Paediatrics: Posterior pituitary: syndrome of inappropriate antidiuretic hormone secretion

Posterior pituitary: syndrome of inappropriate antidiuretic hormone secretion

Heterogeneous disorder characterized by hypotonic hyponatraemia and impaired urinary dilution that cannot be accounted for by a recognized stimulus to ADH secretion. Plasma ADH is elevated or inadequately sup-pressed. Several different types of pathogenic mechanisms are likely to be responsible for this. There are many causes of SIADH.

Up to 15% of children presenting with brain trauma or infection develop SIADH. Clinical features include development of: confusion; headache; lethargy; seizures and coma.

Symptoms do not necessarily depend on the concentration of serum sodium, but on its rate of development. Slow, gradual development of hyponatraemia may be asymptomatic.

SIADH diagnostic criteria

·  Hyponatraemia (serum Na⁺ <135mmol/L)

·  Hypotonic plasma (osmolality <270mOsm/kg)

·  Excessive renal sodium loss (>20mmol/L)

·  No hypovolaemia or fluid overload

·  Normal renal, adrenal, and thyroid function

·  Increased plasma ADH

Management

Treatment of the underlying cause is necessary. Fluid restriction is the mainstay of therapy.

·  Hypertonic (3%) saline solution may be used to correct severe hyponatraemia, or hyponatraemia resistant to fluid restriction.

·  Slow correction of hyponatraemia is essential to avoid rapid overcorrection with possible complication of central pontine demyelination.

Longer-term management/treatment with demeclocycline may be effective for fluid balance by inducing nephrogenic DI.