The age at onset of Alzheimer's disease and an intracranial area measurement. A relationship.

OBJECTIVE To examine the possibility that premorbid brain size may influence the age at onset of symptoms of Alzheimer's disease (AD). DESIGN Retrospective case series. SETTING Outpatients attending a memory disorders clinic in a tertiary referral center. PATIENTS Twenty-eight female patients with the diagnosis of probable AD, selected for the availability of informant derived estimates of age at onset of symptoms and computed tomographic scans of the head satisfying angulation criteria. MAIN OUTCOME MEASURE An average intracranial area of two adjacent computed tomographic scan sections appropriately angled was used as a correlate of premorbid brain size. Strict intracranial volume measurement was not performed. RESULTS Age at onset of symptoms of AD correlated positively (r = .48, P = .009) with our measure of premorbid brain size. There was no confounding by education, height, or ethnicity. CONCLUSION Premorbid brain size may be an important determinant of the age at onset of symptoms of AD. Epidemiologic studies of AD may need to assess the relationship between brain size and putative risk factors, eg, low educational attainment, since there is evidence that brain size is not distributed uniformly across populations.

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