SIADH

Question 1

Definition

Answer 1

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

Question 2

Aetiology/Risk factors (brain and lung)

Answer 2

· Brain
o Haemorrhage/thrombosis
o Meningitis
o Abscess
o Trauma
o Tumour
o Guillain-Barre syndrome

· Lung
o Pneumonia
o TB
o Other: abscess, aspergillosis, small cell carcinoma

Question 3

Aetiology/Risk factors (tumours, drugs and metabolic)

Answer 3

· Tumours
o Small cell lung caner
o Lymphoma
o Leukaemia
o Others: pancreatic cancer, prostate cancer, mesothelioma, sarcoma, thymoma

· Drugs
o Vincristine
o Opiates
o Carbamazepine
o Chlorpropamide

· Metabolic
o Porphyria
o Alcohol withdrawal

Question 4

Epidemiology

Answer 4

· Hyponatraemia is the MOST COMMON electrolyte imbalance seen in hospital

· < 50% of severe hyponatraemia is caused by SIADH

Question 5

Presenting symptoms

Answer 5

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

Question 6

Signs on physical examination

Answer 6

· MILD hyponatraemia - no signs

· SEVERE hyponatraemia:
o Reduced reflexes
o Extensor plantar reflexes

· Signs of underlying cause

· NOTE: the hyponatraemia in SIADH is due to dilution from excessive water reabsorption and not due to a decrease in total body Na+

Question 7

Investigations

Answer 7

· Things to check:

o Low serum sodium

o Creatinine (check renal function)

o Glucose, serum protein and lipids - to rule out pseudohyponatraemia
· Pseudohyponatraemia = when the sodium concentration is actually normal but is erroneously reported as being low because of the presence of either hyperlipidaemia or hyperproteinaemia

o Free T4 and TSH - hypothyroidism can cause hyponatraemia

o Short synacthen test - adrenal insufficiency can cause hyponatraemia

· Investigations for identifying the cause (e.g. CXR, CT, MRI)

Question 8

Investigations for diagnosis

Answer 8

o Low plasma osmolality

o Low serum Na+ concentration

o High urine osmolality

o High urine Na+

o The presence of the above results and the absence of hypovolaemia, oedema, renal failure, adrenal insufficiency and hypothyroidism are required for the diagnosis of SIADH

Question 9

Management plan

Answer 9

· Treat underlying cause

· Fluid restriction

· Vasopressin receptor antagonists (e.g. tolvaptan)

· In SEVERE cases - slow IV hypertonic saline and furosemide with close monitoring

Question 10

Possible complications

Answer 10

· Convulsions

· Coma

· Death

· Central pontine myelinolysis - occurs with rapid correction of hyponatraemia
o Characterised by:
· Quadriparesis
· Respiratory arrest
· Fits

Question 11

Prognosis

Answer 11

· Depends on the CAUSE

· Na+ < 110 mmol/L is associated with a HIGH MORBIDITY and MORTALITY

· 50% mortality with central pontine myelinolysis