The CSF is either normal or shows a few lymphocytes. Pathologically, PML begins with small demyelinative foci at the cortex-white matter junction. PML is a lytic infection leading to oligodendrocyte destruction. Polyoma viruses can also become incorporated into the host genome and cause neoplastic transformation. In PML, astrocytes are also infected by JC virus in a non-lytic manner and show pronounced atypia, suggesting neoplastic change.
Inflammation is usually minimal.
There is no peripheral nerve demyelination or disease in any other organ system. In addition to its clinical significance, PML is interesting as a model of demyelination due to a lytic infection of myelin-producing cells. There is loss of myelin and less severe loss of axons. Neurons of the nuclei pontis are relatively spared. No inflammation is seen.