The spectrum of age-associated brain abnormalities: their measurement and histopathological correlates

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Abstract
Magnetic resonance imaging (MRI) has dramatically increased our ability to detect morphological abnormalities in relation to aging of the brain. Among those changes are alterations of the white matter which display high signal intensity on both proton density and T2-weighted images. They may be seen in the deep and subcortical white matter or in a periventricular location. In clinically asymptomatic individuals the reported prevalence ranges from 20% to 60% for deep and subcortical white matter hyperintensities and from 15% to 94% for periventricular changes. Besides different characteristics of the populations examined these wide ranges are a consequence of quite diverse rating schemes and measurement approaches. Inadequate grading of MRI hyperintensities may also explain some of the inconsistencies in the reported associations of white matter damage with cerebrovascular risk factors or cognitive functions. Therefore development of a commonly accepted rating scheme would be desirable. Histopathologic observations could lay the basis. Hyperintense periventricular capping of the frontal horns and a smooth halo of periventricular hyperintensity have been linked to disruption of the ependymal lining, subependymal gliosis and concomitant loss of myelin. Punctate lesions in the deep and subcortical white matter corresponded to minor perivascular reduction in myelin content possibly because of a lower permeability of thickened arteriolar walls. Larger patchy and confluent hyperintensities, however, appear to indicate more extensive ischemic damage consistent with advanced microangiopathy. In parallel, newer MRI techniques may also contribute to the delineation and separation of these various types of tissue alteration.