Osteopenia, pathological fractures, and increased urinary calcium excretion in schizophrenic patients with polydipsia

Ten male chronic schizophrenic patients with polydipsia and 10 nonpolydipsic controls, matched for gender, diagnosis, duration of illness, age, and race, were studied by dual- and single-photon absorptiometry to estimate bone density of the lumbar spine and radius and by 24-hr urine collections to estimate urinary electrolyte excretion. Bone density was normal in the control group, but was abnormally low in the polydipsic group, which had a markedly increased incidence of fractures. Electrolyte excretion was normal in the control group and in the polydipsic group when water intake was restricted to normal amounts; increased urinary sodium and calcium excretion occurred in proportion to polydipsia. As polydipsia is associated with a number of physiological changes, the cause of the osteopenia is unclear; we suggest that a negative calcium balance caused by increased urinary calcium excretion induced by extracellular space expansion may play an important role in the causation of the skeletal changes.