SIADH and Diabetes Insipidus

Question 1

Pathophysiology of SIADH

Answer 1

• Condition where there is too much ADH
• May result from posterior pituitary secreting too much or can come from elsewhere (i.e. SCLC)
• Excessive ADH leads to excessive water reabsorption in the collecting ducts
• Water dilutes blood causing hyponatraemia
• Causes include:
  
  • Post-operative from major surgery
  • Infection, particularly atypical pneumonia and lung abscesses
  • Head injury
  • Medications (thiazide diuretics, carbamazepine, vincristine, cyclophosphamide, antipsychotics, SSRIs, NSAIDSs,)
  • Malignancy, particularly small cell lung cancer
  • Meningitis

Question 2

Presentation and investigation of SIADH

Answer 2

• Presentation
  
  • Headache
  • Fatigue
  • Muscle aches and cram-ps
  • Confusion
  • Severe hyponatraemia (can cause seizures and reduced conscious level)
• Diagnosis
  
  • Diagnosis of exclusion
  • Urine Na and osmolality high
  • Need to exclude causes of hyponatraemia
    
    • Negative short synachten test
    • No Hx of diuretic use
    • No diarrhoea, vomiting, burns, fistula or excessive sweating
    • No excessive water intake
    • No CKD or AKI

Question 3

Management of SIADH

Answer 3

• Treat cause
• Correct sodium (remember to correct slowly - < 10mmol/24hrs - due to risk of central pontine myelinolysis)
• Fluid restriction
• Tolvaptan (ADH receptor blocker)
• Demeclocycline (tetracycline antibiotic that inhibits ADH)

Question 4

Pathophysiology of diabetes insipidus

Answer 4

• Lack of ADH
• Prevents kidneys concentrating urine
• Can be nephrogenic or cranial
• Nephrogenic causes (CDs do not respond to ADH):
  
  • Drugs, particularly lithium used in bipolar affective disorder
  • Mutations in the AVPR2 gene on the X chromosome that codes for the ADH receptor
  • Intrinsic kidney disease
  • Electrolyte disturbance (hypokalaemia and hypercalcaemia)
• Cranial causes (hypothalamus does not produce ADH):
  
  • Brain tumours
  • Head injury
  • Brain malformations
  • Brain infections (meningitis, encephalitis and tuberculosis)
  • Brain surgery or radiotherapy

Question 5

Presentation and investigation of diabetes insipidus

Answer 5

• Presentation:
  
  • Polyuria (excessive urine production)
  • Polydipsia (excessive thirst)
  • Dehydration
  • Postural hypotension
  • Hypernatraemia
• Investigations:
  
  • Low urine osmolality
  • High serum osmolality
  • Water deprivation test (fluid restrict, measure urine osmolality, give synthetic ADH and then check urine osmolality again in 8 hours)

Question 6

Management of diabetes insipidus

Answer 6

• Treat underlying cause
• Desmopressin (synthetic ADH) can be used in:
  
  • Cranial diabetes insipidus to replace ADH
  • Nephrogenic diabetes insipidus in higher doses under close monitoring