SIADH

Question 1

Define

Answer 1

Characterized by continued secretion of ADH, despite the absence of normal stimuli for secretion

(Stimuli: ↑serum osmolarity or ↓blood volume)

Question 2

Causes

Answer 2

• Malignancy → lung small-cell, pancreas, prostate, thymus, lymphoma
• CNS disorders → meningoencephalitis, abscess, stroke, subarachnoid or subdural haemorrhage, head injury, neurosurgery, Guillain–Barré, vasculitis or SLE
• Chest disease: TB, pneumonia, abscess, aspergillosis, SCC
• Drugs: opiates, psychotropics, SSRIs, cytotoxics
• Other: acute intermittent porphyria, trauma, major

abdominal or thoracic surgery, symptomatic HIV

Question 3

Epidemiology

Answer 3

Hyponatraemia is the most common electrolyte imbalance seen in hospitals
<50% of all severe hyponatraemia are due to SIADH

Question 4

Symptoms

Answer 4

Note: The hyponatraemia in SIADH is dilutional from ↑body water and not ↓ total body Na+

↘ Mild hyponatraemia (125-135 mmol/L) may be asymptomatic

↘ Headache, nausea, vomiting
↘ Muscle cramp/weakness
↘ Irritability, confusion, drowsiness ↘ Convulsions, coma

Question 5

Signs

Answer 5

Mild hyponatraemia – no signs
↘ Severe hyponatraemia – ↓reflexes
↘ Cognitive impairment – memory loss, confusion,

disorientation ↘ Seizures

Question 6

Investigations

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 the absence of 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

Management

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

Complications

Answer 8

Convulsions

Coma

Death

Central pontine myelinolysis - occurs with rapid correction of hyponatraemia

Characterised by:

• Quadriparesis
• Respiratory arrest
• Fits

Question 9

Prognosis

Answer 9

Depends on the CAUSE

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

50% mortality with central pontine myelinolysis