Friday, 1 October 2010

MRCP revision battle 24.5: SIADH

SIADH = syndrome of inappropriate antidiuretic hormone secretion is a condition whose name really says it all: it is ADH being produced and released when it shouldn't be.

Quick bit of physiological background:
  • ADH is made in the magnocellular neurones in the supraoptic and paraventricular nuclei of the hypothalmus
  • ADH is stored in the posterior pituitary
  • it is released in response to raised plasma osmolality (osmoreceptors in hypothalamus) or decreased volume (baroreceptors in carotids, atria)
  • its effect is to increase the insertion of aquaporin 2 channels in the collecting ducts of the kidney, hence increasing water reabsorption.
  • it also increases peripheral vascular resistance

SIADH is diagnosed in the presence of:
  • concentrated urine ( sodium >20mmol/l and osmolality >500mosmol/kg) AND
  • hyponaturaemia (<125) or low plasma osmolality (<260mosmol/kg) AND
  • the absence of hypovolaemia, oedema or diuretics.

Causes of SIADH include:
  • malignancy: small cell lung cancer, pancreas, prostate, lymphoma
  • lung pathology: TB, pneumonia, aspergillosis
  • CNS pathology: injury, GBS, fits, subarachnoid/subdural haemorrhage, abscess, stroke
  • metabolic: porphyria
  • drugs: opiates, carbamazepine, SSRIs, TCA

So lets now finish todays battles by widening this topic out to hyponatraemia...