Diencephalic or hippocampal amnesia: different etiologies, common mechanisms.

ObjectivesTo compare regional volume deficits within the Papez circuit in Alzheimers disease (AD) and Korsakoffs syndrome (KS), taking into account the neurodegenerative nature of AD. Methods18 KS patients, 40 AD patients (20 with Moderate AD (MAD) matched on cognitive deterioration with KS patients and 20 with Severe AD (SAD)), and 70 healthy controls underwent structural MRI examination. Volumes of the hippocampi, thalami, cingulate gyri, mammillary bodies (MB) and mammillothalamic tracts (MTT) were extracted. ResultsFor the anterior and posterior cingulate gyri, and anterior thalamic nuclei, all patient groups were significantly affected compared to controls but did not differ between each other. Lower volumes were observed in all patient groups compared to controls in the entire thalamus, mediodorsal thalamic nuclei and MB, but these regions were more severely damaged in KS compared to AD. MTT volumes were significantly damaged in KS only. Hippocampi were affected in all patient groups but more severely in the SAD than in the KS and MAD. InterpretationThe specificity of KS compared to AD relies on the severity of the MB and mediodorsal nuclei shrinkage, as well as the atrophy of the MTT. Several nodes of the Papez circuit were damaged to the same extent in AD and KS: the anterior thalamic nuclei, the cingulate cortex and the hippocampus (in MAD only). Our results encourage considering common mechanisms in the pathophysiology of amnesia regardless of etiology and question the relevance of the classical distinction between hippocampal and diencephalic amnesia.