OBJECTIVE  Disordered water balance causes substantial morbidity in a subset of schizophrenics and is the consequence of unexplained defects in the regulation of fluid intake and antidiuretic function. We aimed to determine whether oropharyngeal regulation of water balance is altered in these patients.

DESIGN  A 2-hour infusion of 3% saline, followed 35 minute later by an oral water load (10 ml/kg over 3 minutes).

PATIENTS  Age and sex-matched polydipsic schizophrenics with (n = 5) and without (n = 8) hyponatraemia; non-polydipsic schizophrenics (n = 6); and normal controls (n = 13).

MEASUREMENTS  Plasma osmolality, sodium, AVP, and reported desire for water (expressed in cups), determined prior to, and for 30 minutes following, the water load.

RESULTS  Plasma osmolality and sodium were consistently lower in the hyponatraemics (P<0.01). Significant changes did not occur until 20 and 25 minutes, respectively, after oral water loading, and were similar across the four groups. AVP levels were consistently lower in the two polydipsic groups (P<0.001), fell within 5 minutes after drinking and then levelled off. Neither the acute fall nor the overall pattern of the responses differed across groups. Subjective desire for water also decreased within 5 minutes of drinking, and also to a similar extent, in the four groups. Subsequent levels remained suppressed in the non-polydipsic groups, but rebounded toward baseline in the two polydipsic groups. Thus the overall patterns differed (P<0.05). At the end of the study, ad lib intake correlated significantly with reported desire for water (r = 0.51, P<0.002).

CONCLUSIONS  The oropharyngeal regulation of water intake is disrupted in polydipsic schizophrenics with and without hyponatraemia. In contrast, the oropharyngeal regulation of AVP secretion appears preserved in the hyponatraemic subset. Previously observed elevations in plasma AVP in this subset are thus unlikely to be related to defects in oropharyngeal regulation.