HYPONATREMIA

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Hyponatremia (Na⁺ <135 mEq/L)



I. Why is it important?

  • Most common electrolyte abnormality in hospitalized patients.

  • Can be life-threatening → cerebral edema, seizures, herniation.

  • Management depends not just on the number but also:

    • Severity of symptoms

    • Chronicity (acute vs chronic)

    • Volume status

II. Pathophysiology

  • Serum sodium reflects the ratio of total body sodium + potassium to total body water.

  • Hyponatremia usually = excess water relative to sodium, not just sodium loss.

  • Mechanisms:

    1. Excess ADH secretion (SIADH, cirrhosis, CHF, pain, nausea)

    2. Increased water intake (primary polydipsia, low solute diet “tea & toast”)

    3. Renal water handling defect

III. Classification

1. Based on severity

  • Mild: 130–134 mEq/L

  • Moderate: 125–129 mEq/L

  • Severe: <125 mEq/L

2. Based on duration

  • Acute (<48 hrs): brain has not yet adapted → high risk of cerebral edema.

  • Chronic (>48 hrs): brain adapts (↓ intracellular osmolytes) → milder symptoms, but risk of osmotic demyelination if corrected too fast.

3. Based on symptoms

  • Mild: nausea, headache, lethargy

  • Moderate: confusion, vomiting, weakness

  • Severe: seizures, coma, respiratory arrest, herniation

4. Based on serum osmolality

  1. Hypotonic hyponatremia (true hyponatremia)

    • Most common & clinically relevant.

    • Serum osmolality <275 mOsm/kg.

  2. Isotonic hyponatremia (pseudohyponatremia)

    • Due to high lipids or proteins interfering with measurement.

  3. Hypertonic hyponatremia

    • Due to hyperglycemia, mannitol → water shifts from cells → dilutes Na⁺.

IV. Hypotonic Hyponatremia → Evaluate Volume Status

1. Hypovolemic hyponatremia (Na⁺ & water loss, more Na⁺ lost than water)

  • Causes:

    • Renal: diuretics (esp. thiazides), adrenal insufficiency, salt-wasting nephropathy.

    • Extrarenal: vomiting, diarrhea, sweating, burns.

  • Urine sodium:

    • <20 mEq/L → extrarenal loss (GI, skin).

    • 20 mEq/L → renal loss (diuretics, adrenal insufficiency).

  • Treatment: Isotonic saline (0.9% NaCl).

2. Euvolemic hyponatremia (normal volume, water gain without edema)

  • Causes:

    • SIADH (small cell lung cancer, CNS disease, drugs)

    • Hypothyroidism

    • Secondary adrenal insufficiency

    • Primary polydipsia / low solute intake (beer potomania, tea & toast diet)

  • Labs:

    • Urine osmolality >100 mOsm/kg (in SIADH)

    • Urine sodium >40 mEq/L (SIADH)

    • Urine osmolality <100 (primary polydipsia, low solute)

  • Treatment:

    • Fluid restriction (<800 mL/day)

    • Salt tablets, oral urea, loop diuretics (with salt)

    • Demeclocycline or vasopressin antagonists (tolvaptan) in refractory cases

3. Hypervolemic hyponatremia (Na⁺ & water retention, more water than Na⁺)

  • Causes:

    • Cirrhosis

    • Congestive heart failure

    • Nephrotic syndrome

    • Advanced kidney failure

  • Labs:

    • Urine sodium <20 (extrarenal cause, eg CHF, cirrhosis)

    • Urine sodium >20 (renal failure)

  • Treatment:

    • Diuretics (spironolactone + loop in cirrhosis)

    • Stop thiazides

    • Avoid antihypertensives that worsen hypotension

    • Vasoconstrictors (midodrine, terlipressin in cirrhosis/HRS)

    • Fluid restriction usually not helpful except severe cases

V. Emergency Management (Severe Symptomatic Hyponatremia)

  • Indications: seizures, coma, Na⁺ <120 mEq/L.

  • Treatment:

    • 3% hypertonic saline IV bolus (100 mL over 10 min, repeat if needed).

    • Goal: ↑ serum Na⁺ by 4–6 mEq/L in first 24 hrs (enough to stop symptoms).

    • Never correct >8–10 mEq/L in 24 hrs → risk of osmotic demyelination (central pontine myelinolysis).

VI. Key Diagnostic Algorithm (Stepwise)

  1. Confirm true hyponatremia: check serum osmolality.

  2. If hypotonic → assess volume status (clinical + urine sodium/osmolality).

  3. Narrow down cause:

    • Hypovolemic (renal vs extrarenal)

    • Euvolemic (SIADH, hypothyroid, adrenal, polydipsia)

    • Hypervolemic (CHF, cirrhosis, nephrotic, renal failure)

VII. High-Yield Exam Pearls

  • SIADH: low serum osmolality, high urine osmolality, high urine sodium.

  • Primary polydipsia: low serum osmolality, urine osmolality <100.

  • Thiazide diuretics: classic cause of hypovolemic hyponatremia.

  • Correction speed: max 8–10 mEq/L per 24 hrs.

  • Hypertonic saline: reserved for severe symptomatic cases.

✅ In short:

  • Mild = usually no urgent therapy.

  • Severe/symptomatic = hypertonic saline, careful correction.

  • Always identify underlying cause by assessing volume status + urine studies.

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