THE SYNDROME OF INAPPROPRIATE SECRETION OF ANTIDIURETIC-HORMONE

The physiology of the release of antidiuretic hormone (ADH) from the p osterior pituitary is briefly reviewed. The importance of both osmolar and non-osmolar stimuli is emphasised. Osmolar and non-osmolar factor s usually reinforce each other; for example, hydropenia leads to hyper osmolality and hypovolaemia, both promoting ADH release, while hydrati on has the opposite effect. In disease, osmolar and non-osmolar factor s may become dissociated leading to baroreceptor-mediated ADH release in the presence of hyponatraemia and hypo-osmolality. Examples include heart failure, glucocorticoid or thyroxine deficiency, hepatic cirrho sis and nephrotic syndrome with or without the superimposed effect of diuretics, i.e. conditions in which circulatory, and in particular eff ective arterial, volume is reduced. It is dangerous to label such cond itions as 'inappropriate' secretion of ADH since the maintenance of ci rculating volume is at lest as important a physiological requirement a s the defence of tonicity. The syndrome of inappropriate secretion of ADH (SIADH) is uncommon in childhood and should only be diagnosed when physiological release of ADH in response to non-osmolar as well as os molar factors has been excluded. Criteria for the correct identificati on of SIADH are discussed; the presence of continuing urinary sodium e xcretion in the presence of hyponatraemia ad hypo-osmolality is essent ial to the diagnosis. SIADH in children is usually due to intracranial disease or injury. The mainstay of treatment is water restriction whi ch reverses all the physiological abnormalities of the condition. Hype rtonic saline is rarely indicated for the short-term control of neurol ogical manifestations such as seizures. Drugs have little or no place in the treatment of SIADH in children. In many situations labelled as SIADH it is only the diagnosis that is inappropriate.