SIADH and Diabetes Insipidus Flashcards

Altered Hormonal and Metabolic Regulations

Syndrome of Inappropriate Antidiuretic Hormone (SIADH)

• Excessive ADH Release: SIADH is characterized by the excessive release of antidiuretic hormone (ADH).
• Water and Sodium Retention: This leads to water and sodium retention in the body.
• Hyponatremia: Increased water retention occurs without a corresponding increase in sodium, resulting in hyponatremia (low sodium levels in the blood). This dilutes the blood, causing a decrease in Na^+.
• Fluid Balance:
  • ↑ H_2O
  • ↓ Na^+
• Concentrated Urine: The urine becomes concentrated due to increased water retention.
• Euvolemia: Fluid volume increases, often resulting in euvolemia (increased fluid volume).

Pathophysiology and Symptoms

• Excessive ADH Secretion: SIADH involves excessive ADH secretion.
• Hyponatremia and Serum Osmolality: Hyponatremia is observed with low serum osmolality.
• Transient Hypervolemia: Hypervolemia (increased blood volume) is often transient due to natriuretic mechanisms.
• Water Reabsorption: ADH stimulates aquaporins in the kidneys, leading to increased water reabsorption.
• Urine Characteristics: Urine volume decreases, while urine osmolality increases.
• CNS Disorders: Central nervous system disorders can trigger SIADH.
• Tumors: Certain tumors, such as small cell lung cancer (SCLC), can cause SIADH.
• Drugs: Various drugs, including analgesics, antiseizure medications, SSRIs, and antipsychotics, can induce SIADH.
• Surgery: Post-operative states can lead to SIADH.
• Infections: Infections like tuberculosis (TB), pneumonia, and HIV are associated with SIADH.
• Inherited Forms: Inherited forms of SIADH exist.
• Neurological Symptoms: Symptoms range from asymptomatic states to nausea, vomiting, obtundation, headaches, seizures, respiratory arrest, and coma.
• Brain Swelling: Excessive water retention causes brain swelling, leading to neuron dysfunction.

Treatment

• ADH Antagonists: Medications that block the action of ADH.
• Treat the Cause: Address the underlying cause of SIADH.
• Limit Fluids: Restrict fluid intake.

Diagnostic values for SIADH vs Diabetes Insipidus (DI)

Mnemonic

SIADH is "Soaked Inside", while DI is "Dry Inside".

Management of SIADH

• Treat Underlying Cause: Address the primary condition causing SIADH, such as pain, lung pathology, post-operative state, or medications.
• Free Water Restriction: Limit fluid intake to 1 to 1.5 liters per day, but maintaining this restriction can be challenging for patients long-term.
• Salt Tablets:
  • Start with 1g NaCl PO TID (three times a day).
  • Increase the number and frequency of salt tablets with lower sodium levels.
  • Caution: Can cause volume overload.
• Urea Powder:
  • Administer 15-30g PO daily.
  • Induces osmotic water elimination by promoting passive sodium reabsorption in the ascending limb of the loop of Henle.
  • Contraindicated in cirrhosis due to potential metabolism into ammonium by urease-producing bacteria in the colon.
• 0. 9% Normal Saline:
  • If Urine osmolality < 538 mOsm/kg AND UNa+ + UK+ < 154 mEq/L, consider a 250cc bolus of 0.9% NS.
• Vasopressin Receptor Antagonists:
  • Block ADH receptor.
  • Associated with many side effects.

Diabetes Insipidus (DI)

• Definition: A condition characterized by extreme thirst and excessive urination.
• Kidney Function: The kidney is unable to conserve water.
• Blood Glucose: DI does not cause high blood glucose levels.
• ADH Deficiency: It results from a deficiency of ADH (vasopressin).

Symptoms of Diabetes Insipidus

• Extreme Thirst: Intense and persistent thirst.
• Frequent Urination: The need to urinate frequently.
• Nocturia: Frequent urination at night, leading to interrupted sleep or bedwetting.
• Urine Characteristics: High output of very light-colored or clear urine each time you urinate.

Types and Causes of Diabetes Insipidus

1. Central DI:
   • Causes: Brain tumors, pituitary surgery, brain injuries, infections and inflammation (meningitis), autoimmune disorders.
2. Nephrogenic DI:
   • Causes: Medications (lithium), kidney disease, genetic disorders.
3. Gestational DI:
   • Cause: Breakdown of vasopressin by the placenta during pregnancy; usually resolves after delivery.
4. Dipsogenic DI:
   • Causes: Problems with the hypothalamus, mental health conditions (schizophrenia, other psychotic disorders).

Diagnosis of Diabetes Insipidus

1. Initial Exclusions:
   • Exclude diabetes mellitus (urinalysis, capillary blood glucose).
   • Exclude hypokalemia and hypercalcemia.
2. Confirm True Polyuria:
   • Measure 24-hour urine volume to confirm polyuria (>3L).
3. Measure Paired Serum/Urine Osmolality:
   • 24-hour urine volume > 2.5-3L:
     • If serum osmolality > 295 mosmol/kg: Possible diabetes insipidus; refer for specialist investigation.
   • 24-hour urine volume < 2.5-3L:
     • Diabetes insipidus unlikely.