Today, we reviewed an excellent case of a middle aged man with a history of polysubstance abuse and schizophrenia who was found to have hyponatremia, ultimately diagnosed with primary polydipsia.
- Hyponatremia can be classified according to volume status into three categories including: hypovolemic, euvolemic, hypervolemic hyponatremia. (There are other ways to classify too, including by serum ADH!)
- Each category can be subdivided based on either urine Na for hypo/ervolemic hyponatremia or urine osms for euvolemic hyponatremia.
- Urine osm < 100 is consistent with primary polydipsia and urine osm >100 is consistent with SIADH, hypothyroidism or glucocorticoid deficiency. Our patient's urine Osm was 84, consistent with primary polydipsia.
Dr. Larson had several pertinent take-home points including:
- Primary polydipsia is common in psychiatric illness (schizophrenia being the highest) and 10-20% episodes of acute psychosis manifest some degree of polydipsia.
- There is an interplay between antipsychotic use, primary polydipsia and hyponatremia such that pharmacologic therapy can cause an SIADH type picture, while psychosis/stress may cause primary polydipsia.
- Urine studies are helpful in differentiating which is the primary mechanism.