Dissociation of Alzheimer type pathology in a disconnected piece of cortex.

A woman with Alzheimer's disease died at the age of 85 years. A left sphenoid meningioma had been removed 27 years earlier. The tumor and the operation had severely altered the white matter of the frontal lobe and of the anterior part of the temporal lobe on the left side and massively disconnected a small piece of frontal cortex. There were numerous senile plaques and neurofibrillary tangles in the limbic and isocortical samples. The white matter lesions, on the operated (left) side, were associated with a lower density of neuritic plaques and of neuropil threads and with a higher density of beta-amyloid (A beta) deposits. The density of tau-positive neuritic plaques, neurofibrillary tangles and neuropil threads was close to zero, whereas the diffuse deposits of A beta were abundant, in the small disconnected piece of cortex. In this area, the white matter was severely damaged, as in the adjoining cortex, but the continuity of the cortical ribbon was also disrupted. These data show that neuritic and A beta pathologies may be dissociated and suggest that the neuritic alterations mainly involved cortico-cortical fibers coursing tangentially in the cortical ribbon.