At least 30% of neurologically healthy individuals over 60 years of age have focal white matter abnormalities on MRI scans. Patchy areas of high signal intensity on T2-weighted images in the periventricular deep white matter and the centrum semiovale as well as capping of the lateral ventricular margins are findings commonly seen with aging. These abnormalities are better imaged by MRI compared with CT and are best visualized on an intermediate T2-weighted image where CSF is isointense with white matter. Other common changes that may occur with aging include ventricular enlargement, widening of the sulci, multifocal areas of hyperintensity in the basal ganglia, and progressive hypointensity in the putamen and caudate to a level almost equal to the globus pallidus secondary to preferential iron accumulation.
Pathologic correlation has shown that these patchy white matter lesions correspond to clinically silent infarcts, ischemic demyelination, and areas of gliosis. Chronic low-grade vascular insufficiency produces atrophic perivascular demyelination rather than acute tissue necrosis created by ischemia or occlusive vascular disease. The white matter hyperintensities are presumably caused by hypoperfusion and arteriolar disease. Hypoperfusion may be due to episodes of hypotension, hypoxia, or carotid artery disease. Arteriolar disease is most often a result of long term hypertension. Deep white matter is more subject to infarction than gray matter because of its more tenuous blood supply. The periventricular deep white matter and basal ganglia are supplied by long noncollateralizing perforating vessels which arise from large arteries at the base of the brain. The centrum semiovale is fed by the most distal intraparenchymal penetrating arterioles and is a watershed zone. The cerebral cortex has a rich blood supply with short penetrators from the pial arteries which end in the cortex and subcortical white matter (arcuate fibers) and short branches to luxurient capillary networks.
Other processes that involve the white matter such as multiple sclerosis, SLE, and radiation change may mimic the leukoencephalopathic alterations seen with the hypoperfusion/arteriolar changes common with aging. Deep white matter ischemia is often a symmetric process whose lesions have smooth margins allowing differentiation from MS in which the periventricular lesions have an irregular outline-"lumpy-bumpy"- and demonstrate marked asymmetry between right and left sides. Another common observation in deep white matter ischemia is poor gray-white matter contrast (washed- out appearance), a finding not typical of MS. Binswanger disease (subcortical arteriosclerotic encephalopathy) can be distinguished by the presence of dementia or other mental abnormalities and by its relatively more confluent and extensive involvement (see Case 617). However, the MR findings alone, without clinical history, have not shown a strong correlation between the presence of periventricular lesions and the presence or absence of dementia.