Experimental basis of multi-infarct dementia: memory impairments in rodent models of ischemia.

Memory is an intellectual function that is initially and consistently impaired in patients with vascular dementia. Experimental approaches to vascular dementia have so far been confined to investigations of memory impairments in rodent ischemic models. Unilateral middle cerebral artery (MCA) occlusion, multiple small embolization or transient four-vessel occlusion in rats produced acute single or multiple infarctions. In such rats, significant memory impairments occurred during the subacute or chronic phases, but were partially reversible. Permanent stenosis of both common carotid arteries in gerbils caused no ischemic changes at 1 day after stenosis but induced multiple infarctions after 1 week of stenosis, probably due either to chronic recurrent ischemia resulting from transient repetitive obstruction of the carotid arteries or to chronic low perfusion. The memory impairments in this model were persistent. Permanent bilateral common carotid artery occlusion in rats produced multiple infarctions plus white matter changes after 1 week of occlusion. Marked memory impairment was also observed in this model. The results of the above studies suggest that memory impairments due to ischemic causes may be partially reversible provided that the infarctions occur only once and are followed by flow recovery. Memory impairments, however, appear to persist if the brain is exposed to chronic recurrent ischemia or chronic moderately low perfusion. A repetitive or persistent low-flow state appears to be an important factor in determining the irreversibility of cognitive impairments.