Corpus callosum atrophy is a possible indicator of region- and cell type-specific neuronal degeneration in Alzheimer disease: a magnetic resonance imaging analysis.

BACKGROUND: Pathological studies in Alzheimer disease indicate the specific loss of layer III and V large pyramidal neurons in association cortex. These neurons give rise to long corticocortical connections within and between the cerebral hemispheres.
OBJECTIVE: To evaluate the corpus callosum as an in vivo marker for cortical neuronal loss.
METHOD: Using a new imaging technique, we measured region-specific corpus callosum atrophy in patients with Alzheimer disease and correlated the changes with neuropsychological functioning. Total cross-sectional area of the corpus callosum and areas of 5 callosal subregions were measured on midsagittal magnetic resonance imaging scans of 14 patients with Alzheimer disease (mean age, 64.4 years; Mini-Mental State Examination score, 11.4) and 22 healthy age- and sex-matched control subjects (mean age, 66.6 years; Mini-Mental State Examination score, 29.8). All subjects had minimal white matter changes.
RESULTS: The total callosal area was significantly reduced in the patients with Alzheimer disease, with the greatest changes in the rostrum and splenium and relative sparing of the callosal body. Regional callosal atrophy correlated significantly with cognitive impairment in the patients with Alzheimer disease, but not with age or the white matter hyperintensities score.
CONCLUSIONS: Callosal atrophy in patients with Alzheimer disease with only minimal white matter changes may indicate loss of callosal efferent neurons in corresponding regions of the cortex. Because these neurons are a subset of corticocortical projecting neurons, region-specific callosal atrophy may serve as a marker of progressive neocortical disconnection in Alzheimer disease.