Hyponatremia: Causes, Diagnosis, and Management for the USMLE

Hyponatremia (serum sodium < 135 mEq/L) is the most common electrolyte abnormality seen in hospitalized patients and frequently appears in USMLE Step 1 and Step 2 CK exams. This blog will cover the classification, causes, clinical presentation, diagnosis, and management of hyponatremia, along with KOTC science visuals to enhance retention.

Classification of Hyponatremia

Hyponatremia is categorized based on serum osmolality and volume status:

1. Hypotonic Hyponatremia (Most Common)

Serum Osmolality < 280 mOsm/kg

• Hypovolemic Hyponatremia (Low total body water and sodium)

  • Causes: Diuretics (thiazides), vomiting, diarrhea, burns, adrenal insufficiency

  • Diagnosis: Urine sodium < 20 mEq/L (extrarenal loss), > 20 mEq/L (renal loss)

  • Treatment: Isotonic saline to restore volume

• Euvolemic Hyponatremia (Normal total body sodium, increased free water)

  • Causes: SIADH (Syndrome of Inappropriate ADH), psychogenic polydipsia, hypothyroidism, adrenal insufficiency

  • Diagnosis: Urine osmolality > 100 mOsm/kg (SIADH), suppression test

  • Treatment: Fluid restriction, salt tablets, demeclocycline (for SIADH)

• Hypervolemic Hyponatremia (Excess total body water)

  • Causes: Heart failure, liver cirrhosis, nephrotic syndrome, chronic kidney disease

  • Diagnosis: Elevated JVP, edema, low urine sodium

  • Treatment: Fluid restriction, diuretics, correct underlying cause

2. Isotonic Hyponatremia (Pseudo-hyponatremia)

Serum Osmolality 280–295 mOsm/kg

• Causes: Severe hyperlipidemia or hyperproteinemia (multiple myeloma)

• Diagnosis: Normal serum osmolality with lab artifact

• Treatment: Address underlying disorder

3. Hypertonic Hyponatremia

Serum Osmolality > 295 mOsm/kg

• Causes: Hyperglycemia (DKA), mannitol administration

• Diagnosis: Corrected sodium calculation

• Treatment: Manage hyperglycemia or discontinue offending agent

Clinical Presentation of Hyponatremia

Symptoms vary based on severity and rate of onset:

• Mild (Na 130–135 mEq/L): Asymptomatic or mild nausea, headache

• Moderate (Na 120–129 mEq/L): Confusion, lethargy, muscle cramps

• Severe (Na < 120 mEq/L): Seizures, coma, brainstem herniation

🔎 USMLE Tip: Chronic hyponatremia (> 48 hrs) is less symptomatic than acute hyponatremia because the brain adapts by reducing intracellular osmolytes.

Diagnostic Workup for Hyponatremia

1. Serum Osmolality – Determines if hyponatremia is hypotonic, isotonic, or hypertonic

2. Urine Osmolality – Differentiates water retention causes (SIADH) vs. excessive water intake

3. Urine Sodium – Helps determine volume status (pre-renal vs. renal causes)

4. Assess Clinical Volume Status – Edema, JVP, orthostatic changes

Management of Hyponatremia

1. Correcting Sodium Safely

🚨 Avoid Osmotic Demyelination Syndrome (ODS) – Too rapid sodium correction (> 8–12 mEq/L per 24 hrs) can lead to central pontine myelinolysis.

🔹 Mild-Moderate Hyponatremia (Na > 120 mEq/L)

• Fluid restriction (SIADH, hypervolemic states)

• Oral sodium tablets (mild cases)

🔹 Severe Hyponatremia (Na < 120 mEq/L, symptomatic)

• 3% Hypertonic Saline (if seizures, coma, cerebral edema present)

• Frequent sodium monitoring (every 2–4 hours)

🔹 Chronic Hyponatremia

• SIADH: Fluid restriction, demeclocycline (ADH antagonist), Vaptans (vasopressin receptor antagonists)

• Hypovolemic Hyponatremia: Isotonic saline

• Hypervolemic Hyponatremia: Fluid restriction, diuretics

USMLE Question Strategy

💡 Key Clues in Vignettes:

• SIADH? Look for lung cancer (small cell carcinoma), CNS injury, or recent surgery.

• Hypovolemia? Look for diarrhea, vomiting, diuretics.

• Hypervolemia? Look for CHF, cirrhosis, nephrotic syndrome.

• Acute hyponatremia with seizures? Treat with hypertonic saline.

• Chronic hyponatremia? Correct slowly to prevent osmotic demyelination syndrome.

Conclusion

Hyponatremia is a high-yield electrolyte disorder frequently tested on USMLE exams. Understanding its classification, diagnosis, and safe correction strategies is essential for clinical decision-making.

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Frequently Asked Questions (FAQs)

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