What is a large cisterna magna?

Large Cisterna Magna: Definition and Clinical Significance

A large cisterna magna is a developmental variation of the posterior fossa characterized by an enlarged cerebrospinal fluid space (>10 mm in anteroposterior dimension) at the fourth ventricle outlet, without affecting the cerebellar vermis or hemispheres.

Definition and Anatomy

• The cisterna magna (also called cerebellomedullary cistern) is a normal CSF-filled space located between the cerebellum and the medulla oblongata
• Considered "large" or "mega" when it exceeds 10 mm in anteroposterior dimension on imaging
• Distinguished from other posterior fossa abnormalities by:
  • Normal cerebellar vermis and hemispheres
  • No communication with the fourth ventricle
  • Intact posterior fossa structures

Imaging Characteristics

• Best visualized on MRI, especially with special sequences (FIESTA, 3D CISS, BFFE) 1
• Can also be identified on CT scans, though MRI provides superior detail
• Key imaging findings include:
  • Enlarged CSF space (>10 mm) posterior to the cerebellum
  • Normal cerebellar morphology
  • No mass effect on surrounding structures

Clinical Significance

• Most cases of isolated large cisterna magna are benign developmental variations
• When found as an isolated finding during prenatal ultrasound, it is associated with normal pregnancy and neonatal outcomes 2
• May occasionally be associated with:
  • Periventricular nodular heterotopia 3
  • FLNA gene mutations (especially in women with bilateral frontocentral periventricular nodular heterotopia) 3

Associated Conditions

• May be part of a broader spectrum of posterior fossa abnormalities:
  • Can be seen in some cases of panventriculomegaly 4
  • Sometimes associated with cerebral atrophy 5
  • Rarely associated with psychiatric disorders, though causality is not established 6

Management Implications

• Isolated large cisterna magna without other abnormalities typically requires no intervention
• When found during prenatal ultrasound:
  • Should be distinguished from other posterior fossa abnormalities
  • If isolated finding, has good prognosis 2
• When associated with other findings:
  • May require further evaluation based on associated abnormalities
  • Genetic testing may be warranted if associated with periventricular nodular heterotopia 3

Differential Diagnosis

• Must be differentiated from other posterior fossa cystic lesions:
  • Dandy-Walker malformation (involves cerebellar vermis abnormalities)
  • Arachnoid cyst (has mass effect)
  • Blake's pouch cyst (communicates with fourth ventricle)
  • Posterior fossa subarachnoid space enlargement

Follow-up Recommendations

• Isolated large cisterna magna generally requires no specific follow-up
• Long-term studies of children with isolated large cisterna magna on prenatal ultrasound show normal development 2
• If associated with other abnormalities, follow-up should be guided by the associated condition

In summary, a large cisterna magna is typically a benign anatomical variant that requires no intervention when found in isolation. The key to management is accurate differentiation from other posterior fossa abnormalities that may require intervention.