Milky Way sign (of PML)

The milky wayMilky Way sign, also known less memorably merely as the punctate pattern, is an MRI feature described in progressive multifocal leukoencephalopathy (PML) and is particularly useful when distinguishing this from new lesions of multiple sclerosis ^1,2. 

Terminology

Although the term "milk way"Milky Way appearance" has primarily been reported in the context of progressive multifocal leukoencephalopathy, other pathologies share a similar appearance (see differential diagnosis below). Milky Way is capitalised as it refers to our own galaxy which is a proper noun.

Additionally, the term has also been used to denote the punctate enhancement pattern often seen in the same patients on post-contrast T1 weighted images ³. 

This sign should not be confused with the similarly named starry sky appearance (MRI) seen in multiple biliary hamartomas and the starry sky appearance (ultrasound) seen in a variety of liver pathologies. 

Pathology

The sign is believed to be the result of the accumulation of CD8-positive T-cells within the perivascular spaces with ensuing inflammation ².

Radiographic features

MRI

The sign denotes multiple punctate regions of high T2 surrounding the main component of the new lesion, appearing reminiscent of whitethe hazy band of whiteness (formed from the merging of the light of innumerable stars onwhich are not individually visible) stretching across the dark background of the night sky representing our home galaxy, the Milky Way ^1,2.  

Differential diagnosis

Although the term milky wayMilky Way sign is mostly used in the context of progressive multifocal leukoencephalopathy​ a similar pattern has been reported in a number of other conditions including ²: 

• CLIPPERS
• CNS vasculitis
• neurosarcoidosis