Hyponatremia, hyposmolality, and hypotonicity - Tables and fables

The difficulty that nonnephrologists sometimes have with the differential d iagnosis of hyponatremic patients often results from misinterpreting the si gnificance of measured and calculated serum osmolalities, effective serum o smolalities (tonicities), and the influence of various normal leg, serum ur ea nitrogen) and abnormal leg, ethanol) solutes. Among the more commonly he ld misconceptions are that high serum urea or alcohol levels will, by analo gy with glucose, cause hyponatremia, and that a normal (or elevated) measur ed serum osmolality in a hyponatremic patient excludes the possibility of h ypotonicity. This article describes typical and deliberately comparative da ta of the serum levels of sodium, glucose, urea nitrogen, and mannitol and/ or ethanol (if present); calculated and measured osmolality; effective osmo lality; and the potential risk of hypotonicity-induced cerebral edema for e ach of 6 prototypical hyponatremic states. This provides a helpful educatio nal tool for untangling these interrelationships and for clarifying the dif ferences among various hyponatremic conditions.