Amyloid burden and metabolic function in early-onset Alzheimer's disease: parietal lobe involvement.

Alzheimer's disease with early onset often presents with a distinct cognitive profile, potentially reflecting a different distribution of underlying neuropathology. The purpose of this study was to examine the relationships between age and both in vivo fibrillary amyloid deposition and glucose metabolism in patients with Alzheimer's disease. Dynamic [(11)C]Pittsburgh compound-B (90 min) and static [(18)F]fluorodeoxyglucose (15 min) scans were obtained in 100 patients with Alzheimer's disease and 20 healthy controls. Parametric non-displaceable binding potential images of [(11)C]Pittsburgh compound-B and standardized uptake value ratio images of [(18)F]fluorodeoxyglucose were generated using cerebellar grey matter as reference tissue. Nine [(11)C]Pittsburgh compound-B-negative patients were excluded. The remaining patients were categorized into younger (n=45, age: 56 ± 4 years) and older (n=46, age: 69 ± 5 years) groups, based on the median age (62 years) at time of diagnosis. Younger patients showed more severe impairment on visuo-spatial function, attention and executive function composite scores (P<0.05), while we found a trend towards poorer memory performance for older patients (P=0.11). Differences between groups were assessed using a general linear model with repeated measures (gender adjusted) with age as between subjects factor, region (frontal, temporal, parietal and occipital and posterior cingulate cortices) as within subjects factor and [(11)C]Pittsburgh compound-B binding/[(18)F]fluorodeoxyglucose uptake as dependent variables. There was no main effect of age for [(11)C]Pittsburgh compound-B or [(18)F]fluorodeoxyglucose, suggesting that overall, the extent of amyloid deposition or glucose hypometabolism did not differ between groups. Regional distributions of [(11)C]Pittsburgh compound-B binding and [(18)F]fluorodeoxyglucose uptake (both P for interaction <0.05) differed between groups, however, largely due to increased [(11)C]Pittsburgh compound-B binding and decreased [(18)F]fluorodeoxyglucose uptake in the parietal cortex of younger patients (both P<0.05). Linear regression analyses showed negative associations between visuo-spatial functioning and parietal [(11)C]Pittsburgh compound-B binding for younger patients (standardized ß: -0.37) and between visuo-spatial functioning and occipital binding for older patients (standardized ß: -0.39). For [(18)F]fluorodeoxyglucose, associations were found between parietal uptake with visuo-spatial (standardized ß: 0.55), attention (standardized ß: 0.39) and executive functioning (standardized ß: 0.37) in younger patients, and between posterior cingulate uptake and memory in older patients (standardized ß: 0.41, all P<0.05). These in vivo findings suggest that clinical differences between younger and older patients with Alzheimer's disease are not restricted to topographical differentiation in downstream processes but may originate from distinctive distributions of early upstream events. As such, increased amyloid burden, together with metabolic dysfunction, in the parietal lobe of younger patients with Alzheimer's disease may contribute to the distinct cognitive profile in these patients.