Background: Recent studies suggest that larger brain size may offer some protection against the clinical manifestations of Alzheimer's disease. However, this association has not been investigated in population-based studies.
Method: The relationship between head circumference, a measure of premorbid brain size, and score on the Cognitive Abilities Screening Instrument (CASI) was studied in a population of 1985 Japanese-Americans aged 65+ living in King County, Washington, USA.
Results: After adjusting for age, sex and education, head circumference was positively associated with CASI score (b = 3.8, 95% Cl: 2.2, 5.4; P = 0.0000), but not with diagnosis of probable AD (odds ratio = 0.87, 95% Cl: 0.33, 1.87). When the data were stratified by AD status, no association was seen among controls (b = 1.6, 95% Cl: -1.7, 5.1; P = 0.4), whereas a strong effect was present among cases (b = 35.3, 95% Cl: 12.2, 68.4: P = 0.006).
Conclusions: These results suggest that persons with AD with smaller head circumference either had the disease longer or progressed more rapidly than those with larger head circumference. Improvement in environmental factors in prenatal and early life that partially determine completed brain/head size may have consequences for the late-life expression of Alzheimer's disease in vulnerable individuals.