Pathological changes in the parahippocampal region in select non-Alzheimer's dementias.

The transentorhinal and entorhinal regions of the human brain extend over the ambient gyrus and anterior portions of the parahippocampal gyrus. They are important components of the limbic loop which receives its major afferents from the neocortical sensory association areas and generates powerful efferent projections both directly and via intermediary relay stations to the prefrontal cortex. The bilateral structural preservation of limbic loop components is a prerequisite for the maintenance of intact memory functions. In progressive neurodegenerative diseases, such as Alzheimer's disease, argyrophilic grain disease, Pick's disease, idiopathic Parkinson syndrome, and Huntington's disease, the transentorhinal and entorhinal regions are particularly susceptible to severe pathological changes. The transentorhinal region typically registers the initial alterations and becomes the most severely involved. From this transitional region of the mesocortex, the alterations usually invade with decreasing severity both the entorhinal region and temporal proneocortex. Each type of lesion that develops in the above-mentioned neurodegenerative disorders hampers or even interrupts data-transport from the sensory neocortex to the prefrontal neocortex, thereby contributing to the insidious development of progressive changes in personality, cognitive decline, and, ultimately, dementia.