10.3 Disorders of Sodium and Water Balance: SIADH, DI & CSW

10.3 Disorders of Sodium and Water Balance: SIADH, DI & CSW

Clinical Scenario: A 54-year-old male with an aneurysmal subarachnoid hemorrhage (Hunt-Hess 3, Fisher 3) post-coiling develops somnolence on post-bleed day 5. Over the past 12 hours, urine output has totaled 3,600 mL (300 mL/hr). Vital signs: BP 94/56 mmHg, HR 112 bpm, RR 18, and CVP 3 mmHg (baseline 8 mmHg). Exam shows dry mucous membranes and flat neck veins. Labs: serum sodium 126 mEq/L, potassium 4.4 mEq/L, serum osmolality 266 mOsm/kg, urine sodium 92 mEq/L, urine osmolality 510 mOsm/kg, BUN 26 mg/dL, and creatinine 0.9 mg/dL.

Diagnostic Differentiation Matrix

Differentiating SIADH, Diabetes Insipidus (DI), and Cerebral Salt Wasting (CSW) requires integrating volume status and urinary indices:

Syndrome of Inappropriate ADH (SIADH) Management

SIADH results from non-osmotic ADH release caused by CNS disorders (stroke, trauma), pulmonary disease (small cell cancer), or medications (SSRIs, carbamazepine):

1. Pathophysiology: ADH stimulates V2 receptors in collecting ducts, causing aquaporin-2 channel insertion and water reabsorption without sodium. Expanding intravascular volume triggers secondary natriuresis ($U_{Na} > 40$ mEq/L), maintaining euvolemia while worsening hyponatremia.
2. Fluid Restriction: The cornerstone of therapy is fluid restriction to 500–1,000 mL/day for mild hyponatremia.
3. Solute Loading & Loop Diuretics: Oral sodium chloride tablets (1–2 g TID) combined with furosemide (20–40 mg PO/IV) disrupt the medullary gradient, forcing electrolyte-free water clearance.
4. Vasopressin V2 Receptor Antagonists: Tolvaptan (15–30 mg PO daily) or Conivaptan (20 mg IV load, then 20–40 mg/day continuous IV) block V2 receptors, promoting aquaresis. Discontinue fluid restriction when administering vaptans.
5. Severe Symptomatic Hyponatremia (Seizures, Coma):
   • Infuse 3% Hypertonic Saline 100 mL IV over 10 minutes, repeatable twice at 10-minute intervals if seizures persist.
   • The immediate goal is an acute rise of 4–6 mEq/L to arrest brain swelling.
   • Correction Ceiling: Strictly limit sodium correction to $\le 8–10$ mEq/L in 24 hours (and $\le 6–8$ mEq/L in high-risk patients: alcoholism, malnutrition, baseline $Na^+ < 105$ mEq/L). Exceeding these limits risks Osmotic Demyelination Syndrome (ODS), manifesting 2–7 days later with dysarthria, dysphagia, and locked-in syndrome.

Diabetes Insipidus (DI): Central vs. Nephrogenic

Diabetes Insipidus arises from defective ADH synthesis/release (Central DI) or renal collecting duct resistance (Nephrogenic DI):

• Central DI: Caused by hypothalamic-hypophyseal disruption (pituitary surgery, severe TBI, skull fractures, brain death). Post-surgical DI may exhibit a triphasic pattern: acute polyuria, transient interphase SIADH, and permanent central DI.
• Nephrogenic DI: Triggered by chronic lithium therapy, hypercalcemia, hypokalemia, or urinary obstruction.
• Desmopressin (DDAVP) Challenge: In polyuric hypernatremic patients, administer DDAVP 2 mcg IV/subQ. Central DI demonstrates a $> 50\%$ increase in urine osmolality and sharp drop in urine volume. Nephrogenic DI exhibits minimal response ($< 10–45\%$ increase).
• Free Water Deficit Replacement: $\text{FWD} = \text{TBW} \times \left( \frac{\text{Current } [\text{Na}^+]}{140} - 1 \right)$ where $\text{TBW} = \text{weight (kg)} \times 0.6$ (men) or $0.5$ (women). If intravascularly depleted, restore circulation with 0.9% saline first. Then replace the water deficit and ongoing losses with oral/enteral water or appropriate hypotonic IV fluid, adjusting to sodium and urine output. Check sodium about every 4 hours during resuscitation; generally limit the fall to 10 mEq/L/24 h. Recalculate fluid needs when DDAVP reduces urine losses.
• Pharmacotherapy:
  • Central DI: For acute decompensation, desmopressin 1–2 mcg IV/IM with close sodium/fluid monitoring. Consider another dose when high-volume dilute urine returns. Avoid automatic repeat doses or continued high fluid replacement after antidiuresis; maintenance route and schedule are individualized.
  • Nephrogenic DI: Correct electrolytes and stop culprit drugs. Administer Hydrochlorothiazide 25–50 mg daily, Amiloride 5–10 mg daily (blocks lithium uptake via ENaC), and Indomethacin 50 mg PO TID.

Cerebral Salt Wasting (CSW) vs SIADH: The Critical Pitfall

Cerebral Salt Wasting is a primary natriuretic syndrome triggered by intracranial injury, most commonly aneurysmal subarachnoid hemorrhage (SAH):

• Pathophysiology: Cerebral injury provokes BNP release or impairs sympathetic tone, crippling tubular sodium reabsorption. This drives true intravascular hypovolemia and depletional hyponatremia.
• Distinguishing CSW from SIADH: Both present with hyponatremia, high urine sodium ($U_{Na} > 40$ mEq/L), and concentrated urine ($U_{osm} > 300$ mOsm/kg). Assess volume status from the combined findings: CSW produces negative fluid balance and hypovolemia, supported by tachycardia, orthostasis, flat neck veins and relevant laboratory trends. SIADH is clinically euvolemic after other causes are excluded. CVP alone cannot reliably distinguish these states.
• Treatment of CSW:
  • Volume and Sodium Repletion: Resuscitation with 0.9% Normal Saline or 3% Hypertonic Saline to restore intravascular euvolemia and replenish sodium.
  • Mineralocorticoid Therapy: Fludrocortisone acetate 0.1–0.4 mg/day orally promotes distal tubular sodium retention.