Ventricles, Normal Variants

Susan I. Blaser, MD, FRCPC

DIFFERENTIAL DIAGNOSIS

Common

Asymmetric Lateral Ventricles (ALV)
Intraventricular CSF Pulsation Artifact (Flow-Related)
Cavum Septi Pellucidi (CSP) ± Cavum Vergae
Coarctation of Anterior Horns

Less Common

Connatal Cysts
Germinolytic Cysts

Rare but Important

Open Inferior 4th Ventricle (Blake Pouch Remnant)

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Normal variants are asymptomatic
Frequency varies with site
- Lateral ventricle variants common
- Fourth ventricle less common
- Third ventricle variants (such as thick floor) uncommon (rare at imaging) but important for endoscopic third ventriculostomy
Clinical history key!
- Headaches
- Papilledema
- History of prior trauma, infection

Helpful Clues for Common Diagnoses

Asymmetric Lateral Ventricles (ALV)
- Leaflet of septum pellucidum ± “pushed” to smaller ventricle side
- ALV + normal hemisphere
  - Usually normal variant
  - Exclude obstruction at foramen of Monro
  - Cyst or web
  - Tumor (e.g., choroid plexus neoplasm)
- ALV + abnormal hemisphere
  - Larger hemisphere: Hemimegalencephaly (ipsilateral ventricle large, often deformed)
  - Smaller hemisphere: Unilateral atrophy or porencephaly
- ALV = sign of functioning shunt if shunt in smaller ventricle
- Helpful techniques in evaluating ventricles, possible obstruction
  - Sagittal, coronal thin-section T2WI
  - High resolution FIESTA
  - CSF flow study
  - Intraventricular contrast outlines obstruction
  - Intravenous contrast (helpful in detecting small lesions)
Intraventricular CSF Pulsation Artifact (Flow-Related)
- Most common on high field MR
  - FLAIR sequence most commonly affected
  - Look at another sequence or another plane (artifact disappears)
  - Typically occur in phase-encoding axis
  - Look for phase artifact propagating across image
  - When in doubt, change phase-encoding direction and repeat sequence
Cavum Septi Pellucidi (CSP) ± Cavum Vergae
- Developing ventricle closes from posterior → anterior
  - Therefore cavum vergae (CV) does not occur in isolation
  - CSP can exist ± CV but not reverse
- CSP lacks ependymal lining (term “5th ventricle” inaccurate)
- CSP leaflets should be parallel
  - If septal leaflets are not parallel, consider encysted cavum
  - Look for signs of obstructive hydrocephalus
  - Look for evidence of prior trauma with epi-GRE or SWI to detect hemorrhagic residua
Coarctation of Anterior Horns
- Normal variant
- Exclude subependymal pseudocysts seen with inborn errors of metabolism, TORCH, ischemia
- Findings helpful in distinguishing coarcted anterior horns from pathologic subependymal pseudocysts
  - Peroxisomal biogenesis disorder (Zellweger): Cortical dysplasia, hypomyelination, stippled epiphyses, hypotonia
  - Mitochondrial disorders: MRS lactate doublet
  - TORCH (cytomegalovirus): Look for microcephaly, periventricular calcifications
  - Hypoxic ischemic insult of newborn: History of perinatal distress!

Helpful Clues for Less Common Diagnoses

Connatal Cysts
- Considered normal variant
- May be anterior choroid plexus cysts
  - Controversial entity
  - Transient finding
  - Present at birth
  - Spherical form
  - Can be multiple
  - Lined with epithelium
  - Partial “double wall” due to ependymal folding
  - No hemosiderin
  - No septations
Germinolytic Cysts
- Juxtaventricular subependymal pseudocysts
  - Result from germinolysis
  - Lined with germinal/glial cells (not ependymal cells)
  - May have hemosiderin
  - May have septations
- Probably NOT normal variant
  - Rarely isolated, look for other signs of CNS pathology
  - Distinguish from connatal cysts

Helpful Clues for Rare Diagnoses

Open Inferior 4th Ventricle (Blake Pouch Remnant)
- Presence of complete vermis, fastigial recess
  - Differentiates Blake pouch remnant from Dandy-Walker cyst
- Usually non-obstructive
- FIESTA, CSF flow sequences helpful