title: Microstructural white matter changes, not hippocampal atrophy, detect early amnestic mild cognitive impairment.
abstract: BACKGROUND
  Alzheimer's disease (AD) is generally considered to be characterized by pathology in gray matter of the brain, but convergent evidence suggests that white matter degradation also plays a vital role in its pathogenesis. The evolution of white matter deterioration and its relationship with gray matter atrophy remains elusive in amnestic mild cognitive impairment (aMCI), a prodromal stage of AD.
  
  
  METHODS
  We studied 155 cognitively normal (CN) and 27 'late' aMCI individuals with stable diagnosis over 2 years, and 39 'early' aMCI individuals who had converted from CN to aMCI at 2-year follow up. Diffusion tensor imaging (DTI) tractography was used to reconstruct six white matter tracts three limbic tracts critical for episodic memory function - the fornix, the parahippocampal cingulum, and the uncinate fasciculus; two cortico-cortical association fiber tracts - superior longitudinal fasciculus and inferior longitudinal fasciculus; and one projection fiber tract - corticospinal tract. Microstructural integrity as measured by fractional anisotropy (FA), mean diffusivity (MD), radial diffusivity (RD) and axial diffusivity (AxD) was assessed for these tracts.
  
  
  RESULTS
  Compared with CN, late aMCI had lower white matter integrity in the fornix, the parahippocampal cingulum, and the uncinate fasciculus, while early aMCI showed white matter damage in the fornix. In addition, fornical measures were correlated with hippocampal atrophy in late aMCI, whereas abnormality of the fornix in early aMCI occurred in the absence of hippocampal atrophy and did not correlate with hippocampal volumes.
  
  
  CONCLUSIONS
  Limbic white matter tracts are preferentially affected in the early stages of cognitive dysfunction. Microstructural degradation of the fornix preceding hippocampal atrophy may serve as a novel imaging marker for aMCI at an early stage.
authors.affiliation: Centre for Healthy Brain Ageing, University of New South Wales, Randwick, New South Wales, Australia.
authors.forename: Lin
authors.initials: L
authors.lastname: Zhuang
authors.forename: Perminder S
authors.initials: PS
authors.lastname: Sachdev
authors.forename: Julian N
authors.initials: JN
authors.lastname: Trollor
authors.forename: Simone
authors.initials: S
authors.lastname: Reppermund
authors.forename: Nicole A
authors.initials: NA
authors.lastname: Kochan
authors.forename: Henry
authors.initials: H
authors.lastname: Brodaty
authors.forename: Wei
authors.initials: W
authors.lastname: Wen
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D000647:Amnesia / Q000150:complications*; D001284:Atrophy / Q000473:pathology / Q000503:physiopathology; D001921:Brain / Q000473:pathology* / Q000503:physiopathology; D060825:Cognitive Dysfunction / Q000150:complications* / Q000473:pathology* / Q000503:physiopathology; D056324:Diffusion Tensor Imaging; D005260:Female; D006624:Hippocampus / Q000473:pathology / Q000503:physiopathology; D006801:Humans; D021621:Imaging, Three-Dimensional; D008297:Male; D061212:Memory, Episodic; D009929:Organ Size; D013997:Time Factors