The syndrome of psychosis, intermittent hyponatremia, and polydipsia: evidence for diurnal volume expansion.

Seven patients (6 men and 1 woman, mean age 39.1 +/- SD 6.9 years) with psychosis, intermittent hyponatremia, and polydipsia (PIP syndrome) underwent serial determinations of serum sodium (SOD), plasma atrial natriuretic peptide (ANP), and urinary osmolality (UOSM) at 7 AM and 4 PM. There was a diurnal increase in ANP (7 AM 17.9 +/- 5.1 pg/ml and 4 PM 27.7 +/- 9.0 pg/ml, p = 0.02), a diurnal decrease in serum sodium (7 AM 141.1 +/- 1.7 mEq/l, 4 PM 129.9 +/- 3.2 mEq/l, p less than 0.0001) and no diurnal change in UOSM. The diurnal increase in ANP in the the PIP syndrome contrasts to the diurnal decrease in ANP reported in normal subjects. Our data, while preliminary, suggest that patients with the PIP syndrome have increased intravascular volume leading to ANP secretion, natriuresis, and hyponatremia.