In vivo structural neuroanatomy of corpus callosum in Alzheimer's disease and mild cognitive impairment using different MRI techniques: a review.

The corpus callosum (CC), which connects the two cerebral hemispheres, is the largest white matter fiber bundle in the human brain. This structure presents a peculiar myelination pattern: it has small diameter fibers, located in the genu, which myelinate much later in normal development, and large diameter fibers of the splenium, which myelinate early in development. Although the pathology of AD mainly involves the cerebral gray matter structure, there is evidence that white matter may also be involved. To illustrate callosal white matter changes in AD pathology, in this review we summarize in vivo imaging studies in humans, focusing on region of interest, voxel-based morphometry, diffusion-weighted imaging, and diffusion tensor imaging techniques. Our aims were to identify where in the CC, when in the different stages of AD, and how callosal changes can be detected with different MRI techniques. Results showed that changes in the anterior (genu and anterior body) as well as in the posterior (isthmus and splenum) portions of the CC might already be present in the early stages of AD. These findings support the hypothesis that two mechanisms, Wallerian degeneration and myelin breakdown, might be responsible for the region-specific changes detected in AD patients. Wallerian degeneration affects the posterior CC subregion, which receives axons directly from those brain areas (temporo-parietal lobe regions) primarily affected by the AD pathology. Instead, the myelin breakdown process affects the later-myelinating CC subregion and explains the earlier involvement of the genu in CC atrophy.