Distribution of pathology in frontal variant Alzheimer's disease

Atypical presentations of Alzheimer's disease (AD) have been described, including a "frontal" variant (fvAD), which presents with personality change and executive dysfunction similar to that seen in behavioral variant frontotemporal dementia (bvFTD). This clinical variation is thought to reflect the regional distribution of pathology, although few reports include autopsy confirmation. We compared three clinicopathological groups matched for age at diagnosis and disease duration; those with possible bvFTD who at autopsy had only AD (fvAD), those with typical AD clinically and pathologically, and those with typical clinical bvFTD confirmed pathologically. The density of neurons and AD-type pathology was quantified in the frontal association, occipital association, and entorhinal cortices and hippocampal CA1 regions. Immunohistochemistry for phosphorylated tau and amyloid-β deposition was used to detect neurofibrillary tangles and plaques. Of the six core clinical features of the International Consensus Criteria, disinhibition, stereotyped behaviors, and executive dysfunction were most common, occurring in five of the six fvAD patients. Other features were rare. While there was no significant difference in neuron density between groups for any of the four regions, when the ratio of frontal:occipital pathology was examined, neuronal density in fvAD was significantly less than AD but similar to bvFTD. The frontal:occipital ratio of AD-type pathology was also greater in fvAD than AD. The findings of this study suggest a frontal variant of AD exists with features that mimic bvFTD and that this reflects a differential distribution of neurodegeneration with more marked pathology in the frontal cortex compared with the occipital cortex.