Multiple sclerosis (MS) is the most important form of demyelinating disease, in which loss of myelin occurs as the primary event. Myelin loss can be seen, of course, secondary to other insults, e.g. infarcts. In MS, demyelination occurs as stepwise events with episodes of improvement but a relentless progressive downhill deterioration of neurological functions.

The typical morphological feature of MS is a demyelinating plaque. They typically are found in the periventricular white matter. Notice in this picture the well-defined zone of discoloration around the ventricle. It should contain white matter but is down greyish-brown, i.e. demyelinated. The ventricles are secondarily dilated.