SIADH

Question 1

What is the pathophysiology behind SIADH?

Answer 1

Increased levels of ADH leading to excessive water absorption in collecting ducts due to increased binding to V2 receptors which increases the number of aquaporin channels in the membrane
-causes dilution of serum sodium= hyponatraemia BUT not sufficient retention to be classified as fluid overload

Question 2

What changes to serum and urine osmolality and sodium levels would you expect?

Answer 2

Urine:
High osmolality (>100mOsm/kg)
High sodium

Serum 
Low osmolality (<260mOsm/kg)
Low sodium (<135)

Question 3

What are the causes of SIADH?

Answer 3

Post-operative from major surgery 
Atypical pneumonia 
Head injury 
Medications= thiazide diuretics/carbamazepine/vincristine/antipsychotics/SSRIs/NSAIDs 
SCC= paraneoplastic complication 
Meningitis

Question 4

How might someone with SIADH present?

Answer 4

Headaches

Fatigue

Muscle aches and cramps

Confusion

Severe hyponatraemia
-can cause seizures and decreased consciousness

Question 5

How do you diagnose SIADH?

Answer 5

Diagnosis of exclusion

• euvolaemic on examination
• hyponatraemia
• high urine sodium and osmolality
• exclusion of other causes of hyponatraemia

CXR= pneumonia/lung abscess/SCC

CT head= head trauma/tumour effect pituitary

Question 6

How do you manage SIADH?

Answer 6

Fluid restriction
-0.5-1 litre a day

Tolvaptan
-ADH blockers (can rapidly increased serum sodium so need to be approved by specialist when hyponatraemia has persisted despite fluid restriction

Demeclocycline= tetracycline Abx which inhibits ADH (rarely used now we have vaptans)

Review medications to see if possible cause or exacerbating of hyponatraemia (SSRIs/opiates/PPIs/anti-epileptics)

Question 7

Why does central pontine myelinolysis occur in SIADH?

Answer 7

Decreased plasma sodium concentration leads to increased osmotic pressure leading to water moving by osmosis across BBB and causing brain to swell

Brain compensates by reducing solutes in brain cells to try and balance the osmotic gradient and reduce brain swelling. Therefore after few days of hyonatraemia, brain cells will have decreased low osmolality.

Problem occurs when plasma sodium levels rise quickly because this causes water to move from brain cells (now low osmolality) into blood

Question 8

What are the 2 phases of symptoms in central pontine myelinolysis?

Answer 8

1st
-encephalopathic and confusion due to electrolyte imbalance= headache/N+V

2nd

• due to demyelination of neurones in pons
• spastic quadriparesis
• pseudobulbar palsy
• cognitive and behavioural changes