Chronic encephalitis caused by leukoencephalopathy.

As mentioned previously, both MS and PML are demyelinating conditions of the CNS and pose diagnostic difficulties in their differentiation because of similarities in their clinical findings. However, certain features unique to each of these diseases are helpful in clinical diagnosis. MS, unlike PML, is a disease of unknown cause. Polygenetic influences in combination with exposure to an environmental agent and immune-mediated factors may be operative in the pathogenesis of MS. Age of onset peaks in the third to fourth decades with a predominance in women, as contrasted with PML, which peaks in the fifth to sixth decades in most non-AIDS-associated cases with a slight predominance in men. MS is more prevalent in areas farther from the equator: North America, Europe, Australia, and New Zealand. Common initial symptoms seen in MS include bilateral limb weakness (with the legs being affected twice as often as the arms), hyperreflexia, spasticity, optic neuritis, diplopia, incoordination, and paresthesias. (Paresthesias are typically found in the lower limbs in a symmetric pattern, but may follow no obvious anatomic distribution and often do not correspond to the distribution of sensory symptoms. Vibration and position sense are more frequently disturbed than pain and temperature.) Intellectual impairment and mental deterioration are uncommon early in MS, whereas they are a more frequent initial presentation in PML. In addition, the presence of speech impairment and monoparesis or hemiparesis with homonymous hemianopsia is more suggestive of PML. Brain stem involvement is infrequent.(ABSTRACT TRUNCATED AT 250 WORDS)