Cognitive profile and metabolic brain imaging in late-onset PCA compared with early-onset PCA and senile amnesic AD

Objective. Posterior Cortical Atrophy (PCA), the visual variant of Alzheimer's Disease (AD), is typically presenile, but patients with a later onset are also reported. The current study aimed at establishing whether PCA patients with a disease onset after the age of 65 show different cognitive and metabolic imaging features from PCA patients with presenile onset and from AD patients with the more classical (amnesic) presentation of the disease. Material and Methods. We enrolled patients meeting consensus criteria for PCA, stratified by age of onset (< or > 65 years) in a Late-Onset PCA group (LO_PCA; n. 37) and a Early-Onset PCA group (EO_PCA; n. 29), and 30 patients meeting standardized criteria for amnesic AD (aAD). The three groups were comparable for disease duration and MMSE score. LO_PCA patients were comparable to aAD patients for age and age of onset. PCA participants underwent an in-depth assessment of posterior functions and were classified according to the phenotypes outlined by consensus criteria: 1. a simultanagnosic biparietal (biP) variant; 2. a visual agnosic occipitotemporal (OT) variant; 3. a dominant parietal (domP) variant with difficulties in calculation, writing or praxis; 4. a caudal occipital (cO) variant with elementary visual deficits. The prevalence of each of these variants was then compared between the two PCA groups. Brain FDG-PET was available in 33 /37 (89.2%) LO_PCA, 22 /29 (75.9%) EO_PCA patients and all aAD patients. Distribution of hypometabolism for each of the three groups was assessed via comparison with scans from a group of 30 healthy elderly controls using SPM, including age and sex as covariates. Results. The LO_PCA group showed a higher prevalence of OT cases and a lower prevalence of biP and cO cases than the EO_PCA group, while the frequency of the domP variant was similar in the two groups. FDG-PET analysis in LO_PCA patients showed severe hypometabolism in right temporo-parieto-occipital and mesial parietal regions, while aAD patients showed bilateral temporal (lateral and mesial) hypometabolism. On the other hand, EO_PCA hypometabolism was more widespread, with bilateral temporo-parieto-occipital abnormalities. Discussion and conclusion. Behavioral and neuroimaging analyses converged in showing less severe bilateral OP involvement in LO_PCA than EO_PCA patients, associated with a lower prevalence of simultanagnosic cases. The analysis also showed that LO_PCA differs from aAD in terms of distribution of neurodegeneration.