Progressive Multifocal Leukoencephalopathy

      PML has an affinity for subcortical white matter, and the classic distribution is in the parietal-occipital lobes. It is not primarily a periventricular process, but as the disease progresses, the deeper white matter is also affected. Any white matter structures can be involved, but lesions of the corpus callosum are much less common than in multiple sclerosis, for example. Brain stem and cerebellar lesions are found in about one-third of patients; occasionally, they can be the solitary presenting lesion. Basal ganglia and thalamic sites generally represent extension from lesions in the internal capsule or damage to white matter fibers coursing through the gray matter structures. Endnote

      The white matter lesions of PML are patchy and round or oval at first, but then become confluent and large. The process is often distinctly asymmetric and initially involves the peripheral white matter, following the contours of the gray-white matter interface to give outer scalloped margins. Lesions tend to be homogeneous with well-defined margins. The prolonged T1 and T2 relaxation times reflect the loss of myelin and increased water. Mass effect and contrast enhancement are rarely seen. 

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