The influence of the ground substance on the extracellular water of normal and edematous human brain: focal edema and the demyelinating diseases, including multiple sclerosis.

The presence of a ground substance in brain provides a mechanism by which edema localized to one region of the white matter might occur without spreading diffusely into the adjacent tissues. The most common such localization is the sparing of the arcuate white matter when the deeper white matter is markedly edematous. This may be related to the higher concentration of mucopolysaccharides in the former. Petechial hemorrhages in the white matter may be surrounded by a zone free of edema, although the hemorrhagic zone itself is almost certainly edematous. This, and the presence of a central zone within some of the petechiae forming a ring hemorrhage may reflect the influence of the ground substance. Focal lesions of the dorsum of the corpus callosum and similar lesions of the basal surface of the pons, these probably due to traumatization by the contiguous falx or arteries, are characterized by myelin loss and axon preservation, a characteristic of edema; the surrounding tissues are not edematous. Severe hypertension is sometimes associated with the presence of clusters of focal perivenous demyelinating lesions in the white matter, the axons being preserved. These resemble the lesions of acute disseminated encephalomyelitis and may be due to edema; they are surrounded by nonedematous white matter. It is suggested that the same concept may apply to the focal demyelinating lesions of acute disseminated encephalomyelitis, multifocal leukoencephalopathy, central pontine myelinolysis and of multiple sclerosis, i.e. the "true" demyelinating diseases, just as has already been suggested for diffuse sclerosis.