Syndrome Inappropriate ADH Secretion

Malignancy
1. Small Cell Lung Cancer (most common ectopic ADH source)
2. Duodenal Cancer
3. Pancreatic Cancer
4. Head and neck cancers
Lung disease
1. Bacterial Pneumonia
2. Empyema
3. Legionella pneumonia
4. Pulmonary Tuberculosis
5. Acute Respiratory Distress Syndrome (ARDS) or other Acute Respiratory Failure
Neurologic disorders
Miscellaneous causes
1. Psychiatric illness
2. Rocky Mountain Spotted Fever
  1. Hyponatremia is common in this uncommon Tick Borne Illness
3. Post-operative period
  1. Young women have postoperative ADH levels 40x higher than postmenopausal women
  2. Young women are at higher risk of postoperative hyponatremic encephalopathy

Core SIADH findings (see Barrter and Schwartz criteria below)
1. Hyponatremia
  1. See Isovolemic Hypoosmolar Hyponatremia
2. Urine Osmolality > 100 mOsm/kg water
  1. Dilution not maximized despite serum hypoosmolality
3. Urine Sodium > 20-30 mEq/L
4. Serum Osmolality decreased <275 to 280 mOsm
Other supportive features of SIDH diagnosis
1. Hypouricemia (Serum Uric Acid <4 mg/dl)
2. Fractional Excretion of Uric Acid >10%
3. Antidiuretic Hormone (ADH) inappropriately elevated (despite low Serum Osmolality)

Isovolemic Hypoosmolar Hyponatremia (Serum Osmolality <275 mOsm/kg)
Urine is NOT maximally dilute (Urine Osmolality >100 mOsm)
Urine Sodium excretion elevated (>20 to 30 mEq/L), lacking avid Sodium retention
Other causes absent
1. No advanced Kidney disease, Cirrhosis or Congestive Heart Failure
2. No uncorrected Hypothyroidism or Adrenal Insufficiency
3. No Diuretic use

Treating underlying cause is key in SIADH
1. Example: Treat underlying Bronchogenic Carcinoma
2. Relevant Consultation (e.g. oncology, nephrology)
Overall strategy
1. See Isovolemic Hypoosmolar Hyponatremia
2. Fluid restriction (<1000 to 1500 ml/day)
3. Diuretics
  1. Furosemide (Lasix)
  2. Potassium-Sparing Diuretics
4. Slow Serum Sodium correction
  1. See Hyponatremia Management
  2. Do not correct more quickly than 12 mEq/L in 24 hours or 18 mEq/L in 48 hours
5. Avoid Normal Saline infusion
  1. May worsen Hyponatremia with Fluid Shifts intravascularly
SIADH refractory to above measures
1. Induce Nephrogenic Diabetes Insipidus
2. Demeclocycline (600-1200 mg/day)
3. Lithium Carbonate 300 mg PO tid
4. Tolvaptan (Samsca)
  1. Selective Vasopressin (V2) receptor Antagonist
  2. Increases free water excretion
  3. Vaptan use is typically discouraged due to limited evidence of benefit and associated risk
  4. If used, started in hospital setting and NOT combined with water restriction
  5. Adverse effects: Hepatotoxicity, Hypernatremia, Cerebral Demyelination Syndrome