SIADH

Question 1

Definition

Answer 1

Excess release of ADH, dilute euvolemia

Question 2

Epidemiology

Answer 2

SIADH is the most common cause of hyponatremia in patients with cancer = 30% of hyponatremia cases
+ 50

Question 3

Risk factors

Answer 3

Pulmonary conditions
Nursing home residents
Malignancy
Medications associated with SIADH induction
CNS disorder
Post operative state
Endurance exercise

Question 4

Aetiology (SIADH)

Answer 4

SCLC (and other tumours = prostate, pancreatic)
Infection/immunosuppressed = TB, pneumonia, meningitis
Abscesses
Drugs - SSRIS, Sulfonylureas, Carbamazepine
Head trauma

Question 5

Pathology

Answer 5

Increase of ADH independent of RAAS
= Increased insertion of aquaporin 2 channel in apical membrane of the collecting duct = increased BP by water retention
= Vessel vasoconstriction (therefore increase BP)
Excess water retention = dilute blood plasma = hyponatremia as Na+ will decrease = water resorption is not significant enough to cause fluid overload = euvolemic hyponatremia
Urine more concentrated as less water is excreted by the kidney = high urine sodium

Question 6

Signs CRABS-O

Answer 6

Confusion
Raised JVP
Ascites
BRAINSTEM HERNIATION
Severe hyponatremia can cause seizures and reduced GCS
-
Oedema

Question 7

Symptoms

Answer 7

Nausea and Vomiting
Headache
Lethargy
Muscle aches + cramps
Weakness

Question 8

How does brainstem herniation happen

Answer 8

Low Na+ means increased compensatory H20 = enters skull + increased ICP
- Causes hyponatremic encephalopathy
- Risk of brainstem herniating through foramen magnum (tentorial herniation)

Question 9

Diagnosis

Answer 9

No single test to conclusively diagnose SIADH.
Blood test = Low Na+ and NORMAL K+ (serum), low serum osmolality < 280 mOsm/kg
Urinary tests: high urine osmolarity > 100 mOsm/kg
Urine sodium = high urine sodium > 40 mmol/L
DIAGNOSTIC TEST = Give 0.9% saline
- Na+ depletion -> serum will normalize
- SIADH -> Serum fails to normalize

Question 10

Treatment

Answer 10

ACUTE = (<48 Hrs onset)
Fluid restriction + hypertonic saline (3%) - slow infusion to avoid complications = Concentrate blood
Treat underlying cause (tumor excision)
CHRONIC CASES = Drugs:
- FUROSEMIDE (diuretic)
- TOLVAPAN (Vasopressin antagonist) - severe symp.
- DEMECLOCYCLINE (Tetracyclic antibiotic)