Median Aperture

The median aperture is a single midline opening in the inferior portion of the roof of the fourth ventricle. It is traditionally known as the foramen of Magendie.

The aperture provides a direct route for cerebrospinal fluid (CSF) to pass from the fourth ventricle into the subarachnoid space. It opens primarily into the large cerebellomedullary cistern, also called the cisterna magna, located between the cerebellum and medulla.

Together with the paired lateral apertures, the median aperture forms one of the principal outlets through which ventricular CSF enters the external subarachnoid CSF compartment surrounding the brain and spinal cord.

Location

The median aperture lies in the midline of the inferior roof of the fourth ventricle.

It is positioned near the caudal portion of the ventricular cavity, where the fourth ventricular roof is closely related to the cerebellum, medulla, and tela choroidea.

Foramen of Magendie

The foramen of Magendie is the traditional eponym for the median aperture.

Unlike the paired foramina of Luschka, which lie laterally, the foramen of Magendie is a single midline opening.

Fourth Ventricle

The fourth ventricle is a CSF-filled cavity situated between the pons and upper medulla anteriorly and the cerebellum posteriorly.

Its roof is formed by thin neural and membranous structures, while its floor is formed by the dorsal surfaces of the pons and medulla.

Inferior Roof of the Fourth Ventricle

The inferior portion of the fourth ventricular roof is associated with the tela choroidea and inferior medullary velum.

The median aperture develops within this thin roof and provides communication with the subarachnoid space.

Tela Choroidea

The tela choroidea of the fourth ventricle is a thin vascular membrane associated with the inferior ventricular roof.

It supports the fourth ventricular choroid plexus and is closely related to the formation and position of the median and lateral ventricular apertures.

Relationship to the Medulla

The median aperture lies dorsal to the medulla oblongata in the caudal fourth ventricular region.

CSF passing through the opening enters the subarachnoid compartment posterior to the medulla.

Relationship to the Cerebellum

The cerebellum lies posterior and superior to the fourth ventricular cavity.

The median aperture opens into a CSF-filled space between the inferior cerebellum and dorsal medulla.

Cerebellomedullary Cistern

The cerebellomedullary cistern, commonly called the cisterna magna, is a large subarachnoid cistern located between the inferior surface of the cerebellum and the dorsal aspect of the medulla.

The median aperture opens directly into this cistern, making it an important route for ventricular CSF to enter the subarachnoid space.

Role in CSF Circulation

The median aperture is one of the three principal outlets of the fourth ventricle.

CSF passing through it enters the cisterna magna and can then circulate through interconnected subarachnoid cisterns and around the brain and spinal cord.

CSF Flow through the Median Aperture

A simplified route of CSF flow is:

CSF is produced predominantly by the choroid plexuses of the ventricles.
It passes from the lateral ventricles through the interventricular foramina.
It enters the third ventricle.
It flows through the cerebral aqueduct.
It enters the fourth ventricle.
Some CSF exits through the median aperture.
It enters the cerebellomedullary cistern and wider subarachnoid space.

Fourth Ventricular Outlets

The fourth ventricle communicates with the subarachnoid space through three major apertures:

One median aperture
Two lateral apertures

Opening	Number	Traditional Name	General Destination
Median aperture	One	Foramen of Magendie	Cerebellomedullary cistern
Lateral apertures	Two	Foramina of Luschka	Subarachnoid space near the cerebellopontine angles

Median Aperture Compared with Lateral Apertures

Feature	Median Aperture	Lateral Apertures
Number	One	Two
Position	Midline	Lateral
Eponym	Foramen of Magendie	Foramina of Luschka
Associated subarachnoid region	Cerebellomedullary cistern	Cerebellopontine angle region
Function	CSF outflow from the fourth ventricle

Connection with the Subarachnoid Space

The median aperture creates a direct anatomical communication between the ventricular system and subarachnoid space.

This communication is essential because most CSF must leave the ventricles before circulating around the external surfaces of the central nervous system.

Relationship to the Central Canal

Inferiorly, the fourth ventricular cavity also narrows toward the central canal of the spinal cord.

The central canal is extremely small and is not the principal route for ventricular CSF to reach the spinal subarachnoid space. The fourth ventricular apertures provide the major outflow pathways.

Relationship to the Choroid Plexus

The choroid plexus of the fourth ventricle lies within the inferior roof region and is closely related to the ventricular apertures.

CSF produced by this plexus joins fluid arriving from more rostral ventricles before leaving through the median and lateral apertures.

Median Aperture Compared with Interventricular Foramen

Feature	Median Aperture	Interventricular Foramen
Number	One	Two
Associated ventricle	Fourth ventricle	Lateral and third ventricles
Destination	Subarachnoid space	Third ventricle
Primary role	Exit from ventricular system	Passage within ventricular system

Median Aperture Compared with Cerebral Aqueduct

Feature	Median Aperture	Cerebral Aqueduct
Location	Inferior roof of fourth ventricle	Midbrain
Connects	Fourth ventricle with subarachnoid space	Third ventricle with fourth ventricle
Position	Midline opening	Midline canal
Function	Ventricular outflow	Intraventricular CSF passage

Embryological Development

The fourth ventricle develops from the cavity of the embryonic hindbrain.

