What is the Cisterna Magna?
The cisterna magna is an enlarged subarachnoid space located in the posterior fossa of the brain, positioned between the inferior margin of the cerebellar vermis and the foramen magnum, serving as a major cerebrospinal fluid (CSF) reservoir.
Anatomical Definition and Location
The cisterna magna represents a specific CSF-containing space within the subarachnoid compartment. It is anatomically bounded superiorly by the inferior aspect of the cerebellar vermis and extends inferiorly to the level of the foramen magnum 1, 2. This space is part of the larger subarachnoid system where CSF flows after being produced by the choroid plexus 3.
Normal Dimensions and Characteristics
In adults, the cisterna magna demonstrates specific measurable dimensions:
- Depth measurements: The normal midsagittal depth averages approximately 5 ± 3 mm, with the upper limit of normal being around 10 mm 4
- Volume considerations: The cisterna magna volume shows gender differences, being smaller in females than males, and correlates with age and clivus length 2
- Configuration: The space can be classified into five distinct anatomical types representing normal variants in the general population 2
Functional Significance
The cisterna magna serves several critical functions:
CSF circulation pathway: CSF flows from the fourth ventricle through the foramina of Magendie and Luschka into the cisterna magna at the base of the brain, then travels both caudally toward the lumbar spine and rostrally over the cerebral convexities 3
Clinical access point: The cisterna magna provides a crucial site for CSF sampling when lumbar puncture is contraindicated or when ventricular CSF may not be representative of disease activity 5. Cisternal aspiration can be performed when evaluating conditions like leptomeningeal metastases or central nervous system infections.
Clinical Relevance
Diagnostic Considerations
- Imaging evaluation: The cisterna magna is routinely visualized on brain MRI and CT scans, particularly when assessing for posterior fossa pathology 6
- Abnormal findings: An enlarged cisterna magna (mega cisterna magna) may represent a variant within the Dandy-Walker continuum spectrum 7
- CSF leak detection: CT cisternography and MR cisternography can evaluate CSF flow through the cisterna magna region when investigating suspected CSF leaks 6, 8
Important Distinctions
The cisterna magna should be distinguished from the occipito-atlantal cistern (OAC), which is a separate subarachnoid space extending from the foramen magnum to the level of C2 vertebra 1. While historically these spaces may have been conflated, they represent distinct anatomical compartments with different dimensions and clinical significance.
Developmental Anatomy
During fetal development, the cisterna magna can be identified on prenatal ultrasound, with normal measurements established for various gestational ages 4. The cisterna magna septa, representing vestigial remnants of Blake's pouch, serve as markers of normal rhombencephalon development 7.