SIADH

Question 1

Define SIADH?

Answer 1

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

Question 2

What are some of the causes related to the brain that can cause SIADH?

Answer 2

Haemorrhage/thrombosis 
Meningitis 
Abscess 
Trauma 
Tumour
Gullain-Barre Syndrome

Question 3

What are some of the causes related to the lung that can cause SIADH?

Answer 3

Pneumonia
TB
Other: abscess, aspergillosis, small cell carcinoma

Question 4

What tumours can cause SIADH?

Answer 4

Small cell lung cancer
Lymphoma
Leukaemia
Pancreatic Cancer
Prostate Cancer 
Mesothelioma
Sarcoma 
Thymoma

Question 5

What drugs can cause SIADH?

Answer 5

Vincristine
Opiates
Carbamazepine
Chlorpropamide

Question 6

What are some of the Metabolic causes of SIADH?

Answer 6

Porphyria

Alcohol withdrawal

Question 7

What is the epidemiology of SIADH?

Answer 7

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

Question 8

What are the presenting symptoms of SIADH?

Answer 8

Mild hyponatraemia may be ASYMPTOMATIC 
Headache
Nausea/vomiting
Muscle cramp/weakness 
Irritability 
Confusion
Drowsiness 
Convulsions
Coma 
Symptoms of underlying cause

Question 9

What are the signs of SIADH on physical examination?

Answer 9

Mild Hyponatraemia - no signs
Severe Hyponatraemia - Reduced Reflexes, Extensor Plantar Reflexes
Signs of underlying cause

Question 10

Why is there Hyponatraemia in SIADH?

Answer 10

Due to dilution from excessive water reabsorption and not due to a decrease in total body Na+

Question 11

What investigations would you do for SIADH?

Answer 11

Check things
SIADH Diagnosis
Investigations for idendtifying the cause

Question 12

What do you check with SIADH?

Answer 12

Low serum sodium
Creatinine (check renal function)
Glucose, serum protein and lipids - to rule out pseudohyponatraemia 
Free T4 and TSH 
Short Synacthen Test

Question 13

What is Pseudohyponatraemia?

Answer 13

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

Question 14

Why do we do thyroid tests for SIADH?

Answer 14

Hypothyroidism can cause hyponatraemia

Question 15

Why do we do a Short Synacthen Test for SIADH?

Answer 15

Adrenal Insufficiency can cause hyponatraemia

Question 16

What investigations would you do to diagnose SIADH?

Answer 16

Low plasma osmolality
Low serum Na+ concentration
High urine osmolality
High urine Na+

Question 17

What is needed for the diagnosis of SIADH?

Answer 17

The presence of all the results earlier mentioned, and the absence of:

• Hypovolaemia
• Oedema
• Renal Failure
• Adrenal Insufficiency
• Hypothyroidism

Question 18

What are some examples of investigations do you do to identify the cause of SIADH?

Answer 18

CXR
CT
MRI

Question 19

What is the management plan for SIADH?

Answer 19

Treat underlying cause
Fluid restriction
Vasopressin receptor antagonists (e.g. tolvaptan)
In SEVERE cases - slow IV hypertonic saline and furosemide with close monitoring

Question 20

What are the possible complications of SIADH?

Answer 20

Convulsions
Coma
Death
Central pontine myelinolysis

Question 21

When does Central Pontine Myelinolysis occur?

Answer 21

With rapid correction of hyponatraemia

Question 22

What is Central Pontine Myelinolysis characterised by?

Answer 22

Quadriparesis
Respiratory arrest
Fits

Question 23

What is the prognosis for patients with SIADH?

Answer 23

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