SIADH

Question 1

What is SIADH?

Answer 1

Continued secretion of ADH, despite the absence of normal stimuli for secretion (i.e. increased serum osmolality or decreased blood volume)

Question 2

List broad areas that when effected can lead to SIADH

Answer 2

Brain
Lungs
Drugs
Tumours
Metabolic state

Question 3

List 6 “brain” related causes of SIADH

Answer 3

Haemorrhage/thrombosis  
Meningitis 
Abscess 
Trauma  
Tumour  
Guillain-Barre syndrome

Question 4

What 4 pulmonary pathologies can lead to SIADH?

Answer 4

Pneumonia
TB
Abscess
Aspergillosis

Question 5

What cancerous pathologies can lead to SIADH?

Answer 5

Small cell lung caner  
Breast cancer 
Lymphoma  
Leukaemia  
Pancreatic cancer 
Prostate cancer 
Mesothelioma 
Sarcoma 
Thymoma

Question 6

List 4 drugs that may lead to SIADH

Answer 6

Vincristine
Opiates
Carbamazepine
Chlorpropamide

Question 7

What metabolic states may lead to SIADH?

Answer 7

Porphyria

Alcohol withdrawal

Question 8

Describe the epidemiology of SIADH

Answer 8

Hyponatraemia is the MOST COMMON electrolyte imbalance seen in hospital
< 50% of severe hyponatraemia is caused by SIADH

Question 9

List 9 symptoms of SIADH

Answer 9

Headache  
N+V 
Muscle cramp/ weakness 
Irritability  
Confusion  
Drowsiness  
Convulsions  
Coma  
Symptoms of underlying cause

Question 10

List signs of severe hyponatraemia

Answer 10

Reduced reflexes
Extensor plantar reflexes
Signs of underlying cause

Question 11

What is the hyponatraemia caused by in SIADH?

Answer 11

Dilution from excessive water reabsorption + not due to a decrease in total body Na+

Question 12

Why does SIADH cause a euvolaemic hyponatraemia?

Answer 12

Hypervolaemia causes heart distention, so BNP + ANP are secreted which cause salt+ water extcretion

Question 13

What investigations are performed in suspected SIADH?

Answer 13

Serum sodium
Serum osmolarity
Serum urea
Urine osmolarity
Creatinine (check renal function)
Glucose, serum protein + lipids: to exclude pseudohyponatraemia
Free T4 + TSH: hypothyroidism can cause hyponatraemia
Short synACTHen test: adrenal insufficiency can cause hyponatraemia

Question 14

What is Pseudohyponatraemia?

Answer 14

When Na+ concentration is actually normal but is erroneously reported as being low because of the presence of either hyperlipidaemia or hyperproteinaemia

Question 15

Describe the results necessary for diagnosis of SIADH

Answer 15

Low plasma osmolality  
Low serum Na+ conc.
High urine osmolality 
High urine Na+  
\+ Absence of hypovolaemia, oedema, renal failure, adrenal insufficiency + hypothyroidism

Question 16

List 3 investigations that may be used to identify the cause of SIADH

Answer 16

CXR
CT
MRI

Question 17

Describe the management of SIADH

Answer 17

1st line: Treat underlying cause
1st line: Fluid restriction to 1L per day
2nd line: Vasopressin receptor antagonists (e.g. tolvaptan)
3rd line: NaCl + furosemide
4th line: demeclocycline

Question 18

List 4 complications of SIADH

Answer 18

Convulsions
Coma
Death
Central pontine myelinolysis

Question 19

What is the prognosis of SIADH?

Answer 19

Depends on the CAUSE
Na+ < 110 mmol/L is associated with a HIGH MORBIDITY + MORTALITY
50% mortality with central pontine myelinolysis

Question 20

What is Central pontine myelinolysis? What is it caused by? What is it characterised by?

Answer 20

Demylination of pons
Occurs with rapid correction of hyponatraemia  
Quadriparesis  
Respiratory arrest  
Fits

Question 21

Describe the treatment of SIADH in severe/ acute cases

Answer 21

Slow IV hypertonic saline + furosemide with close monitoring