n CME : Your SA Journal of CPD - Water and sodium disturbances : main article

Volume 24, Issue 7
  • ISSN : 0256-2170



ECF [Na<sup>+</sup>] is the main contributor of the ECFC tonicity. <BR>When there is pure water loss from the ECFC due to diabetes insipidus, this loss is shared across both intracellular and extracellular compartments. Dehydration is therefore only a late sign. <BR>When there is loss of both water and sodium from the ECFC, as in diarrhoea and vomiting, this loss is borne mainly by the ECFC. Dehydration is therefore an early sign. <BR>Hypernatraemia is always associated with an increase in the measured plasma osmolality. <BR>Hyponatraemia can be associated with a normal, increased or decreased measured plasma osmolality. <BR>Rapid correction of hyponatraemia or hypernatraemia after cellular adaptation has taken place can lead to irreversible demyelination of pontine and extrapontine neurones (myelinolysis). <BR>SIADH is only diagnosed after excluding cardiac, pituitary, adrenal, thyroid and renal disorders as well as the effect of drugs. <BR>Biochemical features of SIADH are true hyponatraemia, an inappropriately concentrated urine and a urine [Na<sup>+</sup>] > 20 mmol/l (plasma and urine samples collected at the same time).

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