Macrostructural and Microstructural White Matter Alterations Are Associated with Apathy across the Clinical Alzheimer's Disease Spectrum

Abstract

Apathy is the commonest neuropsychiatric symptom in Alzheimer's disease (AD). Previous findings suggest that apathy is caused by a communication breakdown between functional neural networks involved in motivational-affective processing. This study investigated the relationship between white matter (WM) damage and apathy in AD. Sixty-one patients with apathy (AP-PT) and 61 without apathy (NA-PT) were identified from the Alzheimer's Disease Neuroimaging Initiative (ADNI) database and matched for cognitive status, age and education. Sixty-one cognitively unimpaired (CU) participants were also included as controls. Data on cognitive performance, cerebrospinal fluid biomarkers, brain/WM hyperintensity volumes and diffusion tensor imaging indices were compared across groups. No neurocognitive differences were found between patient groups, but the AP-PT group had more severe neuropsychiatric symptoms. Compared with CU participants, only apathetic patients had deficits on the Clock Drawing Test. AP-PT had increased WM damage, both macrostructurally, i.e., larger WM hyperintensity volume, and microstructurally, i.e., increased radial/axial diffusivity and reduced fractional anisotropy in the fornix, cingulum, anterior thalamic radiations and superior longitudinal and uncinate fasciculi. AP-PT showed signs of extensive WM damage, especially in associative tracts in the frontal lobes, fornix and cingulum. Disruption in structural connectivity might affect crucial functional inter-network communication, resulting in motivational deficits and worse cognitive decline.

Keywords: Alzheimer’s disease; apathy; white matter.

Figure 1

Differences in relative WM hyperintensity volume (WMHV/TIV) across groups (error bars are standard deviations). AP-PT: Patients with apathy, CU: Cognitively unimpaired, NA-PT: Patients without apathy, TIV: Total intracranial volume, WM: White matter, WMHV: White matter hyperintensity volume.

Figure 2

WMH maps created by means of a one-sample t-test (cluster-forming threshold p < 0.001; FWE-corrected at cluster level): (A) AP-PT group; (B) NA-PT group; (C) CU group. L: Left, R: Right.

Figure 3

Clusters of altered WM microstructural integrity in the AP-PT compared with the CU group: (A) fractional anisotropy—AP-PT < CU; (B) axial diffusivity—AP-PT > CU; (C) radial diffusivity—AP-PT > CU (the significant small cluster in the fornix is highlighted by the blue circle).