Syndrome of inappropriate antidiuretic hormone syndrome (SIADH)

Question 1

What is syndrome of inappropirate antidiuretic hormone syndrome (SIADH)?

Answer 1

Increased antidiuretic hormone (ADH) release from posterior pituitary or an ectopic source

Question 2

What are some of the neurological causes of SIADH?

Answer 2

Meningitis, encephalitis, or cerebral abscess
Intracranial haemorrhage e.g. subarachnoid or subdural
Stroke

Question 3

What are some of the malignant causes of SIADH?

Answer 3

Small cell carcinoma of the lung (most common)

Others: breast cancer, head + neck tumours are rarer causes

Question 4

What are some of the infectious causes of SIADH?

Answer 4

Pneumonia

Tuberculosis

Question 5

What are some of the endocrine causes of SIADH?

Answer 5

Hypothyroidism

Question 6

What are some of the drugs that can cause SIADH?

Answer 6

SSRIs + TCAs
Proton pump inhibitors
Carbamazepine
Sulfonylureas (glimperide + glipizide)

Question 7

What are some of the miscellaneous causes of SIADH?

Answer 7

Porphyria

Positive end-expiratory pressure

Question 8

What is classified as mild SIADH and what are the symptoms?

Answer 8

Mild (130-135 mmol/L)

Nausea, vomiting, headache, lethargy

Question 9

What is classified as moderate SIADH and what are the symptoms?

Answer 9

Moderate (125-129 mmol/L)

Weakness, muscles aches, confusion, ataxia

Question 10

What is classified as severe SIADH and what are the symptoms?

Answer 10

Severe (<125 mmol/L)

Reduced consciousness, seizures, respiratory arrest

Question 11

What is the role of ADH in the body?

Answer 11

ADH is responsible for free water reabsorption by acting on the collecting ducts of the kidneys resulting in water retention (but not reabsorption of the solutes)

Question 12

What does an increase in ADH result in?

Answer 12

Increase in free water retention and therefore dilution of the blood and a decrease in solutes in the blood

Question 13

Why are patients with SIADH typically euvolaemic?

Answer 13

Water distributes throughout the entire intravascular and extravascular compartment

Question 14

In SIADH why is there a continuous ADH production irrespective of serum osmolarity?

Answer 14

SIADH results in a loss of an effective feedback mechanism, resulting in low serum sodium + relatively high urinary sodium with concentrated urine

Question 15

What is classified as hyponatremia?

Answer 15

Sodium concentration < 135 mmol/L

Question 16

What does it mean if a patient is euvolaemic?

Answer 16

Patient has normal circulatory or blood fluid volume within their body

Question 17

Clinical features in SIADH depend on degree of what?

Answer 17

Hyponatraemia + rate of change in serum sodium levels

Question 18

What is the essential diagnostic criteria for SIADH?

Answer 18

Low plasma osmolarity <275 mOsm/kg
High urine osmolarity > 100 mOsm/kg
High urine sodium >30 mmol/L
Clinical euvolaemia
Exculsion of glucocorticoid deficiency or hypothyroidism (rare)

Question 19

In health people why should low serum osmolarity cause low urine osmolarity?

Answer 19

Kidney attempts to retain solute

Question 20

What is the management of acute onset (<48 hrs) hyponatraemia?

Answer 20

Hypertonic (3%) saline

Question 21

What is the management of chronic onset (>48 hrs) hyponatraemia?

Answer 21

Correction should not occur too quickly - aim for maximum increase of 10 mmol/L per day

Question 22

What is the management of chronic onset (>48 hrs) hyponatraemia in mild to moderate asymptomatic cases?

Answer 22

Fluid restriction

Question 23

What is the management of chronic onset (>48 hrs) hyponatraemia in severe or symptomatic cases?

Answer 23

Demeclocycline or ADH receptor antagonists (e.g. tolvaptan) may be considered

N.B. Guidelines differ

Question 24

What are some of the complications of SIADH?

Answer 24

Cerebral odema

Central pontine myelinolysis

Question 25

How does SIADH cause central pontine myelinolysis?

Answer 25

Rapid correction of sodium can cause osmotic demyelination, particularly in chronic hyponatraemia Therefore slow correction is vital

Question 26

What are the symptoms of central pontine myelinolysis?

Answer 26

Tremors, dysarthria, quadriplegia, opthalmoplegia, seziures + extra pyramidal symptoms May result in 'locked-in syndrome' - patients are awake but unable to verbally communicate or move

Question 27

How is a diagnosis of central pontine myelinolysis made?

Answer 27

MRI

Question 28

How does SIADH result in cerebral odema?

Answer 28

Hyponatraemia lowers the osmolarity of the blood causing an osmotic shift of water into the brain tissue with subsequent odema + risk of herniation