Diabetes Insipidus Lecture Notes

Key Information

• Lecturer: Tom from Zero Finals
• Resource: Zero Finals website and Medicine book.

Diabetes Insipidus Overview

• Cause: Lack of Antidiuretic Hormone (ADH) or lack of response to ADH.
• Function of ADH: Acts on kidney collecting ducts to reabsorb water from urine.
• Symptoms:
  • Polyuria (excessive urination)
  • Polydipsia (excessive thirst)
  • Dehydration
  • Postural hypotension
  • Possible hypernatremia (high blood sodium concentration)

Types of Diabetes Insipidus

Nephrogenic Diabetes Insipidus

• Cause: Kidneys don't respond to ADH.
• Factors:
  • Drug-induced (e.g., Lithium for bipolar disorder)
  • Genetic mutations (e.g., AVPR2 gene on the X chromosome)
  • Chronic kidney disease
  • Electrolyte imbalances (hypokalemia, hypercalcemia)

Cranial Diabetes Insipidus

• Cause: Insufficient ADH production by the hypothalamus.
• Factors:
  • Idiopathic (unknown cause)
  • Brain tumors
  • Head injuries
  • Brain malformations
  • Infections (e.g., meningitis, TB)
  • Brain surgery or radiotherapy

Key Differential Diagnosis

• Primary Polydipsia: Excessive water intake leading to polyuria and polydipsia without diabetes insipidus.

Investigation & Diagnosis

Tests

• Blood Tests: Check kidney function, look for hypernatremia.
• Urine Osmolality: Low due to dilution by excessive water.
• Serum Osmolality: High due to concentrated blood solutes.
• Water Deprivation Test (Desmopressin Stimulation Test):
  • Fluid avoidance for 8 hours.
  • Measure urine osmolality before and after synthetic ADH (desmopressin).

Interpretation of Water Deprivation Test

• Cranial Diabetes Insipidus:
  • Low urine osmolality after deprivation, high after ADH.
• Nephrogenic Diabetes Insipidus:
  • Low urine osmolality after deprivation, remains low after ADH.
• Primary Polydipsia:
  • High urine osmolality after deprivation, remains high.

Treatment

• General Approach: Correct underlying cause if possible.
• Cranial Diabetes Insipidus: Desmopressin to replace missing ADH.
• Nephrogenic Diabetes Insipidus: High doses of desmopressin, requires close monitoring.

Additional Resources

• Zero Finals Website: Comprehensive notes, illustrations, test questions, blog articles.
• YouTube Channel: Thumbs up, comments, and subscriptions for future updates.