Syndrome of inappropriate anti-diuretic hormone secretion (SIADH)
SIADH is characterised by the persistence of ADH secretion despite decreased plasma osmolality and normal or increased extracellular fluid volume.
See Table 11.13.
ADH is a peptide hormone similar to oxytocin which is normally secreted from the posterior pituitary, in response to an increase in plasma osmolality. It acts on the collecting tubules in the kidney to make them more permeable to water molecules. Hence its secretion causes water retention (see Fig. 11.12).
Patients present with headache, confusion, behavioural changes, convulsions and coma. On examination there is no peripheral oedema. There may be muscle twitching with an extensor plantar reflex.
Electrolyte analysis will reveal hyponatraemia with reduced plasma osmolality, and high urinary osmolality and sodium. Plasma osmolality can be estimated from the sodium, urea and glucose concentrations (âˆ¼2x [Na] + [Urea] + [glucose]).
Fluid restriction is the mainstay of treatment, although this is unpleasant for the patient and often difficult to enforce. It is also a useful diagnostic test. Demeclocycline, an ADH antagonist at the renal collecting ducts can be used, but is nephrotoxic, especially in the elderly. If water intoxication is severe, diuretics with hypertonic saline infusion is used. Any underlying cause should be identified and treated.
In many cases the syndrome is temporary.