SIADH

Question 1

What maintain serum sodium concentration?

Answer 1

• Thirst
• ADH
• RAAS
• Renal handling

Question 2

What is the most common electrolyte abnormality in clinical practice?

Answer 2

• Hyponatraemia

Question 3

What causes SIADH?

Answer 3

• Malignancy
  
  • small-cell lung cancer
  • pancreas
• CNS disorder
  
  • meningoencephalitis
  • abcess, stroke, subarachnoid/subdural haemorrhage
• Endocrine
  
  • hypothyroidism
• Drugs
  
  • opiates
  • psychotropics
  • SSRI
  • cytotoxic

Question 4

What is the Px of SIADH leading to hyponatraemia?

Answer 4

1. Excessive ADH > excessive water reabsoprtion in collecting ducts
2. Water dilutes Na in blood > euvolaemic hyponatraemia
3. Urine becomes more concentrated > high urine osmolality

Question 5

What are the sx of SIADH?

Answer 5

• Mild - anorexia, headache, nausea, vomiting, lethargy.
• Moderate - personality change, muscle cramps and weakness, confusion, ataxia.
• Severe - drowsiness, seizures

Question 6

How would you diagnose SIADH?

Answer 6

• Urine
  
  • Na > 20mmol/L
  • osmolality > 100mosmol/kg
• Plasma
  
  • Na < 125mmol/L
  • osmolality < 260mosmol/kg

Question 8

What other Ix are performed to exclude other causes?

Answer 8

• Serum potassium - if raised, think Addison’s disease
• TSH, T3, T4 - exclude hypothyroidism
• Random cortisol test
• ACTH stimulation test

Question 9

How would you mx SIADH?

Answer 9

*Treat the cause

• Correct sodium slowly - prevent central pontine myelinolysis
  
  • >10mmol/L increase over 24h
• Salt + loop diuretics
• Fluid restrict
• Demeclocycline
• Vaptans

Question 10

What is central pontine myelinolysis (osmotic demyelination syndrome)?

Answer 10

• Cx of long term severe hyponatraemia (<120mmols/L) being treated too quickly (>10mmol/L inc in 24h)

Question 11

What is the Px of CPM?

Answer 11

1. Blood Na falls > water moves by osmosis across BBB into brain cells
2. Brain cells swell
3. Brain adapts by reducing solutes in cells (takes few days)
4. If blood Na rapidly rise > water rapidly shift out of cells into blood
5. First phase: encephalopathic, confused, N & V
6. Second phase: demyelination of neurones

Question 12

How will Central Pontine Myelinolysis present?

Answer 12

• spastic quadriparesis
• pseudobulbar palsy
• cognitive behavioural change