Anatomy

Median Aperture of the 4th Ventricle

Anatomy

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Structure:

The median aperture of the fourth ventricle, also known as the Foramen of Magendie, is a midline opening located at the inferior aspect of the fourth ventricle. The fourth ventricle lies within the posterior fossa of the skull, positioned between the brainstem (the pons and medulla anteriorly) and the cerebellum posteriorly.[1] The median aperture is situated at the lower end of the fourth ventricle and opens directly into the cisterna magna, one of the largest cerebrospinal fluid (CSF)-filled spaces within the subarachnoid space.[2]

The fourth ventricle contains:

One median aperture in the midline

Two lateral apertures located superolaterally[1]

Together, these openings allow CSF to exit the ventricular system. The median aperture forms a direct anatomical communication between the ventricular system and the subarachnoid space.[3] In terms of spatial relationships, the median aperture lies just superior to the upper cervical spinal cord, adjacent to the rhomboid fossa (which forms the floor of the fourth ventricle), and near the posterior vermis of the cerebellum. Its midline position and relative size create a natural corridor used in certain neurosurgical approaches to access the fourth ventricle and its floor.[3]

Although modern imaging modalities such as MRI clearly demonstrate the fourth ventricle, the apertures themselves may be difficult to visualize due to their small size and complex three-dimensional orientation.[1]

Image Coming Soon

Function:

The primary function of the median aperture is to allow CSF to exit the fourth ventricle and enter the subarachnoid space.[3] CSF is produced within the ventricular system and flows sequentially through the lateral ventricles, third ventricle, cerebral aqueduct, and the fourth ventricle.[3] From the fourth ventricle, CSF exits through the median aperture and the two lateral apertures.[3]

Specifically, the median aperture allows CSF to enter the cisterna magna. From there, CSF circulates around the brain and spinal cord within the subarachnoid space, providing cushioning, facilitating nutrient exchange, removing metabolic waste, and maintaining a stable extracellular environment. A small portion of CSF that does not exit through these apertures continues into the central canal of the spinal cord.[1]

Clinically, obstruction of the median aperture by tumor, cysts, inflammation, or congenital malformation may impair CSF outflow.[2,3] This can result in fourth ventricle outlet obstruction (FVOO), a form of noncommunicating hydrocephalus characterized by ventricular enlargement and increased intracranial pressure.[1] Since the fourth ventricle is the most distal ventricle in the system, obstruction at this level may cause dilation of all upstream ventricles. The median aperture also has neurosurgical significance. Its midline location and anatomical relationships make it an important landmark for accessing lesions of the rhomboid fossa, fourth ventricular tumors or cysts, and posterior cerebellar pathology.[2]

References:

Roesch ZK, Tadi P. Neuroanatomy, Fourth Ventricle. In: StatPearls. StatPearls Publishing; 2023. Updated August 7, 2023. https://www.ncbi.nlm.nih.gov/books/NBK546577/
Ciołkowski M, Sharifi M, Tarka S, Ciszek B. Median aperture of the fourth ventricle revisited. Folia Morphol (Warsz). 2011;70(2):84-90. https://journals.viamedica.pl/folia_morphologica/article/viewFile/19311/15204
Spierer R. The debated neuroanatomy of the fourth ventricle. J Anat. 2023;243(4):555-563. doi:10.1111/joa.13885.

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