Renal · Year 2 · from Renal

Case 3: SIADH from Small Cell Lung Cancer

Patient Presentation

A 68-year-old male with a 50-pack-year smoking history presents with confusion and lethargy. His family notes progressive cognitive decline over 2 weeks.

History of Present Illness

  • 15-pound weight loss over 3 months
  • Chronic cough with occasional blood-tinged sputum
  • Progressive confusion and somnolence
  • Decreased appetite
  • No history of head trauma or recent illness

Physical Examination

  • Blood pressure: 128/78 mmHg
  • Heart rate: 72 bpm
  • No peripheral edema
  • Lungs: Decreased breath sounds right upper lobe
  • Neurologic: Oriented to person only, asterixis present
  • Euvolemic on examination

Workup

Laboratory Studies:

  • Serum sodium: 118 mEq/L (severely low)
  • Serum osmolality: 248 mOsm/kg (low)
  • Urine osmolality: 450 mOsm/kg (inappropriately concentrated)
  • Urine sodium: 65 mEq/L (elevated)
  • TSH: 2.1 mIU/L (normal)
  • Morning cortisol: 18 mcg/dL (normal)

Imaging:

  • Chest CT: 4 cm mass in right upper lobe with mediastinal lymphadenopathy
  • Brain MRI: No metastases

Biopsy:

  • Small cell lung carcinoma

Diagnosis

SIADH Secondary to Ectopic ADH Production from Small Cell Lung Cancer

Discussion

This case demonstrates inappropriate ADH secretion:

  • SIADH Pathophysiology: The lecture explains that ADH normally suppresses when plasma osmolality falls below 280-285 mOsm/kg. In SIADH, ADH continues to be secreted despite hypoosmolality, causing water retention and dilutional hyponatremia.
  • Urine Findings: The urine is inappropriately concentrated (>100 mOsm/kg when it should be maximally dilute) because ADH continues to drive AQP2 insertion and water reabsorption.
  • Euvolemia: The initial water retention causes mild volume expansion, which triggers natriuresis. The patient reaches a new steady state with hyponatremia but clinical euvolemia.

SIADH Diagnostic Criteria

  1. Serum osmolality <275 mOsm/kg
  2. Urine osmolality >100 mOsm/kg (inappropriately concentrated)
  3. Urine sodium >40 mEq/L
  4. Clinical euvolemia
  5. Normal thyroid and adrenal function
  6. No diuretic use

Treatment

  • Fluid restriction to 1000-1200 mL/day
  • Treat underlying malignancy (chemotherapy for SCLC)
  • For severe/symptomatic hyponatremia: Hypertonic saline (3% NaCl)
  • Consider tolvaptan (V2 receptor antagonist) if refractory
  • Correction rate: <8-10 mEq/L in 24 hours to avoid osmotic demyelination syndrome

Clinical Pearl

Small cell lung cancer is the classic malignancy causing ectopic ADH production. The mnemonic for SIADH causes: "SIADH" = Small cell lung cancer, Intracranial pathology, Aspirin (and other drugs), Drugs (SSRIs, carbamazepine), Head trauma.

Image Reference

For visual reference of concentration/dilution concepts, see:

  • Radiopaedia: Diabetes insipidus - Clinical and imaging information
  • Wikimedia Commons: Loop of Henle - Countercurrent mechanism diagrams
  • Radiopaedia: SIADH - Clinical presentation and causes

Learning Points

  1. Water Deprivation Test: Central DI responds to DDAVP with urine concentration; nephrogenic DI does not respond to DDAVP.
  1. Countercurrent Multiplication: The thick ascending limb creates a 200 mOsm/kg "single effect" that is multiplied by countercurrent flow geometry to achieve 1200 mOsm/kg at the papillary tip.
  1. Urea Recycling: Urea contributes ~50% of inner medullary osmolality. Protein malnutrition reduces urea and impairs concentrating ability.
  1. SIADH vs Hypovolemic Hyponatremia: Both have low serum sodium, but SIADH has high urine sodium (>40 mEq/L) while hypovolemic hyponatremia has low urine sodium (<20 mEq/L).
  1. Correction Rate: Rapid correction of chronic hyponatremia risks osmotic demyelination syndrome. Limit to 8-10 mEq/L per 24 hours.

All cases for this lecture as Markdown