White Matter Degeneration in Atypical Alzheimer Disease

To assess white matter (WM) tract damage in patients with atypical Alzheimer disease (AD), including early-onset AD (EOAD), logopenic variant of primary progressive aphasia (lvPPA), and posterior cortical atrophy (PCA), by using diffusion-tensor magnetic resonance (MR) imaging and to identify similarities and differences across the AD spectrum.

This study was approved by the local ethical committees on human studies, and written informed consent from all subjects was obtained prior to enrollment. WM tract damage and cortical atrophy were assessed by using diffusion-tensor MR imaging and voxel-based morphometry, respectively, in 28 patients with EOAD, 12 patients with lvPPA, and 13 patients with PCA relative to age- and sex-matched healthy subjects. Conjunction and interaction analyses were used to define overlapping and syndrome-specific patterns of brain damage.

Patients with EOAD, lvPPA, and PCA shared a common pattern of WM damage that involved the body of the corpus callosum, fornix, and main anterior-posterior pathways (P < .05). They also shared cortical atrophy of the left temporoparietal regions and precuneus (P < .05, family-wise error corrected). Patients with EOAD also had specific damage to the genu and splenium of the corpus callosum and parahippocampal tract bilaterally (P < .05). In all patients with AD, particularly in the two focal forms (lvPPA and PCA), WM damage was more severe and widely distributed than expected on the basis of cortical atrophy.

In atypical AD clinical phenotypes, the distribution of WM damage exceeds cortical atrophy and may reflect the pathologic dissemination through structural connections from atrophic to unaffected cortical regions. WM degeneration may be an early marker of AD pathologic changes in EOAD and focal AD forms.