Hypotonic Hyponatremia

🧂Hypotonic Hyponatremia

<280 mOsm/kg (hypotonic), urine osmolality >100 mOsm/kg, and urinary sodium level ≥20 mEq/L.

🚰 Primary (Psychogenic) Polydipsia

Case:
A 45-year-old woman with psychiatric history (e.g., schizophrenia) presents with excess water drinking, dry mouth, and seizures. On exam: moist mucous membranes and normal skin turgor. Labs show hyponatremia, urine osmolality <100.

🛠️ Management:
- Restrict water intake.
- Psychiatric evaluation.

⚠️ Complications: High risk of rapid correction; risk of osmotic demyelination syndrome.

🔎 High-Yield Fact (HYF):
Water deprivation test can help distinguish from DI. Free water restriction normalizes serum sodium because kidneys concentrate urine normally.

🩸 True Hypovolemia

Case:
A 30-year-old woman with vomiting and diarrhea. On exam: dry mucous membranes, ↓ skin turgor. Labs: urine osmolality >100, urine Na <30.

🛠️ Management:
- IV fluids (NS or LR).
- Correct underlying cause.
- Antiemetics or antidiarrheals if needed.

⚠️ Complications: Acute kidney injury.

🔎 HYF:
If volume loss suspected, empirically trial IV fluids. Persistent hyponatremia? Test urine Na. Urine Na <30 confirms active RAAS (kidney conserving sodium).

❤️ Heart Failure, Cirrhosis

Case:
A 75-year-old man with CHF or cirrhosis, presenting with SOB and peripheral edema. On exam: JVD, crackles, edema. Labs: hyponatremia.

🛠️ Management:
- Treat exacerbations with loop diuretics, water restriction.
- Cirrhosis management: spironolactone + furosemide.

⚠️ Complications: Cardiorenal or hepatorenal syndrome.

🔎 HYF:
Urine Na low (<30) and urine osmolality >100 suggest volume overload rather than depletion.

💊 Diuretic-Induced Hyponatremia

Case:
A 55-year-old man with heart failure worsening after diuretic dose increase. On exam: dry mucous membranes, ↓ skin turgor. Labs: urine Na >30.

🛠️ Management:
- Hold diuretics.
- IV fluids if necessary.

⚠️ Complications: Worsening of heart failure if diuretics are stopped without monitoring.

🔎 HYF:
Urine Na >30 indicates RAAS not active. Important to differentiate diuretic hyponatremia from SIADH or adrenal failure.

🧠 Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH)

Case:
A 54-year-old man with smoking history presents with confusion, weight loss, hyponatremia (serum sodium 121). On exam: euvolemic, normal skin turgor. Labs: urine osmolality 250, urine sodium 120.

🛠️ Management:
- Fluid restriction.
- Hypertonic saline if severe hyponatremia (careful correction!).
- Loop diuretics and salt tablets.
- Consider ADH antagonists (e.g., conivaptan, tolvaptan).

⚠️ Complications: Osmotic demyelination if corrected too fast!

🔎 HYF:
Common in lung cancers (small cell carcinoma), CNS diseases, pulmonary infections, and medications (antidepressants, antipsychotics).

🥣 Decreased Solute Intake

Case:
A 75-year-old woman with poor social support, fatigue, weight loss. On exam: moist mucous membranes, normal turgor. Labs: hyponatremia with urine osmolality <100.

🛠️ Management:
- Increase solute intake via normal diet.
- Assess social support needs.

⚠️ Complications: High risk of rapid correction; osmotic demyelination.

🔎 HYF:
Classic in elderly ("tea and toast" diet) and chronic alcoholics ("beer potomania").

📚 Important References:

Hypothyroidism: See pp. 137-139.
Adrenal Insufficiency: See pp. 154-156.