[Neuroradiological Diagnosis of Progressive Multifocal Leukoencephalopathy (PML): Pathology of Extending/expanding Demyelinating Lesions Detected by MRI].

Although progressive multifocal leukoencephalopathy (PML) is known as a fatal disease, some recent cases have shown favorable prognosis with early diagnosis. Therefore, detection of early PML lesions using magnetic resonance imaging (MRI) is important. PML lesions are divided into 4 groups based on distinct developing patterns: A)cerebral lesion, B)central lesion including deep gray matter, C)infratentorial lesion of the brain stem and cerebellum, and D)punctate lesions in the deep white matter. These lesions develop in 3 steps: 1)initiation of a small demyelinating lesion, 2)extension/expansion, and 3)fusion. It is likely that the viruses first reach the brain via the bloodstream and form small demyelinating foci (initiation). Second, the demyelinating foci spread along nerve fibers or expand at the sites (extension/expansion). Finally, the foci fuse with one another to form larger demyelinating lesions (fusion). Understanding the spreading patterns of the virus could help early MRI diagnosis of PML, which is important for favorable prognosis. (Received May 7, 2020; Accepted May 11, 2020; Published September 1, 2020).