Reduced size of right hippocampus in 39- to 80-year-old normal subjects carrying the apolipoprotein E epsilon4 allele.

Hippocampal size on magnetic resonance imaging was compared between normal subjects with the apolipoprotein E (apo E) epsilon4 allele (epsilon4/4, epsilon4/3, and epsilon4/2) and those without the epsilon4 allele (epsilon3/3, epsilon3/2, and epsilon2/2) in the age range of 39-80 years. The Mini-Mental State Examination (MMSE) scores did not differ between the two groups. The right hippocampal area and its ratio to hemisphere area and intracranial cavity area were significantly smaller in epsilon4 carriers than non-carriers, whereas hemisphere area did not differ between the two groups. These results suggest that as early as their forties, apo E epsilon4 allele carriers have a markedly smaller right hippocampus with no apparent cognitive impairment, which may have some significance in the high prevalence of the epsilon4 allele in Alzheimer's disease as well as other conditions that cause dementia.