SIADH Flashcards
What does SIADH stand for
Syndrome of Inappropriate Anti-Diuretic Hormone (SIADH)
What is the definition of Syndrome of Inappropriate Anti-Diuretic Hormone (SIADH)
Characterised by excess release of ADH leading to retention of water and hyponatraemia
ADH = Causes water
a) reabsorption
b) excretion
a) reabsorption
Define plasma osmolality
Refers to the concentration of dissolved particles in a body of fluid.
If plasma osmolality is high what does this mean?
Means there are more particles or less fluid.
What is the function of ADH excess
ADH excess results in reduced diuresis - water excretion and urinary output are reduced
This leads to an increase in total body water which dilutes the sodium concentration causing hyponatraemia.
What is the other name for Anti-Diuretic Hormone (ADH)
Vasopressin
What produces ADH
Produced in the hypothalamus
Where is ADH stored and secreted
Posterior pituitary gland
ADH is released in response to what?
Rising plasma osmolality
ADH stimulates water reabsorption from where?
The collecting ducts and distal convoluted tubule in the kidneys
How does ADH cause water reabsorption
ADH stimulates water reabsorption from the collecting ducts and distal convoluted tubule in the kidneys
ADH stimulates the insertion of aquaporin-2 channels onto the luminal membrane, allowing the free entry of water
Define paraneoplastic syndrome
When autoimmune disorders are triggered by tumours.
Give examples of autoimmune conditions that can cause paraneoplastic syndrome
Cushing’s syndrome
SIADH
Lambert-Eaton syndrome
Hyperparathyroidism
Name some causes of SIADH
Either the posterior pituitary is secreting too much ADH or ADH is being produced somewhere else e.g. small cell lung cancer
- Malignancy – small cell lung cancer (paraneoplastic syndrome)
- Recent major surgery
- CNS – stroke, SAH, head injury
- Infection – TB, pneumonia
- Drugs – sulfonylureas, SSRIs
Name the two main causes of SIADH
Malignancy – small cell lung cancer (paraneoplastic syndrome)
Recent major surgery
Name some of the clinical features of SIADH
A large proportion of cases will be asymptomatic
- Headache
- Fatigue
- Muscle aches and cramps
- Confusion
- Severe hyponatraemia can cause seizures and reduced consciousness
How is SIADH diagnosed
Diagnosis of exclusion
No reliable test to measure ADH activity
Clinical examination of a patient with SIADH will show
a) Euvolaemia
b) Hypovolaemia
c) Hypervolaemia
a) Euvolaemia
In SIADH, urine sodium and osmolality will be
a) high
b) low
a) high
What is the mainstay of management in SIADH
Fluid restriction (500mls – 1 L per day)
Why should hyponatraemia be slowly corrected?
Rapid increases in plasma sodium may result in central pontine myelinolysis
If fluid restriction is not enough to manage SIADH what can be given?
Vaptans e.g. tolvaptan – ADH receptor antagonists.
Initiated by a specialist endocrinologist and requires close monitoring
What is Central pontine myelinolysis
Also known as osmotic demyelination syndrome
Caused by rapid correction of hyponatraemia
Causes irreversible neurological damage
What causes Central pontine myelinolysis
Caused by rapid correction of hyponatraemia
