Cognitive impact of neuronal pathology in the entorhinal cortex and CA1 field in Alzheimerʹs disease

The relative contribution of Alzheimerʹs disease (AD) hippocampal neuronal pathology in cognitive decline is still a matter of debate. To address this issue, we performed a stereological analysis of layer II of the entorhinal cortex and the CA1 field of the hippocampus in 34 autopsy cases covering the whole spectrum of old age and Clinical Dementia Rating (CDR) scores. In both areas, the proportion of neurofibrillary tangle (NFT)-containing neurons increased steadily as a function of the CDR score. Questionable dementia was associated with a 1.9% neuronal loss in the entorhinal cortex and 26% in the CA1 field. NFT numbers predicted only 38% of the neuron number variability in the entorhinal cortex and 55% in the CA1 field. Neuron counts in the entorhinal cortex and both neuron and NFT counts in the CA1 field were significantly associated with cognitive status explaining 25% and 44% of the CDR variability, respectively. Our data reveal a dissociation between the patterns of progression of NFT and neuronal loss in the entorhinal cortex and CA1 field. Moreover, they show that less than 50% of the cognitive variability may be attributable to AD neuronal pathology in these areas.