Subacute dementia with necrotising encephalitis selectively involving the fornix and splenium. Retrograde development of Alzheimer's neurofibrillar tangles in the subiculum.

An elderly woman developed subacute progressive dementia with predominant impairment in recent memory and CT findings indicating an enhancing lesion near the splenium of the corpus callosum. Autopsy four months after the onset of the symptoms revealed necrotising encephalitis selectively involving the bilateral fornices and the adjacent splenium. Degeneration was marked in the contiguous right fimbria hippocampi. Particularly numerous Alzheimer's neurofibrillar tangles (NFTs) were present in the right subiculum, while they were scattered in Sommer's sectors and parahippocampal gyri and were practically nonexistent elsewhere. Senile plaques were rare. Electron microscopy disclosed that the right subicular tangles were composed mostly of 15-nm straight tubules which were also frequently observed in the myelinated axons. Since the major projection fibers to the fornix originate in the subiculum, the distinctive pattern of NFT distribution might be derived from the retrograde neuronal changes secondary to the fornix-fimbria lesion. This case represents a rare form of amnesia-dementia confirmed anatomically to have been caused by fornix lesions.