We investigated if there are individuals at a preclinical stage of idiopathic normal pressure hydrocephalus (iNPH) in a general population. All the residents (n = 1142) aged 61 years (n = 306, men/women = 156/150) and 70-72 years (n = 836, men/women = 356/480) in the two communities of Japan were requested to take brain MRI examination. The "iNPH features on MRI" were defined as an Evans index of > 0.3 and a narrowing of the subarachnoid space and cortical sulci at the high convexity of the cerebrum. "Possible iNPH" was defined as the presence of one or more symptoms of iNPH, together with such MRI features. 790 (69.2%) of the 1142 residents participated in this study. Among them, 51 individuals (men/women = 35/16) (6.46%) had the enlarged ventricles (Evans index of > 0.3), 12 (men/women = 7/5) (1.52%) of which showed the iNPH features on MRI. Of the 12 individuals, 8 (men/women = 4/4) (1.01%) were asymptomatic, while 4 (men/woman = 3/1) (0.51%) had gait disturbance and/or dementia (possible iNPH). During a follow-up period of 4-8 years, two of the 8 asymptomatic subjects developed dementia and/or gait disturbance with worsening of ventriculomegaly on brain MRI in one case. The prevalence of possible iNPH was 0.51% (4/790) among Japanese elderly (> 61 years of age). Asymptomatic ventriculomegaly with the iNPH features on MRI (AVIM) may represent a preclinical stage of iNPH.
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