The Hypotonic Syndrome

Abstract

Hypotonicity (P_osm < 280 mOsm/kg) reflects a relative increase in body water in relation to the body solute content, i.e. it is in a simplistic sense always dilutional. Sodium is the only plasma solute the concentration of which can decrease sufficiently, upon plasma dilution, to induce hypotonicity. Hypotonicity therefore always means also hyponatremia. However, it should be remembered that the opposite needs not be always true. Hence if low plasma concentration of sodium has been found in a patient (P_Na < 130 mmol/l) it should be decided first whether the condition represents “true” hypotonic hyponatremia or whether some of the non-hypotonic forms of hyponatremia is present (Fig. 20.1).