What causes a dilated perivascular space on a head computed tomography (CT) scan?

Causes of Dilated Perivascular Spaces on Head CT

Dilated perivascular spaces (DPVS) on head CT are primarily caused by age-related changes, vascular risk factors, and rarely by pathological processes that disrupt normal interstitial fluid drainage pathways.

Normal Anatomy and Pathophysiology

Perivascular spaces, also known as Virchow-Robin spaces, are pial-lined, interstitial fluid-filled structures that surround cerebral vessels as they penetrate brain tissue 1. These spaces:

• Are normally <2mm in diameter in healthy individuals
• Contain interstitial fluid, not cerebrospinal fluid
• Function as drainage pathways for interstitial fluid
• Are typically found in characteristic locations:
  • Basal ganglia (along lenticulostriate vessels)
  • Centrum semiovale (high convexity white matter)
  • Midbrain

Common Causes of Dilation

1. Age-related changes: The most common cause of DPVS is normal aging 2, 3

   • Small dilations in high convexity areas represent an anatomic variant called "état criblé"
   • Prevalence increases with advancing age

2. Vascular risk factors:

   • Hypertension
   • Small vessel disease
   • Cerebrovascular disease

3. Impaired interstitial fluid drainage:

   • Defects in the drainage of interstitial fluid into the ventricular system 1
   • Not necessarily related to increased intraventricular pressure

Pathological Causes

1. Giant or tumefactive DPVS:

   • Rare massive enlargement of perivascular spaces
   • Can cause mass effect and neurological symptoms
   • May require neurosurgical intervention 1
   • Can cause hydrocephalus by compressing adjacent structures

2. Inflammatory conditions:

   • Chronic lymphocytic inflammation with pontine perivascular enhancement responsive to steroids (CLIPPERS)
   • Vasculitis 4

3. Infectious processes:

   • Cryptococcosis
   • Neurocysticercosis
   • Toxoplasmosis 3

4. Association with intracranial tumors:

   • Some cases show regression of DPVS following resection or regression of intracranial tumors 5

Imaging Characteristics on CT

On head CT, dilated perivascular spaces appear as:

• Hypodense (CSF-density) lesions
• Well-defined margins
• No contrast enhancement
• No surrounding edema (unless giant/tumefactive)
• Typically <5mm (unless giant/tumefactive)
• Located in characteristic regions (basal ganglia, centrum semiovale, midbrain)

Special Variants

1. Opercular perivascular cysts:

   • Associated with vascular loops of middle cerebral artery branches
   • Found in anterior temporal lobe and frontal operculum
   • May show perilesional T2/FLAIR signal changes on MRI 6

2. Poirier's type IIIb "expanding lacunae":

   • Giant dilations in basal ganglia and midbrain
   • Can cause symptomatic hydrocephalus requiring treatment 2

Clinical Significance

Most DPVS are incidental findings without clinical significance. However:

• Giant/tumefactive DPVS may cause symptoms through mass effect
• Symptoms can include headache, seizures, and focal neurological deficits
• When causing hydrocephalus, treatment with CSF diversion (e.g., ventriculoperitoneal shunt) may be necessary 1

Differential Diagnosis

DPVS must be differentiated from:

• Lacunar infarcts
• Cystic neoplasms (e.g., dysembryoplastic neuroepithelial tumor)
• Multinodular and vacuolating neuronal tumor
• Cystic infections 3

Management Considerations

• Most DPVS require no treatment
• Regular follow-up imaging may be appropriate for larger DPVS
• Surgical intervention is only indicated for symptomatic cases causing hydrocephalus or significant mass effect
• Treatment should target the underlying cause when identified

When evaluating DPVS on head CT, consider advanced imaging with MRI for better characterization, particularly for atypical or symptomatic cases.