Although total water is increased in SIAD, it is not clinically demonstrable since water is distributed throughout intracellular and extracellular compartments. In contrast, hypervolaemic hyponatraemia is recognised by features of extracellular fluid overload, e.g. dependent oedema and hypovolaemic hyponatraemia by signs of dehydration and hypotension. In all types of hyponatraemia there is an excess of water relative to extracellular sodium, and the three categories of hyponatraemia are distinguished by the extracellular sodium content. Examples of causes of the different types of hyponatraemia are given in Fig. 5.21.
