Endocrine - SIADH Flashcards
What is Syndrome of inappropriate antidiuretic hormone?
Increased ADH from posterior pituitary
Increased water reabsorption from urine, diluting the blood causing hyponatraemia
What are the potential sources of excess ADH?
- Increased secretion by posterior pituitary
- Ectopic ADH via small cell lung
Outline SIADH pathophysiology
Excessive ADH causes increased water reabsorption in collecting ducts, diluting blood
This causes dilutional hyponatraemia
Extra water is not enough to cause fluid overload
SIADH results in euvolaemic hyponatraemia
Why do patients with SIADH have high urine osmolality and high urine sodium?
Urine becomes more concentrated as kidneys excrete less water
How do patients with SIADH present?
Symptoms relate to hyponatraemia
- Headache
- Fatigue
- Muscle aches and cramps
- Confusion
Severe hyponatraemia
- Seizures
- Reduced consciousness
What are the causes of SIADH?
SIADH
Small cell lung cancer
Infection- pneumonia and lung abscesses
After surgery (post-op)
Drugs (SSRIs+ carbamazepine)
HIV
How do you diagnose SIADH?
Euvolaemia
Hyponatraemia
Low serum osmolality
High urine sodium
High urine osmolality
What causes of hyponatraemia must be excluded?
Adrenal insufficiency
No diuretic use
No CKD
No HF
No liver disease
No excessive water intake
What is primary polydipsia?
Excessive water consumption with no underlying cause
Dilutes blood and urine
Euvolaemic hyponatraemia
What imaging is used for SIADH?
CXR- for infection and lung cancer
CTtap
MRI head
Both to find the malignancy
How is SIADH managed?
Treat underlying cause
Fluid restriction
Vasopressin receptor antagonists (tolvaptan)- causes rapid sodium rise
Why do you correct sodium slowly in SIADH?
Osmotic Demyelination Syndrome
AKA
Central pontine myelinolysis
Complication of long-term severe hyponatraemia
What is osmotic demyelination syndrome?
As blood sodium drops water moves across the BBB
As the brain has a high concentration of solutes this causes the brain to swell with fluid
Brain adapts by reducing solutes to balance water across the BBB
So if hyponatraemia has been present and severe for a long time, brain cells with have low osmolality
First phase
Rapid rise causes sudden shift of water
Patient presents as encephalopathic and confused
Vomiting, headaches and seizures
Second phase
Demyelination of neurones in the pons
Occurs a few days after rapid correction of sodium
- Spastic quadriparesis
- Pseudobulbar palsy
- Cognitive and behavioural changes
- Significant risk of death
Prevent as treatment is only supportive
Most patients will have neurological deficit after
