A perivascular space, also known as a Virchow–Robin space, is a fluid-filled space surrounding certain blood vessels in several organs, including the brain, potentially having an immunological function, but more broadly a dispersive role for neural and blood-derived messengers. The brain pia mater is reflected from the surface of the brain onto the surface of blood vessels in the subarachnoid space. In the brain, perivascular cuffs are regions of leukocyte aggregation in the perivascular spaces, usually found in patients with viral encephalitis. Perivascular spaces vary in dimension according to the type of blood vessel. In the brain where most capillaries have an imperceptible perivascular space, select structures of the brain, such as the circumventricular organs, are notable for having large perivascular spaces surrounding highly permeable capillaries, as observed by microscopy. The median eminence, a brain structure at the base of the hypothalamus, contains capillaries with wide perivascular spaces. In humans, perivascular spaces surround arteries and veins can usually be seen as areas of dilatation on MRI images. While many normal brains will show a few dilated spaces, an increase in these spaces may correlate with the incidence of several neurodegenerative diseases, making the spaces a topic of research.
Structure Perivascular spaces are gaps containing interstitial fluid that span between blood vessels and their host organ, such as the brain, which they penetrate and serve as extravascular channels through which solutes can pass. Like the blood vessels around which they form, perivascular spaces are found in both the brain subarachnoid space and the subpial space. Perivascular spaces surrounding arteries in the cerebral cortex and the basal ganglia are separated from the subpial space by one or two layers of leptomeninges, respectively, as well as the pia mater. By virtue of the leptomeningeal cell layer, the perivascular spaces belonging to the subarachnoid space are continuous with those of the subpial space. The direct communication between the perivascular spaces of the subarachnoid space and the subpial space is unique to the brain's arteries, as no leptomeningeal layers surround the brain's veins. Use of the scanning electron microscope has determined that the spaces surrounding blood vessels in the subarachnoid space are not continuous with the subarachnoid space because of the presence of pia mater cells joined by desmosomes. Perivascular spaces, especially around fenestrated capillaries, are found in many organs, such as the thymus, liver, kidneys, spleen, bones, and pineal gland. Particularly within the brain circumventricular organs – subfornical organ, area postrema, and median eminence – large perivascular spaces are present around fenestrated capillaries, indicating that the spaces serve a dispersive role for brain- or bloodborne messengers. Perivascular spaces may be enlarged to a diameter of five millimeters in healthy humans and do not imply disease. When enlarged, they can disrupt the function of the brain regions into which they project. Dilation can occur on one or both sides of the brain. Dilated perivascular spaces are categorized into three types:
Type 1 are located on the lenticulostriate arteries projecting into the basal ganglia Type 2 are located in the cortex following the path of the medullary arteries Type 3 are located in the midbrain Perivascular spaces are most commonly located in the basal ganglia and white matter of the cerebrum, and along the optic tract. The ideal method used to visualize perivascular spaces is T2-weighted MRI. The MR images of other neurological disorders can be similar to those of the dilated spaces. These disorders are:
cystic neoplasms lacunar infarctions cystic periventricular leukomalacia cryptococcosis multiple sclerosis mucopolysaccharidoses neurocysticercosis arachnoid cysts neuroepithelial cysts Perivascular spaces are distinguished on an MRI by several key features. The spaces appear as distinct round or oval entities with a signal intensity visually equivalent to that of cerebrospinal fluid in the subarachnoid space. In addition, a perivascular space has no mass effect and is located along the blood vessel around which it forms.
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