Overview
This lecture reviews the key differences between Syndrome of Inappropriate Antidiuretic Hormone (SIADH) and Diabetes Insipidus (DI), focusing on pathophysiology, hallmark findings, nursing priorities, and related exam questions.
Pathophysiology & Memory Tricks
- SIADH: Excess ADH leads to water retention—“soaked inside.”
- DI: Lack of ADH causes massive water loss—“dry inside.”
- ADH (antidiuretic hormone) is made in the posterior pituitary and retains water in the body.
- Synthetic ADH drugs end in “-pressin” (e.g., desmopressin, vasopressin).
SIADH Key Features (7 S's)
- Stop urinating: Very low urine output, thick/sticky (high specific gravity >1.030).
- Soaked inside: Fluid overload with swelling/edema.
- Low labs: Low serum osmolality and hyponatremia (sodium <135).
- Swelling in brain: Headaches, confusion, agitation—early signs of hyponatremia.
- Seizures: Due to low sodium; institute seizure precautions.
- Severely high BP: Fluid overload increases blood pressure.
- Salt supplementation: May require 3% saline for severe hyponatremia; diuretics rarely used.
Diabetes Insipidus (DI) Key Features (7 D's)
- Diuresis/Draining: Excessive urine output (up to 20L/day), very dilute (low specific gravity <1.005).
- Dry inside: Dehydration, dry mucous membranes, poor skin turgor.
- High & dry labs: High serum osmolality and hypernatremia (sodium >145).
- Drink more: Increased thirst is common.
- Decreased BP: Hypotension due to volume loss.
- Desmopressin: Synthetic ADH reduces urine output, increases BP; risk of hyponatremia if overdosed.
- Damage: Often caused by pituitary or brain damage (tumors, trauma, post-surgery).
Causes
- SIADH: Small cell lung cancer, certain lung tumors, CNS trauma, infections (meningitis, pneumonia).
- DI: Brain/pituitary trauma, tumors, post-brain surgery, increased intracranial pressure.
Nursing Care & Key Exam Concepts
- Strictly monitor intake/output (I&Os) and daily weights (not weekly).
- SIADH: Restrict fluids, monitor sodium, seizure precautions, supplement sodium with IV saline if needed.
- DI: Replace lost fluids, monitor for dehydration and hypernatremia, administer desmopressin as ordered.
Key Terms & Definitions
- ADH (Antidiuretic Hormone) — Hormone that retains water in the body, produced in the posterior pituitary.
- Serum Osmolality — Measure of blood concentration; low in SIADH, high in DI.
- Specific Gravity — Urine concentration; high in SIADH, low in DI.
- Hyponatremia — Low blood sodium (<135), seen in SIADH.
- Hypernatremia — High blood sodium (>145), seen in DI.
- Desmopressin — Synthetic ADH used to treat DI.
Action Items / Next Steps
- Review SIADH and DI study guides.
- Practice answering NCLEX-style questions on SIADH and DI.
- Memorize the 7 S's for SIADH and 7 D's for DI for quick recall.
- Strictly monitor daily weights and intake/output for patients with fluid imbalances.