Treatment of psychosis, intermittent hyponatremia, and polydipsia (PIP syndrome) using lithium and phenytoin

Six patients [5 men and 1 woman, mean age 37.3 +/- 8.2 (SD) years] with psychosis, intermittent hyponatremia, and polydipsia (PIP syndrome) underwent a sequence of treatments in an effort to normalize basal serum sodium levels and thereby protect the patients against complications, including hyponatremic seizures and coma. The morning baseline group mean basal serum sodium value was 132.5 +/- 3.8 meq/liter. Over a 20-month period, the sequence of treatments was salt-added diet, lithium and phenytoin, and lithium alone. Each treatment program yielded morning group mean basal serum sodium determinations superior to baseline values, except for the program of lithium alone, which could not be tolerated. The combination of lithium and phenytoin provided a morning group mean basal serum sodium level of 140.6 +/- 3.2 meq/liter, which was superior (p less than 0.01) to all other treatment modalities. Early morning hyposthenuria persisted throughout the 20-month period of observation.