SIADH

Question 1

Define SIADH

Answer 1

Syndrome of Inappropriate ADH

Plasma ADH/vasopressin is inappropriately high for the existing plasma osmolality. Characterised by hypotonic hyponatraemia, concentrated urine and a euvolaemic state in the absence of stimuli for secretion

Question 2

Aetiology of SIADH

Answer 2

Malignancy: small cell lung carcinoma, pancreas, prostate, thymus
CNS: Stroke, SAH/SDH, meningoencephalitis, head injury, SLE
Chest disease: TB, pneumonia, abscess, aspergillosis,
Endocrine: hypothyroidism, Addison’s
Drugs: opioids, psychotropics, SSRIs
Other: acute intermittent porphyria, trauma, major abdominal or thoracic surgery, HIV

Question 3

Symptoms of SIADH

Answer 3

Mild (125-135) - asymptomatic

Nausea and vomiting
Altered mental status
Headache
Muscle cramp/weakness
Irritability 
Confusion
Drowsiness
Seizure 
Coma 

P[Na+]<120mM = generalised weakness, poor mental function 
P[Na+}<110mM = confusion leading to coma and ultimately death

Question 4

Signs of SIADH on examination

Answer 4

Mild - no signs
Severe:
Hypo-reflexia
Extensor plantar reflexes

Question 5

Investigations for SIADH

Answer 5

Urinalysis: raised osmolality, raised sodium

Serum osmolality: reduced
U+Es: reduced sodium, reduced urea
TFTs: normal (exclude hypothyroidism)
Short synacthen test: exclude adrenal insufficiency
Renal function: exclude renal failure
Proteins and lipid profile + glucose: exclude pseudo-hyponatraemia

Diagnostic trial with normal saline infusion: serum sodium level does NOT improve after normal saline infusion

CXR/CT CAP/MRI: identify the cause

Question 6

Criteria for SIADH diagnosis

Answer 6

1. Reduced ↓ plasma osmolality
2. Reduced ↓ Na+
3. Raised ↑ urine osmolality (>100mosmol/kg, Na+ >20mmol/L)
4. Absence of hypovolaemia/hypotension, oedema, renal failure, adrenal insufficiency and hypothyroidism

Question 7

Management for SIADH

Answer 7

1. Address hyponatraemia -> fluid restriction
   
   • If ineffective -> democlocyclin
2. Treat underlying cause
3. Vasopressin (V2) receptor antagonists e.g. tolavaptan in chronic hyponatraemia

Severe (Seizure, coma): slow IV hypertonic 3% saline + furosemide

Question 8

Complications of SIADH

Answer 8

Convulsions
Coma
Death
Central pontine myelinolysis (quadriparesis, respiratory arrest, fits)

Question 9

Prognosis of SIADH

Answer 9

Depends on underlying cause
High morbidity and mortality if Na <110
central pontine myelinolysis mortality 50%