Progressive Multifocal Leucoencephalopathy
Abnormal T2 hyperintense signal is observed in the white matter of the left high posterior frontal and parietal lobes. The lesions are non enhancing and are not associated with mass effect.
PML is a rapidly progressive fatal demyelinating disease in patients with impaired immune system (chronic lymphocytic leukemia, lymphoma, Hodgkins disease, carcinomatosis, AIDS, tuberculosis, sarcoidosis, organ transplant) . With the increasing prevelance of malignancy and AIDS , and the increasing use of steroids and chemotherapy, PML is becoming more and more common.
Etiology : It is caused by a common human polyomavirus, JC virus .
Pathophysiology : The virus attacks the oligodendrocytes, which are responsible for manufacturing myelin, leading to their destruction and hence, areas of demyelination and edema.
Histopathology: Intranuclear inclusion bodies are seen with swollen oligodendrocytes (viral particles in nuclei) and there is absence of significant perivenous inflammation.
Clinical Features: progressive neurological deficits, visual disturbances, dementia, ataxia, spasticity.
CSF fluid Examination is normal
Location : The disease has predilection for the parieto-occipital region and predominantly involves the subcortical white matter.
CT shows multicentric confluent white matter lesions of low attenuation with scalloped borders along cortex and no mass effect.
MR shows characteristic patchy high intensity lesions of white matter away from ependyma in asymmetric distribution on T2 w images with sparing of cortical gray matter.
It is typically seen in parieto-occipital region.
Initially , the lesions tend to be focal and circumscribed as the name "multifocal" implies. Later, they become confluent and large.
Typically, no mass effect is seen.
Lesions often spare the periventricular region , unlike other diseases such as multiple sclerosis that affect the white matter.
Contrast enhancement is rare.
While PML occurs in parieto – occipital distribution, it increasingly occurs in unusual locations when found in patients with AIDS. Primary involvement of the cortex, the basal ganglia, or the cerebellum and the brain stem ( all structures that PML was thought initially to spare) is no longer uncommon and may present in upto 50% of AIDS patient with PML .
Prognosis: is bad. Death occurs usually within 6 months.
Differential diagnosis : in early stages : Primary CNS lymphoma.