Progressive multifocal leukoencephalopathy (PML) is a rare opportunistic infection of the central nervous system caused by the JC (John Cunningham) virus.1 This virus is common and is generally innocuous in an immunocompetent host. However, in individuals with innate, acquired, or iatrogenic immunodeficiency, the JC virus can become active and infect oligodendrocytes, leading to their lysis. Oligodendrocyte lysis leads to central nervous system demyelination, which may then result in focal neurologic deficits including hemiparesis, visual field deficits, and cognitive impairment. Progressive multifocal leukoencephalopathy is usually irreversible and fatal.
Korman BD, Tyler KL, Korman NJ. Progressive Multifocal Leukoencephalopathy, Efalizumab, and ImmunosuppressionA Cautionary Tale for Dermatologists. Arch Dermatol. 2009;145(8):937-942. doi:10.1001/archdermatol.2009.175