SIADH

Question 1

What is SIADH?

Answer 1

SIADH is a condition characterised by a continuous secretion of ADH despite absence of stimuli to secrete ;

in circumstances where there is no increase in serum osmolality or reduction in blood volume, there would be no need for ADH release.

The body makes too much antidiuretic hormone (ADH).

This leads to excessive fluid retention

Question 2

Explain the aetiology/risk factors of SIADH

Answer 2

• Brain
○ Haemorrhage/thrombosis
○ Meningitis
○ Abscess (infections)
○ Trauma
○ Tumour
○ Guillain-Barre syndrome

• Lung
○ Pneumonia
○ TB
○ Other: abscess, aspergillosis, small cell carcinoma
Several lung disorders including pneumonia can cause SIADH by unknown mechanisms.

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

• Drugs
○ Vincristine
○ Opiates
○ Carbamazepine (anti epileptics)
○ Chlorpropamide (sulfonylurea)

• Metabolic
○ Porphyria
○ Alcohol withdrawal

Question 3

Summarise the epidemiology of SIADH

Answer 3

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

Question 4

Recognise the presenting symptoms of SIADH

Answer 4

• Mild hyponatraemia may be ASYMPTOMATIC
• Headache
• Nausea/vomiting
• Muscle cramp/Weakness
• Irritability
• Confusion
• Drowsiness
• Convulsions
• Coma

• Symptoms of underlying cause

Question 5

Recognise the signs of SIADH on physical examination

Answer 5

• MILD hyponatraemia - no signs
• SEVERE hyponatraemia:
○ Reduced reflexes
○ 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 6

Identify appropriate investigations for SIADH

Answer 6

• Things to check:
○ Low serum sodium
○ Creatinine (check renal function)
○ 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

○ Free T4 and TSH - hypothyroidism can cause hyponatraemia
○ Short synacthen test - adrenal insufficiency can cause hyponatraemia

• SIADH Diagnosis
○ Low plasma osmolality
○ Low serum Na+ concentration
○ High urine osmolality
○ High urine Na+
○ The presence of the above results and NO hypovolaemia, oedema, renal failure, adrenal insufficiency and hypothyroidism are required
for the diagnosis of SIADH

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

Question 7

How do you manage SIADH?

Answer 7

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

Question 8

Identify possible complications of SIADH

Answer 8

• Convulsions
• Coma
• Death

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

Question 9

Summarise the prognosis for patients with SIADH

Answer 9

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