SIADH

Question 1

what is SIADH?

Answer 1

is characterised by a physiologically inappropriate release of antidiuretic hormone (ADH) either from the posterior pituitary gland, or an ectopic non-pituitary source

Question 2

what is the aetiology of SIADH?

Answer 2

• primary brain injury (e.g. meningitis, subarachnoid haemorrhage)
• malignancy (e.g. SCLC)
• drugs (e.g. carbamazepine, SSRIs, amitriptyline)
• infectious (e.g. atypical pneumonia, cerebral abscess)
• hypothyroidism

Question 3

what are the symptoms of SIADH?

Answer 3

• mild hyponatraemia: nausea, vomiting, headache, anorexia, lethargy
• moderate hyponatraemia: cramps, weakness, confusion, ataxia
• severe hyponatraemia: drowsiness, seizures, coma

Question 4

why might patients with chronic hyponatraemia have fewer symptoms than those with acute hyponatraemia, despite having very low sodium levels?

Answer 4

• due to a compensatory process known as cerebral adaptation, in which brain cells adapt their metabolism to cope with abnormal sodium levels
• can only occur if the change in sodium concentration is gradual, explaining the more severe symptoms associated with acute hyponatraemia and the potentially less severe symptoms associated with chronic hyponatraemia

Question 5

what are the signs on examination of SIADH?

Answer 5

• decreased level of consciousness
• cognitive impairment (e.g. short-term memory loss, disorientation, confusion)
• seizure
• brain stem herniation (e.g. coma, respiratory arrest)
• hypervolaemia (e.g. pulmonary oedema, peripheral oedema, raised JVP, ascites)

Question 6

what are the investigations for SIADH?

Answer 6

• U&Es
• plasma osmolality
• TFTs
• serum cortisol
• urine osmolality
• urine sodium
• CXR/CT

Question 7

what are the diagnostic criteria for SIADH?

Answer 7

• hyponatraemia
• low plasma osmolality
• inappropriately elevated urine osmolality
• urine [Na+] >40 mmol/L despite normal salt intake
• euvolaemia
• euthyroid
• euadrenal

Question 8

what is the management of SIADH?

Answer 8

• treat the underlying cause (e.g. malignancy, drugs)
• fluid restriction (e.g. 0.5-1L a day)
• ? tolvaptan/demeclocycline
• ? hypertonic 3% saline (e.g. severe or symptomatic hyponatraemia)