Association of hippocampus head diffusivity and episodic memory performance in early Alzheimer's disease

Abstract

Hippocampal atrophy is a major structural imaging finding in early Alzheimer's disease (AD). Corresponding to the neuropathological finding of early neurofibrillary tangle accumulation within the CA1 subfield recent studies showed pronounced volume loss within the lateral hippocampal regions in mild cognitive impairment and mild AD. There is also increasing evidence that the anterior hippocampus plays a crucial role in the formation of memories. To investigate whether structural disturbances of the anterior part of the hippocampus are associated with impaired episodic memory function we correlated several global and regional hippocampal diffusivity and hippocampal volume measurements in 12 patients with early AD (MMSE 25.3 ± 1.8) with the cognitive performance of the patients. Compared to a group of 16 age- and education-matched normal controls all global and regional hippocampal volumes were significantly decreased bilaterally in early AD with a pronounced volume difference within the anterior hippocampus (hippocampus head). Early AD patients showed significant diffusivity increases only within the hippocampus head bilaterally. Episodic memory performance (delayed verbal recall test) correlated most strongly with increased left hippocampal head diffusivity of the patients group (r = - 0.72, p = 0.008). Significant correlations of the verbal delayed recall test with decreased hippocampal volume in the body-tail regions bilaterally (left: r = 0.59, p = 0.045; right: r = 0.58, p = 0.048) could also be found in the patient group. Our findings suggest that elevated diffusivity is a sensitive and early marker of functional relevant structural disturbances of the hippocampus in AD. Moreover, increases of cross-sectional left anterior hippocampal diffusivity seem to be more closely related to verbal memory impairment than global or regional hippocampal volume reductions. Additionally, these data support the assumption that the anterior hippocampus contains necessary neuronal substrates of episodic memory function.