As development proceeds, the thin roof of the fourth ventricle establishes openings that connect the ventricular cavity with the surrounding subarachnoid space.

Development of the Median Aperture

The median aperture forms in the thin caudal roof of the fourth ventricle.

Its development allows CSF within the ventricular system to communicate freely with the external subarachnoid spaces.

Developmental Importance

Normal formation of the fourth ventricular outlets is necessary for appropriate CSF circulation.

Congenital abnormalities affecting the posterior fossa or fourth ventricular roof can alter these pathways and contribute to ventricular enlargement.

Imaging

The region of the median aperture can be evaluated with high-resolution MRI, particularly sequences that clearly demonstrate CSF spaces of the posterior fossa.

Imaging can assess the fourth ventricle, cisterna magna, cerebellum, brainstem, and possible causes of impaired CSF outflow.

CSF Flow Imaging

Specialized MRI techniques can be used in selected clinical settings to evaluate CSF movement through narrow intracranial pathways.

These studies may help assess abnormal CSF dynamics near the fourth ventricular outlets and craniocervical junction.

Clinical Significance

Obstruction of the Median Aperture

Obstruction of the median aperture can reduce CSF flow from the fourth ventricle into the cerebellomedullary cistern.

The severity of the effect depends partly on whether the paired lateral apertures remain patent.

Obstructive Hydrocephalus

If outflow through the fourth ventricular apertures is sufficiently impaired, obstructive hydrocephalus may develop.

CSF accumulates proximal to the obstruction, potentially enlarging the ventricular system and increasing intracranial pressure.

Fourth Ventricular Tumors

Tumors arising within the fourth ventricle can obstruct the median aperture or other ventricular outlets.

Posterior fossa tumors may therefore produce hydrocephalus by interfering with normal CSF flow even when CSF production itself remains normal.

Ependymoma

Ependymomas may arise within the fourth ventricle, particularly in pediatric patients.

They can fill the ventricular cavity and extend through its apertures, producing obstruction and characteristic patterns of tumor spread.

Medulloblastoma

Medulloblastomas arise in the posterior fossa and may compress or project toward the fourth ventricular region.

Mass effect on the ventricle and its outlets can interfere with CSF circulation and cause hydrocephalus.

Intraventricular Hemorrhage

Blood entering the fourth ventricle can form clots that obstruct narrow CSF outlets, including the median aperture.

This can contribute to acute impairment of CSF circulation.

Inflammation and Adhesions

Inflammatory processes involving the meninges or ventricular system may produce scarring and adhesions near the fourth ventricular outlets.

Such changes can impair communication between the ventricular system and subarachnoid space.

Posterior Fossa Malformations

Congenital abnormalities affecting the posterior fossa may alter the anatomy of the fourth ventricle, its roof, and its outlets.

These abnormalities can be associated with disturbed CSF circulation, although the precise mechanism varies among conditions.

Dandy-Walker Spectrum

Abnormalities within the Dandy-Walker spectrum involve development of posterior fossa structures, including the cerebellar vermis and fourth ventricular region.

Altered fourth ventricular anatomy may be accompanied by enlargement of posterior fossa CSF spaces and hydrocephalus in some patients.

Cisterna Magna and Lumbar CSF

After CSF enters the cisterna magna through the median aperture, it can circulate through connected subarachnoid spaces around the brainstem and spinal cord.

This continuity ultimately allows CSF within cranial cisterns to communicate with the spinal subarachnoid compartment sampled during lumbar puncture.

Surgical Anatomy

The median aperture and surrounding cerebellomedullary region are important in surgical approaches to the fourth ventricle and posterior fossa.

Knowledge of the tela choroidea, fourth ventricular roof, cerebellum, medulla, choroid plexus, and neighboring vascular structures is essential when operating in this region.

Key Anatomical Features

Feature	Key Point
Structure	Single opening of fourth ventricle
Traditional name	Foramen of Magendie
Position	Midline, inferior fourth ventricular roof
Destination	Cerebellomedullary cistern
Primary function	CSF outflow into subarachnoid space
Related membrane	Tela choroidea
Related brainstem structure	Medulla oblongata
Related posterior structure	Cerebellum
Other fourth ventricular outlets	Paired lateral apertures
Major clinical consequence of severe obstruction	Obstructive hydrocephalus

Anatomical Importance

The median aperture forms a critical connection between the fourth ventricle and the subarachnoid space. By opening into the cerebellomedullary cistern, it provides a major route for CSF to leave the internal ventricular system and enter the external fluid spaces surrounding the central nervous system.

Its function is complemented by the paired lateral apertures. Together, these three openings allow CSF arriving in the fourth ventricle to distribute into the basal cisterns and subsequently around the brain and spinal cord.

The median aperture is also clinically important because obstruction at the fourth ventricular outlets can disrupt CSF circulation and contribute to hydrocephalus. Its relationships with the medulla, cerebellum, tela choroidea, choroid plexus, and cisterna magna make it an important landmark in posterior fossa anatomy, imaging, and neurosurgery.