Syndrome of inappropriate ADH secretion Flashcards
What is the main cause of hyponatraemia in SIADH?
(blood sodium levels are too low (typically <135 mmol/L))
Dilution due to excessive water retention
Name three causes of SIADH
- Ectopic ADH secretion (tumors)
- Primary pituitary disorder
- Paraneoplastic syndrome
(eg, small cell lung carcinoma)
What hormone is excessively released in SIADH?
Antidiuretic hormone (ADH)/vasopressin
How does SIADH lead to dilutional hyponatraemia?
Excess ADH causes water retention, expanding volume and diluting sodium levels
What are non-osmotic stimuli that trigger ADH release?
- Hypovolemia
- Hypotension
- Pain
- Nausea
- Vomiting
- Muscle cramps
How is SIADH often discovered in hospital patients?
Incidentally, when low sodium is found on routine U+Es
What conditions should be ruled out before diagnosing SIADH?
Adrenal insufficiency and other sources of sodium loss.
What investigations do you use for SIADH?
(1) Urea and electrolytes
= shows a hyponatraemia
(2) Plasma osmolality
= will be low (<270mOsm/kg)
(3) Urine sodium
= will be high (>20mmol/L)
(4) Urine osmolality
= will be inappropriately concentrated (>100mOsmol/kg)
Use MRI brain for pituitary tumour
A 60-year-old man presents to the emergency department feeling confused. He has a history of small cell lung cancer with SIADH. He complains of muscle weakness and nausea and vomiting.
What is the most likely cause of his confusion?
hyponatraemia
A 24-year-old man presents to the Emergency Department. He is experiencing muscle cramps, nausea and a headache. He feels more tired than normal and appears confused. He was recently started on a new medication by his GP.
A urine sample reveals high urine osmolality and high urinary sodium. Blood tests show low sodium, urea, and serum osmolality.
Which medication is most likely to have caused this presentation?
Citalopram, Carbamazepin, sulfonylureas
A 65-year-old woman presents with two months of nausea, fatigue, dizziness, and intermittent confusion. She has a worsening productive cough with blood-streaked sputum and unintentional weight loss. She denies trauma and uses salbutamol for asthma.
On examination, she has a heart rate of 90bpm, respiratory rate 22/min, saturations 94% on room air, blood pressure 108/75 mmHg and temperature 37.3°C. She has reduced air entry to the right base with a dull percussion note, and bilateral pitting oedema is noted. She is unsteady whilst walking and needs to sit to avoid falling over.
What is the most likely electrolyte disturbance in this patient?
Hyponatraemia
First-line management for SIADH is what?
fluid restriction to 500-1000 ml/day, as well as treating the underlying cause (eg, stopping medications that cause SIADH
The two most important differentials to exclude in SIADH. These are?
- Severe hypothyroidism
= thyroid function test - Glucocorticoid insufficiency
= Serum cortisol
A 78-year-old woman is admitted to hospital with an infective exacerbation of chronic obstructive pulmonary disease (COPD). On admission, she is noted to be hyponatremic with a sodium of 122 mmol/L (normal range 135-145 mmol/L).
On examination, she is euvolemic. The following morning she has paired urine and serum osmolality measurements:
(1) Serum osmolality
= 260 (275-295 mOsmol/kg)
(2) Urine osmolality
>500mOsm/kg
(3) Urine sodium
>50 mmol/L
What is the next most appropriate investigation?
Serum cortisol first to rule out adrenal insufficiency (life-threatening). If normal, then check thyroid function
