CSF, Ventricles & Brain Anatomy
Foramen of Monro (interventricular foramen)
The foramen of Monro is the channel connecting each lateral ventricle to the third ventricle.
In short: Foramen of Monro (interventricular foramen)
- There is one on each side, connecting a lateral ventricle to the third ventricle.
- Blockage of one causes a single lateral ventricle to enlarge.
- It is a key landmark in the CSF pathway.
- It can be involved in cysts or tumors.
What it is
The foramen of Monro (or interventricular foramen) is the channel that connects each lateral ventricle to the central third ventricle. There is one foramen of Monro on each side.
Understanding Foramen of Monro (interventricular foramen)
CSF from each lateral ventricle drains through its foramen of Monro into the third ventricle. If one foramen becomes blocked — for example, by a cyst (such as a colloid cyst) or a tumor — the lateral ventricle on that side cannot drain and enlarges on its own, producing unilateral hydrocephalus. Because both foramina lead into the single third ventricle, this region is an important junction in the CSF pathway and a relevant landmark in surgery and imaging.
Why it matters
Understanding the foramen of Monro helps families see how a one-sided blockage produces enlargement of a single ventricle, and why this junction is important in evaluating and treating hydrocephalus.
Common questions
What does the foramen of Monro connect?
Each lateral ventricle to the third ventricle.
How many are there?
Two — one on each side.
What happens if one is blocked?
The lateral ventricle on that side enlarges (unilateral hydrocephalus).
Bottom line
The foramen of Monro connects each lateral ventricle to the third ventricle; blockage of one causes unilateral enlargement.
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