdivided into 3 categories based on volume status: 1. Hypovolemic, 2. Euvolemic, and 3. Hypervolemic.
- If a patient is hypovolemic, the differential includes both renal and GI losses, as well as other pathologic states that can lead to dehydration (i.e. burns).
- Causes of hyponatremia in a euvolemic patient include SIADH, adrenal insufficiency, and hypothyroidism.
- SIADH is often triggered by respiratory illness, infection, surgery, pain, anxiety, or medications such as SSRIs, anticonvulsants, and antiretroviral therapy. If the hyponatremia is not improving with treatment of the underlying cause, it is also important to broaden your differential to include malignancy and/or CNS lesion(s).
- Hypervolemic hyponatremia refers to a state in which total body fluid is elevated, but the kidneys aren’t necessarily “seeing” this volume as the effective arterial blood volume is decreased. This occurs in diseases such as heart failure, liver failure/cirrhosis, and nephrotic syndrome.
- Neonates are unable to concentrate urine as efficiently as older children (they are less responsive to ADH). We are therefore more likely to see ADH-independent causes of hyponatremia in this age group. Dr. Starr uses a urine osm threshold of ~150 to determine whether a neonate is ADH-responsive.
- If asymptomatic, there is no urgency to fix the sodium. In fact, aggressive normalization of the sodium may lead to patient harm, particularly if the hyponatremia is chronic.
- If symptomatic (altered mental status, seizures, etc.), need to correct sodium urgently.
- For the asymptomatic patient who we suspect has a more chronic hyponatremia, you usually want to raise the sodium slowly. In general, a cutoff of raising the sodium no more than 10 mEq in a 24 hour period is an acceptable standard. Dr. Starr’s approach is to aim for an increase in roughly 5-6 mEq per day to account for correcting faster than expected.
- While the patient is admitted, you should check serum sodium at least every 4-6 hours (or more frequently if clinically indicated) as you want to avoid overcorrection which can lead to central pontine myelinolysis.
- Remember, IV fluids are a drug. If you are suspicious of SIADH, patients should be fluid-restricted as the first step. As long as fluid out > fluid in, patients will eventually re-achieve sodium balance.
- Dr. Starr’s approach is to closely monitor daily output and restrict fluid by a number less than the output amount. Some providers will restrict intake to 60% of the urine output, but this isn’t necessarily evidence-based.
- Salt tabs are also useful in conjunction with fluid restriction for treatment of SIADH. Salt tabs increase a patient’s daily solute load which drives the kidneys to clear more free water. You can use either NaCl or urea in order to achieve this.
- In patients with a readily identifiable cause of SIADH, you can sometimes tolerate lower sodium levels (i.e. > 130) while treating the underlying cause and following serial lab values. This may help avoid complications associated with aggressive sodium correction.
Listeners will understand the basic approach, workup, and management of hypotonic hyponatremia in order to improve inpatient clinical care.
After listening to this episode listeners will…
- Be able to distinguish the difference between tonicity and osmolality, and appreciate how this applies to the workup of hyponatremia.
- Be familiar with the different types of hyponatremia, and how to distinguish between them based on serum osm.
- Recognize hypotonic hyponatremia and be able to determine whether it is ADH-dependent, or ADH-independent.
- Remember the 3 categories of ADH-dependent hypotonic hyponatremia, and list the differential diagnosis for each based on the patient’s volume status.
- Feel comfortable initiating treatment for hyponatremia and monitoring sodium values based on the specific clinical circumstance.
Dr Starr reports no relevant financial disclosures. The Cribsiders report no relevant financial disclosures.
Wilson S, Starr M, Masur S, Chiu C, Berk J. “#96: Saltshakers and Stethoscopes: A Practical Approach to Hyponatremia”. The Cribsiders Pediatric Podcast. https:/www.thecribsiders.com/ October 25, 2023.