SIADH

Question 1

Define SIADH

Answer 1

Inappropriately high ADH release/levels for physiology -> increased water reabsorption ->
impairment of free water excretion -> euvolaemic hyponatraemia and concentrated urine
(high urine sodium >20mmol/l and osmolality)

Question 2

What are the causes/risk factors of SIADH?

Answer 2

CNS
• Haemorrhage/thrombosis
• Meningitis
• Abscess
• Trauma
• Tumour
• Guillain-Barré Syndrome

Pulmonary
• Pneumonia
• TB
• Abscess
• Aspergillosis

Malignancy
• Small lung cell carcinoma
• Lymphoma
• Leukaemia
• GI cancer
• Sarcoma

Drugs
• SSRIs
• Vincristine
• Opiates
• Carbamazepine
• Chlorpropramide

Question 3

What are the symptoms of SIADH?

Answer 3

• Asymptomatic if mild (125-135mmol/l)
• Headache
• Nausea
• Vomiting
• Muscle cramps/weakness
• Irritability
• Confusion
• Drowsiness
• Coma

Question 4

What are the signs of SIADH?

Answer 4

• Hyporeflexia
• Upgoing plantar reflexes
• Convulsions

Question 5

What investigations are carried out for SIADH?

Answer 5

• Serum Sodium - hyponatraemia <135 mmol/L.
• Serum Osmolality - hypotonic < 280 mosmol/kg H2O. SIADH presents with hypotonic hyponatraemia: low serum sodium and osmolality.
• Urine Sodium - high, > 40 mosmol/L.
• Urine Osmolality- hypertonic > 100 mosmol/kg H2O.

The presence of the above and absence of hypovolaemia/hypotension, oedema, renal failure, adrenal insufficiency and hypothyroidism are required for a diagnosis of SIADH.

• TFTs - normal, to exclude hypothyroidism.
• Short Synacthen - normal, to exclude Addsion’s disease.
• U&Es - creatinine must be normal to exclude renal failure.
• Serum Lipids and Proteins - to exclude pseudohyponatraemia seen with ­ protein or lipids

Question 6

What is the management for SIADH?

Answer 6

• Treat the underlying cause.
• Fluid restrict the patient
• Water restriction (0.5-1 L/day):
• If ineffective, give demeclocycline (Reduces responsiveness of the collecting tubule cells to ADH).
• Vaptans –Vasopressin (V2) receptor antagonists e.g. tolvaptan are likely to be useful in moderate chronic hyponatraemia if water restriction is insufficient.
• In severe cases (seizures and reduced consciousness), give slow IV hypertonic (3%) saline (and furosemide) with close monitoring.
• Demcoclocycline – desensitiser

Question 7

What are the complications of SIADH?

Answer 7

• convulsions
• coma
• death
• central pontine myelinolysis (quadreparesis, respiratory arrest, fits) occurs with rapid correction of hyponatraemia