Virchow-Robin Perivascular Spaces

       When nutrient vessels penetrate the brain substance, the pia mater is carried along with the vessel down to the capillary level. The small subarachnoid space that follows the pia is called the Virchow-Robin (VR) space. These perivascular CSF spaces appear as punctate areas of high signal on T₂-weighted images. 

      Occasionally, very large (1 cm or more) VR spaces will be observed in the basal ganglia region. If the patient is young and has no risk factors for vascular or degenerative disease, these large VR spaces are probably normal variants, representing a confluence of penetrating arteries and veins.

      Not infrequently, VR spaces in the brainstem will be sufficiently large to be seen on MR images. On T₂-weighted axial images they are sectioned longitudinally and appear as hyperintense linear structures coursing in a ventrolateral direction. Their linear character generally distinguishes them from small brainstem infarcts.

      If the brain become atrophic and loses volume, it retracts away from the vessels and extracellular fluid fills the space. On postmortem studies, these perivascular fluid spaces appear like a network of tunnels within the brain substance. These changes have been termed état criblé (sieve-like). These fluid spaces simply represent dilated VR spaces. Brain atrophy results in dilated VR spaces in the same manner that the cortical sulci become enlarged. As expected, in older patients with atrophic brains the VR spaces are larger and appear more numerous.  

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