Do Answer as a short note for final year MBBS Medicine exam in Kerala
I need a topic or question to answer! It seems your message may have been cut off.
Could you please share the specific topic or question you'd like me to cover as a short note for your Kerala final year MBBS Medicine exam?
SIADH
Reading File
Finding Sources
Finding Sources
Reading File
Reading File
Reading File
Reading File
Finding Sources
Reading File
I now have comprehensive information to write a thorough short note. Let me compile the answer.
SIADH (Syndrome of Inappropriate Antidiuretic Hormone Secretion)
Definition
SIADH is a condition in which ADH (vasopressin) is secreted autonomously - without an osmotic stimulus - leading to excess water retention, dilutional hyponatremia, and inappropriately concentrated urine despite low plasma osmolality. - Goldman-Cecil Medicine, p. 1201
Pathophysiology
- ADH acts on V2 receptors in the renal collecting duct, increasing aquaporin-2 channels and causing excess water reabsorption
- Plasma becomes hypo-osmolar; a subclinical increase in extracellular volume suppresses proximal tubular Na+ reabsorption
- Result: low serum Na+, low plasma osmolality, high urine osmolality, high urine Na+, low serum uric acid
- The normal negative feedback (low plasma osmolality suppressing ADH) is absent because ADH is secreted autonomously - Costanzo Physiology, p. 311
Causes (Mnemonic: PULMONARY CNS DRUG TUMOR)
Tumors (ectopic ADH):
- Bronchogenic carcinoma (most common) - especially SCLC
- Carcinoma of pancreas, duodenum, ureter
- Non-Hodgkin lymphoma, Hodgkin disease, thymoma
Pulmonary disease:
- Tuberculosis, pneumonia, lung abscess, aspergillosis
- Positive pressure mechanical ventilation
CNS disease:
- Meningitis, encephalitis, brain abscess, brain tumour
- Head injury, subarachnoid haemorrhage, Guillain-Barre syndrome
- Acute intermittent porphyria
Drugs:
- AVP analogues: desmopressin, oxytocin
- Carbamazepine, valproate (anticonvulsants)
- Antipsychotics, antidepressants (SSRIs, TCAs)
- Nicotine, opiates, MDMA (Ecstasy)
- Chlorpropamide, cyclophosphamide, vincristine
Diagnostic Criteria (Goldman-Cecil / Bartter-Schwartz Criteria)
| Criterion | Value |
|---|---|
| Plasma osmolality | < 275 mOsm/kg H2O |
| Urine osmolality | > 100 mOsm/kg H2O (inappropriately high) |
| Urine Na+ | > 20-30 mmol/L on normal salt/water intake |
| Clinical volume status | Euvolemic (no oedema, no dehydration) |
| Serum Na+ | < 136 mmol/L |
| Serum uric acid | Low (due to decreased proximal tubular reabsorption) |
| Absence of | Hypothyroidism, glucocorticoid deficiency, renal failure, diuretic use |
Clinical Features
Symptoms reflect the severity and rate of onset of hyponatraemia:
- Mild (Na+ 125-135): Nausea, malaise, headache, lethargy
- Moderate: Confusion, muscle cramps, weakness
- Severe/Acute (Na+ < 120 or rapid drop): Seizures, coma, cerebral oedema - can be fatal, especially in women of reproductive age
- Gradual onset allows osmotic adaptation; patients may be surprisingly asymptomatic even with Na+ < 120
Investigations
- Serum Na+ < 136 mmol/L
- Serum osmolality < 275 mOsm/kg
- Urine osmolality > 100 mOsm/kg (inappropriately concentrated)
- Urine Na+ > 20-30 mmol/L
- Serum uric acid - LOW (characteristic and differentiates from other causes of hyponatraemia like CCF, cirrhosis where uric acid is elevated)
- BUN/creatinine - normal
- TFT, cortisol - normal (to exclude hypothyroidism and Addison's)
Treatment
1. Treat the underlying cause
2. Fluid restriction (first-line for mild-moderate)
- Restrict fluid intake to less than urine output + insensible losses (~800-1000 mL/day)
- Gradual correction is essential to avoid osmotic demyelination syndrome (ODS) / Central Pontine Myelinolysis (CPM)
3. Rate of correction
- Target: 0.5-1 mmol/L per hour, maximum 8-10 mmol/L in 24 hours
- Do NOT exceed 12 mmol/L/day to prevent CPM
4. Specific treatments
| Severity | Treatment |
|---|---|
| Mild (asymptomatic) | Fluid restriction alone |
| Moderate | Fluid restriction + salt tablets ± demeclocycline |
| Severe / symptomatic (seizures, coma) | 3% hypertonic saline IV + IV furosemide |
| Chronic/resistant | Vaptans (tolvaptan 15 mg PO OD; conivaptan IV) |
5. Demeclocycline (150-300 mg 3-4 times daily)
- Inhibits ADH action on renal principal cells
- Used in chronic SIADH; onset of action slow (1-2 weeks)
- Largely replaced by vaptans now
6. Vaptans (V2 receptor antagonists)
- Tolvaptan 15 mg PO daily - selective V2 antagonist
- Conivaptan 20-120 mg PO or 10-40 mg IV - blocks V1A and V2
- Cause aquaresis (free water excretion without Na+ loss)
- Effective in euvolemic and hypervolemic hyponatraemia
Key Differentials
| Feature | SIADH | Volume Depletion | CCF/Cirrhosis |
|---|---|---|---|
| Volume status | Euvolemic | Hypovolemic | Hypervolemic |
| Urine Na+ | High (>20) | Low (<20) | Low (<20) |
| Serum uric acid | LOW | HIGH | HIGH |
| BUN/Creatinine | Normal | Elevated | Elevated |
Important Points for Exam
- Most common cause of euvolemic hyponatraemia
- SCLC is the most common malignancy causing SIADH
- Low serum uric acid is the key distinguishing lab finding vs. other causes of hyponatraemia
- Rapid correction of hyponatraemia causes Central Pontine Myelinolysis (osmotic demyelination) - characterised by paraparesis, dysarthria, dysphagia, locked-in state
- 3% NaCl is reserved for severe symptomatic hyponatraemia (seizures/coma only)
- Tolvaptan is FDA-approved for SIADH and hypervolemic hyponatraemia
Sources: Goldman-Cecil Medicine, p. 1200-1201 | Harrison's Principles of Internal Medicine 22E | Costanzo Physiology 7th Ed., p. 311
This is a shared conversation. Sign in to Orris to start your own chat.