What imaging finding is characteristic of PML?

Progressive multifocal leukoencephalopathy (PML) is a demyelinating disease caused by the reactivation of the JC virus, primarily in immunocompromised individuals. The characteristic imaging findings seen on magnetic resonance imaging (MRI) in patients with PML include multiple non-enhancing lesions in the white matter. These lesions are typically asymmetric and may be situated in the periventricular regions.

The reason multiple demyelinating lesions are the hallmark of PML is that the disease affects oligodendrocytes, the cells responsible for the formation of myelin in the central nervous system. As these cells are destroyed, demyelination occurs, leading to the appearance of multiple lesions on imaging studies. The lesions are often not associated with significant mass effect or enhancement, distinguishing PML from other processes such as glioblastomas or primary CNS lymphomas, which can present with single or enhancing lesions and might exert mass effect.

Understanding why multiple demyelinating lesions are a defining characteristic helps in the differential diagnosis of white matter pathology, especially when assessing patients with compromised immune systems. This is critical for management and treatment considerations in those presenting with neurological symptoms.