Perivascular spaces (PVS), also known as Virchow-Robin spaces, are naturally occurring channels found throughout the brain. Although they are a normal feature of brain anatomy, they are typically microscopic and invisible on standard imaging. A “prominent perivascular space” refers to a state where these channels have enlarged enough to be visible on a magnetic resonance imaging (MRI) scan. Understanding their significance requires examining their anatomy, function in fluid dynamics, and how their visible enlargement relates to brain health.
The Anatomy and Structure of Perivascular Spaces
Perivascular spaces are fluid-filled sleeves surrounding small blood vessels as they penetrate the brain tissue from the surface. These channels follow the penetrating arteries, arterioles, venules, and veins, extending deep into the brain parenchyma. The PVS is an extension of the subpial space, the region between the pia mater (the innermost layer of the meninges) and the brain itself.
Astrocytes line the channels, forming the glia limitans, which creates a boundary between the vessel wall and the surrounding brain tissue. The PVS itself is not part of the vessel wall, and PVS are not true lymphatic vessels. On imaging, they are most frequently identified in the basal ganglia and the white matter of the cerebral hemispheres.
Physiological Role in Brain Waste Clearance
PVS are intrinsically tied to the brain’s unique fluid handling and waste removal system, known as the glymphatic system. This system uses PVS as conduits for fluid exchange. Cerebrospinal fluid (CSF) flows into the brain parenchyma along the perivascular spaces surrounding the arteries, driven partially by arterial pulsation.
Inside the brain tissue, this fluid mixes with interstitial fluid (ISF), which contains metabolic waste products. The glymphatic system is important for clearing potentially harmful proteins, such as amyloid beta and tau, implicated in neurodegenerative conditions. The fluid mixture is then channeled out along the perivascular spaces surrounding the veins. This process is facilitated by the water channel protein Aquaporin-4 (AQP4) located on the astrocyte endfeet.
Defining “Prominent” Perivascular Spaces on Imaging
A perivascular space is defined as “prominent” when its diameter exceeds the typical microscopic size and becomes visible on MRI. The threshold is generally a diameter greater than 2 or 3 millimeters. On an MRI scan, prominent PVS appear as distinct, sharply delineated structures that are round, ovoid, or linear, depending on the angle of the penetrating vessel.
These structures exhibit a signal intensity identical to CSF, confirming their fluid content. Radiologists confirm the diagnosis because they do not cause a mass effect, meaning they do not compress the surrounding brain tissue. To quantify prominence, radiologists use visual rating scales, such as the Potter or Wardlaw scale. These scales grade severity based on the number of spaces present in specific brain regions. For example, in the basal ganglia or centrum semiovale, a Grade 1 score may represent 1–10 visible PVS, while a Grade 4 indicates more than 40.
Prominence and Association with Specific Neurological Conditions
A few small prominent PVS are a common, incidental finding, particularly in older individuals, and are rarely concerning on their own. However, a high burden or widespread presence of prominent PVS is recognized as an imaging marker of underlying cerebral small vessel disease (SVD). SVD affects the tiny blood vessels in the brain and is often linked to chronic hypertension and aging.
Extensive prominence, especially in the basal ganglia, correlates strongly with vascular risk factors like high blood pressure, suggesting a link to arterial stiffening. The pulsatile force from stiffened arteries is theorized to enlarge the PVS over time, impairing fluid movement. This impaired fluid dynamics can lead to the glymphatic system failing to adequately clear waste products.
Poor clearance of metabolic waste, including amyloid beta, links a high burden of prominent PVS to neurodegenerative disorders, such as Alzheimer’s and Parkinson’s disease. Prominent PVS frequently coexist with other SVD markers, including white matter hyperintensities and lacunar infarcts. While prominent PVS are not usually the direct cause of symptoms, a high number is often interpreted as a visual sign of chronic brain pathology and impaired waste clearance.