Fourth ventricle outlets: foramina of Luschka and Magendie
Most cerebrospinal fluid leaves the fourth ventricle for the subarachnoid space (the space around the brain and spinal cord) through three apertures (openings): the foramina of Luschka and Magendie.
Remember
Most cerebrospinal fluid leaves through the lateral apertures (Luschka) and the median aperture (Magendie) of the fourth ventricle, so dilatation of all four ventricles with normal absorption points to an obstruction at these outlets.
Memory aid
Lateral and Luschka both start with ‘L’ and Median and Magendie both start with ‘M’
Pair the lateral apertures with Luschka and the median aperture with Magendie by their shared first letters.
The ventricular system is made of four ventricles, connected cavities deep in the brain, and the fourth is the last of them. Cerebrospinal fluid (CSF) is made by the choroid plexus (a fringe of blood vessels and lining cells), which produces the fluid but does not drain it.
Most CSF leaves the fourth ventricle through three apertures (openings) into the subarachnoid space, the space around the brain and spinal cord. The two paired lateral apertures are the arm-like openings at either side, called the foramina of Luschka. The single median aperture lies on the dorsal (back) side and is the foramen of Magendie. A small amount also passes into the central canal of the spinal cord, but that channel is very narrow, so it carries little CSF.
From the subarachnoid space, the CSF moves through arachnoid granulations (small projections that push into the venous sinuses) into the superior sagittal sinus, a large vein running along the top of the brain, and so into the venous system.
If the outlets of the fourth ventricle are blocked, CSF made in every ventricle cannot get out, so it backs up and dilates (enlarges) the lateral, third and fourth ventricles together. This is obstructive hydrocephalus (hydrocephalus is a build-up of CSF in the brain), also called non-communicating because the block lies within the ventricular system. Absorption at the arachnoid granulations is normal in this setting. If the lateral and third ventricles are dilated but the fourth ventricle stays a normal size, that points instead to a block at the cerebral aqueduct, the channel that carries CSF from the third ventricle into the fourth.
Key facts
- Lateral apertures
- the paired foramina of Luschka, arm-like openings at either side of the fourth ventricle.
- Median aperture
- the single foramen of Magendie, on the dorsal (back) side of the fourth ventricle.
- Main route
- most CSF drains through these three apertures into the subarachnoid space.
- Minor route
- the central canal of the spinal cord is very narrow and carries little CSF.
- Next step
- from the subarachnoid space, CSF moves through arachnoid granulations into the superior sagittal sinus.
- Imaging clue
- all four ventricles dilated with normal absorption points to a block at the outlets of the fourth ventricle, while dilated lateral and third ventricles with a normal fourth ventricle point to a block at the cerebral aqueduct.
Exam traps
- The interventricular foramina (the foramina of Monro) do not lead out of the ventricular system, because they connect each lateral ventricle to the third ventricle, so CSF is still inside the system after passing through them.
- The median aperture (foramen of Magendie) is one of three exits from the fourth ventricle, so naming it by itself leaves out the paired lateral apertures (foramina of Luschka), which also carry CSF into the subarachnoid space.
- A blocked cerebral aqueduct does not dilate all four ventricles, because it stops CSF passing from the third to the fourth ventricle, so the fourth ventricle stays a normal size.
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