Electrolyte-induced demyelination in rats - 2. Ultrastructural evolution

This study presents the electron microscopic evolution of lesions in electrolyte-induced demyelination (EID) in rats, a lesion which bears striking histological and clinical similarity to central pontine myelinolysis. The earliest change was observed during the hyponatremic phase and consisted of minimal intracellular edema present throughout the brain. Following the injection of hypertonic saline, additional changes were observed which were restricted to sites previously reported to be frequently involved in EID. Early dilatation of the inner tongue of oligodendrocyte cytoplasm in myelinated nerve fibers was observed at 3h post hypertonic saline injection (PHS). This was followed, at 48h PHS, by the appearance of degenerative changes consistent with dying oligodendrocytes. Well-delineated, vacuolar and spongy lesions, seen by light microscopy, were present by 48h PHS at the same sites as above. Electron microscopically, this appearance was found to be due to striking intramyelinic edema. By 96h PHS, macrophages containing myelin and other cellular debris were frequently present at these sites. Concomitantly, myelin sheaths underwent vesicular disruption and disintegration. This sequence of events suggest a lesion of the oligodendrocyte-myelin complex, secondary to initial blood-brain barrier damage and edema.