perivascular spaces

Perivascular spaces, also known as Virchow-Robin spaces, are pial-lined interstitial fluid-filled spaces in the brain that surround perforating vessels. They do not have a direct connection with the subarachnoid space and in fact, the fluid within them has a slightly different composition as compared to CSF.

On axial imaging, they appear as smoothly demarcated fluid-filled cysts, typically less than 5 mm in diameter, showing the same attenuation or intensity as CSF, and often found in the basal ganglia or midbrain.

Terminology

When perivascular spaces are very numerous the brain can have a colander-like appearance, referred to as état criblé (as opposed to numerous lacunar infarcts, sometimes referred to as état lacunaire).

When perivascular spaces are very large, they are referred to as tumefactive perivascular spaces.

When located in the anterior temporal lobe and related to a vascular loop, they are known as anterior temporal lobe perivascular spaces; however, these likely represent a different entity compared to typical scattered perivascular spaces.

Epidemiology

Perivascular spaces are very common, and increasingly seen with better MRI image resolution. Depending on defining criteria, they are seen in 50-100% of patients .

Clinical features

They are almost invariably asymptomatic, even when quite large. Rarely, they can cause mass effect and can result in obstructive hydrocephalus.

Pathology

Perivascular spaces are normal structures which consist of a single or double layer of invaginated pia surrounding small cerebral blood vessels . They are mostly microscopic, although more enlarged spaces may be detected on CT or MRI .

Perivascular spaces which are visible on imaging are typically less than 5 mm in diameter but can reach much larger sizes. A so-called "giant" perivascular space or tumefactive perivascular space, and can exert enough mass effect to be symptomatic . They tend to enlarge with age and hypertension.

Previously, perivascular spaces were believed to be entirely incidental findings, mostly significant so as not to be mistaken for a more sinister pathology. This is especially true with a tumefactive or unusual in appearance (e.g. anterior temporal lobe perivascular spaces which are often mistaken for tumors).

More recently, a few studies have suggested an association between extensive basal ganglia perivascular spaces (état criblé) and changes of chronic microvascular ischemic disease . This is related to the observation that, although a few scattered perivascular spaces are a nearly ubiquitous imaging finding, the number and prominence of these spaces increases with aging and along with other findings of microvascular disease, e.g. periventricular white matter lesions and lacunar infarcts. The association remains controversial .

Location

Most commonly, perivascular spaces are located in the lower half of the basal ganglia, especially in the anterior perforated substance along the line of the anterior commissure . They are also found in :

• substantia nigra
• dentate nucleus
• subinsular region: "subinsular bright spots"
• corpus callosum and cingulate gyrus

The cortical regions are spared. In contrast to lacunar infarcts encountered more frequently in the upper half of the putamen, perivascular spaces are seen more often in the lower half.

A special type of perivascular space occurs in the anterior temporal lobe and can mimic a cystic tumor. These are discussed separately: anterior temporal lobe perivascular spaces .

Types

Perivascular spaces are divided into three main types :

• type 1: located in the area supplied by the lenticulostriate arteries entering the basal ganglia
• type 2: located in the area supplied by the perforating medullary arteries as they enter the cortical grey matter
• type 3: located in the midbrain

Associations

They are usually idiopathic, although they are seen in greater frequency in :

• mucopolysaccharidoses, e.g. Hunter disease, Hurler disease
• some muscular dystrophies
• traumatic brain injury
• hypertensive encephalopathy 
• cerebral amyloid angiopathy
• Sener syndrome

Radiographic features

As they are filled with a fluid similar to CSF, perivascular spaces have appearances akin to water on all imaging modalities and sequences. Differentiating them from foci of encephalomalacia that result from chronic lacunar infarcts can be challenging but is important as imaging findings of ischemia can lead to patients being put on medication such as antiplatelet therapy.

Perivascular spaces should be considered if:

• patient is young
• absence of vascular risk factors
• no other changes of chronic small vessel ischemia

CT

• well-circumscribed fluid-density spaces
• no enhancement
• no calcification
• CT angiography occasionally demonstrates a traversing vessel

MRI

They follow CSF signal on all pulse sequences . When small, the adjacent white matter is normal, thus helping to distinguish perivascular spaces from lacunar infarcts, which have surrounding gliosis (best seen on T2 FLAIR sequence).

Although generally cyst-like on axial sequences, on sagittal and coronal sequences a linear radiating morphology may be seen consistent with their role in surrounding vessels.

In a minority of cases, especially when they are large, a thin increased T2-signal halo may be seen. Usually, they will have a positive mass effect. On T2 sequences, a traversing vessel is sometimes seen.

The exception to the 'no surrounding high T2 signal' rule is anterior temporal lobe perivascular spaces .

History and etymology

Virchow-Robin spaces are named after German pathologist Rudolf Virchow (1821–1902) and French anatomist Charles-Philippe Robin (1821–1885) .

Differential diagnosis

For small perivascular spaces, consider:

• choroid fissure cyst
• lacunar infarcts and striatocapsular infarcts
  • a rim of gliosis seen best on FLAIR
  • neutral or negative mass effect
  • typically upper two-thirds of basal ganglia (due to infarcts of perforating end arteries)
• chronic small vessel ischemic disease
  • typically periventricular and subcortical
• neurocysticercosis
  • cyst with a dot sign
  • enhancement
  • calcification
• CNS cryptococcosis: if multiple

For giant perivascular spaces consider:

• cystic neoplasms
  • they rarely exhibit intensity similar to CSF on all MRI sequences
• ventricular diverticula
• cystic encephalomalacia
• non-neoplastic neuroepithelial cyst(s)