Incidence of idiopathic normal pressure hydrocephalus (iNPH): a 10-year follow-up study of a rural community in Japan.

Background: The epidemiology and pathophysiology of iNPH remain unclear. We aimed to investigate the incidence of iNPH in elderly inhabitants of the community, and to identify how ventriculomegaly develops on brain MRIs and how symptoms develop in iNPH patients.

Methods: In 2000, 350 inhabitants, all 70-year-olds living in the community of Takahata in Japan, were asked to participate in a survey that included a questionnaire, physical examinations, cognitive screenings, and brain MRI studies. Using brain MRI as a screening, we defined having both Evans index of >0.3 and a narrow subarachnoid space and cortical sulci at high convexity (tight high convexity, THC) as suspicious findings for iNPH. Among the subjects who showed the iNPH feature on brain MRI, those who had gait disturbance and/or dementia were defined as possible iNPH. Twice during the 10 years, we administered the same check-up.

Results: In the first survey, 271 inhabitants participated. During the 10 years, three new possible iNPH patients were found. The incidence of iNPH above 70 years old was estimated at 1.2/1000 persons per year. The iNPH patients developed their symptoms and brain MRI findings as follows; first, only THC without ventriculomegaly was observed on their brain MRIs, next, asymptomatic ventriculomegaly with features of iNPH on brain MRIs (AVIM) was seen, and then a final expression of symptoms of iNPH was shown.

Conclusions: The estimated incidence of iNPH in a community was higher than those estimated by previous studies where they collected patients at hospitals. There were subclinical or preclinical states before the development of iNPH.