Psychogenic polydipsia is associated with significantly reduced left insula volume in patients with schizophrenia, researchers report.
The team also found that schizophrenia patients with polydipsia have poorer cognitive function than those without excessive water intake.
"It is possible that polydipsia or the secondary hyponatremia might induce left insula volume reduction," comment Tomohisa Nagashima (Nara Medical University, Kashihara, Japan) and colleagues.
"Furthermore, this structural brain change may indirectly induce more severe neuropsychological impairments in polydipsic patients," they add.
The researchers studied eight polydipsic schizophrenia patients, eight schizophrenia patients without polydipsia, and eight mentally healthy controls. All of the participants were men, and there were no significant differences among the groups regarding demographic characteristics, or between the two patient groups regarding clinical characteristics.
Patients with polydipsia had a history of at least two hyponatremic states induced by polydipsia on available medical records, exhibited low sodium levels for at least 3 years, and were continuously treated with water restrictions, even on the day of testing.
All of the participants underwent magnetic resonance imaging of the brain, and neuropsychologic function was assessed using the Brief Assessment of Cognition in Schizophrenia, Japanese version (BACS-J).
The team found that, compared with controls, schizophrenia patients with polydipsia showed widespread gray- and white-matter volume reductions in the right inferior temporal gyrus, the left and right superior temporal gyrus, the left insula, the left medial and inferior frontal gyrus, the right lingual gyrus, the right cuneus, the right parahippocampal gyrus, the left uncus, and the right cerebellar posterior lobe.
They also showed significant gray- and white-matter volume reductions in the left insula compared with nonpolydipsic schizophrenia patients.
Polydipsic patients had significantly lower BACS-J scores than controls for verbal memory, working memory, motor speed, verbal fluency, attention/speed of processing, and executive function. Polydipsic patients were also more impaired in all categories of BACS-J than nonpolydipsic patients, although not significantly.
Writing in BMC Psychiatry, Nagashima et al conclude that "insula abnormalities might contribute to the pathophysiology of polydipsic patients."
They add that further imaging studies "may help elucidate the brain networks disrupted or the extent of pathophysiology in polydipsia."
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