SIADH and Diabetes Insipidus Flashcards

Altered Hormonal and Metabolic Regulations

Syndrome of Inappropriate Antidiuretic Hormone (SIADH)

• Excessive release of Antidiuretic Hormone (ADH).
• Water and sodium retention.
• Hyponatremia: Increased water retention without a corresponding increase in sodium (Na+). This dilutes the blood, leading to decreased Na+ levels.
• ↑ H_2O ↓ Na+
• Concentrated urine.
• Euvolemia: Increased fluid volume.

Details of SIADH

• Excessive ADH secretion leads to hyponatremia with low serum osmolality.
• Hypervolemia is often transient due to natriuretic mechanisms.
  • Natriuretic mechanisms are the body's way of trying to restore normal fluid balance by increasing sodium and water excretion.
• Aquaporins: Water channels that are affected by ADH, leading to increased water reabsorption in the kidneys.

Causes of SIADH

• CNS disorders
• Tumors (Small Cell Lung Cancer - SCLC)
• Drugs: Analgesics, antiseizure medications, SSRIs, antipsychotics.
• Surgery
• Infections: Tuberculosis (TB), Pneumonia, HIV.
• Inherited forms

Symptoms of SIADH

• Asymptomatic cases.
• Nausea and vomiting.
• Obtundation (decreased alertness).
• Headaches.
• Seizures.
• Respiratory arrest.
• Coma.
• Excess water causes brain swelling and neuron dysfunction.

Management of SIADH

• Treat the underlying cause.
• ADH antagonists.
• Limit fluid intake.

Lab values: SIADH vs DI

Management of SIADH

• Treat underlying cause: Causes can include pain, primary lung pathology, post-operative phenomenon, medications, etc. Treating the underlying cause is the only management strategy that addresses the root problem, rather than just managing symptoms.
• Free water restriction: Restrict intake to 1 to 1.5L per day. This can be difficult for patients to maintain long-term.
• Salt tablets: Start with 1g NaCl PO TID (three times a day). With lower Na, increase the number and frequency of salt tablets administered. Can cause volume overload.
• Urea powder: 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 given the potential for it to be metabolized into ammonium by urease-producing bacteria in the colon.
• 0.9% Normal Saline: If Urine osmolality < 538 AND UNa+ + UK+ < 154, can try giving a 250cc bolus of 0.9% NS.
• Vasopressor receptor antagonists: Block ADH receptor. Has many side effects.

Diabetes Insipidus

• A condition that causes extreme thirst and excessive urination.
• The kidney is unable to conserve water.
• Does not cause high blood glucose levels.
• Deficiency of ADH (Vasopressin).

Symptoms of Diabetes Insipidus

• Extreme thirst.
• The need to urinate frequently.
• Having to get up several times at night to urinate or wet the bed.
• High output of very light-colored or clear urine each time you urinate.

Types and Causes of Diabetes Insipidus

1. Central DI:
   • Brain Tumors
   • Pituitary surgery
   • Brain injuries
   • Infection and inflammation (meningitis)
   • Autoimmune disorders
2. Nephrogenic DI:
   • Medications (lithium)
   • Kidney disease
   • Genetic disorders
3. Gestational DI:
   • Breakdown of vasopressin by the placenta during pregnancy, but usually resolves after delivery.
4. Dipsogenic DI:
   • Problems with the hypothalamus, mental health conditions (schizophrenia, other psychotic disorders).

Diagnosis of Diabetes Insipidus

• Exclude diabetes mellitus (urinalysis, capillary blood glucose).
• Exclude hypokalemia and hypercalcemia.
• Confirm true polyuria (24-hour urine volume >3L).
• Measure paired serum/urine osmolality.
• If 24h urine volume
• If 24h urine volume >2.5-3L, possible diabetes insipidus. Refer for specialist investigation.