Peri vascular space

Peri vascular spaces (PVS), also known as Virchow-Robin spaces, are pial lined intersitital fluid filled spaces that surround perforating vessels ¹. They do not have a direct connection with the subarachnoid space and in fact the fluid within them is slightly different.

Epidemiology

Overall PVS's are very common, and increasingly seen with better MRI image quality. Depending on your definition, they are seen in 50-100% of patients ^2-3 and their relevance is mainly in not mistaking them for pathology.

Pathology

They are typically less than 5 mm in diameter, but can reach much larger sizes, so called giant peri vascular space or tumefactive perivascular space, and can exert enough mass effect to be symptomatic. When very numerous the brain can have a colander-like appearance referred to as etat crible (as opposed to numerous lacunes = etat lacunaire)

Location

Most commonly they are located in the lower half of the basal ganglia especially along the line of the anterior commissure ³. They are also found in ¹:

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

The cortical regions are spared

Associations

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

• mucopolysaccharidoses ¹, e.g. Hunter disease, Hurler disease 
• some muscular dystrophies

Radiographic features

As they are filled with fluid similar to CSF they have appearances akin to water on all imaging modalities and sequences.

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 PVS from lacunar infarcts which have surrounding gliosis. In a minority of cases, especially when they are large thin increased T2 halo may be seen. Usually will have positive mass effect. On T2 sequences a traversing vessel is sometimes seen.

Complications

Rarely they can cause obstructive hydrocephalus.

Differential diagnosis

For small "run of the mill" PVSs consider

• choroid fissure cyst
• lacunar infarcts
  • rim of gliosis seen best on FLAIR
  • neutral or negative mass effect
• neurocysticercosis
  • cyst with a dot sign
  • enhancement
  • calcification
• CNS cryptococcosis : if multiple

For giant PVS's consider

• cystic neoplasm(s) ¹
• ventricular diverticula
• cystic encephalomalacia
• non-neoplastic neuroepithelial cyst(s)