Approach to Hyponatremia

Last week we discussed hyponatremia in morning report and noon rounds. The focus was having an approach to hyponatremia to help you know how to work up and manage it.

Approach to hyponatremia

Hyponatremia tells you nothing about the total body salt (i.e. the volume status). Hyponatremia is more about an imbalance in free water compared to salt. What this means is that there is too much water for how much sodium there is in the blood (i.e. the sodium concentration is low). In order for this to happen there needs to be (1) free water intake and (2) ADH.

What to think about when faced with hyponatremia:  

1. Is it real?

- Recheck the sodium and send a serum osmolality

- Usually hyponatremia is hypo-osmolar hyponatremia.

- This means the serum osmolality is low

- If the serum osmolality is not low, you need to think of causes of hyperosmolar hyponatremia (i.e. hyperglycemia (for each 10 mmol/L the glucose is above normal the serum sodium lowers by 3) or mannitol, etc.) or euosmolar hyponatremia (rare – causes include paraproteinemia or hyperlipidemia).

2. Is it acute or chronic? Is it symptomatic?

While we will not get into treatment of hyponatremia in this blog, these are important questions to ask to know how fast you need to correct the sodium and if there are concerns about rapid correction (overrapid correction of chronic hyponatremia can cause osmotic demyelination syndrome)

3. What is the patient’s volume status? This is a common framework that is used and helps greatly with the differential diagnosis and ultimately how you will manage the hyponatremia.

Hypovolemic:

Differential diagnosis includes:

Renal losses (i.e. diuretic use)

GI losses (vomiting and diarrhea)

Other losses (third spacing (pancreatitis), sweating, poor intake)

Euvolemic:

Differential diagnosis includes:

SIADH: malignancy, brain processes, lung processes, pain, post-operative states

Medications: many can cause hyponatremia. Classic examples include thiazide diuretics and SSRIs

Endocrine: hypothyroidism, adrenal insufficiency

*Don’t forget to include TSH and 8am cortisol as part of a work up for euvolemic hyponatremia*

Other: psychogenic polydipsia, low solute intake (beer potomania, “tea and toast”), osmostat reset

Hypervolemic:  

Differential diagnosis includes:

Heart Failure, Cirrhosis, Nephrotic  Syndrome

4. How do the urine measurements help you?

Urine osmolality: if <100 this is consistent with a low ADH state which means there is another reason for the hyponatremia (i.e. excess water intake (i.e. psychogenic polydipsia) or low solute intake (i.e. beer potomonia or “tea and toast”))

Urine Sodium <20 mEq/L: this is consistent with a hypovolemic hyponatremia

Urine Sodium >40 mEq/L: this is more consistent with SIADH

*note: if a patient is on a diuretic it makes the urine electrolytes difficult to interpret