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# Cerebral aqueduct

The cerebral aqueduct is a thin channel through the midbrain joining the third and fourth ventricles; a narrowing here causes hydrocephalus.

Official name: Aqueduct of midbrain

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## What it is

A thin channel through the midbrain.

## What it does

Joins the third and fourth ventricles; a narrowing here causes hydrocephalus.

## Key points

- Obstructive hydrocephalus at the aqueduct: lateral and third ventricles enlarge while the fourth stays small, the sign that locates the block
- Communicating hydrocephalus is different: CSF flows freely through the ventricles but is not absorbed (after meningitis or subarachnoid haemorrhage), so all four ventricles enlarge
- Aqueductal stenosis is a leading cause of congenital hydrocephalus; some cases are X-linked, with thumbs held across the palm
- Acquired blockage: pineal and tectal tumours, bleeding, infection and parasitic cysts such as neurocysticercosis

## Key facts

**Where**
Midbrain, from the third to the fourth ventricle; about 15 mm long and 1–2 mm wide, narrowest in its middle

**Function**
Carries CSF; narrowest point of the system (aqueductal stenosis causes hydrocephalus)

## Relations

**Course**
From the back of the [third ventricle](https://durgical.com/en/anatomy/third-ventricle/), under the posterior commissure, down through the [midbrain](https://durgical.com/en/anatomy/midbrain/) to the upper angle of the [fourth ventricle](https://durgical.com/en/anatomy/fourth-ventricle/)

**Behind**
Tectum: the superior and inferior colliculi

**In front**
Tegmentum with the oculomotor and trochlear nuclei and the medial longitudinal fasciculus

**Contents**
CSF only: it has no [choroid plexus](https://durgical.com/en/anatomy/choroid-plexus/), ringed by the periaqueductal grey

## How to find it

On a midline sagittal MRI it is the thin channel between the tegmentum in front and the tectum (colliculi) behind, linking the back of the third ventricle with the top of the fourth; a dark flow void within it on T2 images suggests it is open. At endoscopy its opening is seen at the back of the third ventricle, below the posterior commissure.

## On imaging

Thin-slice, strongly T2-weighted sagittal MRI shows a web or narrowing; normal flow leaves a dark flow void on T2. Phase-contrast MRI measures CSF flow through it, which is often exaggerated in normal-pressure hydrocephalus.

## Clinical notes

- Endoscopic third ventriculostomy bypasses aqueductal obstruction without a shunt
- A slow-growing tectal glioma in a child or teenager can narrow it and present late with hydrocephalus
- Dilation of the aqueduct and third ventricle presses on the dorsal midbrain, adding upgaze palsy (Parinaud syndrome)

## Development

It is the narrowed cavity of the embryonic midbrain, squeezed as the tectum and tegmentum thicken around it.

## Related conditions

- [Hydrocephalus](https://durgical.com/en/conditions/hydrocephalus/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which ventricles enlarge when the cerebral aqueduct is blocked?

The lateral and third ventricles enlarge while the fourth stays small, the sign that locates the block.

## How does communicating hydrocephalus differ from aqueductal obstruction?

CSF flows freely through the ventricles but is not absorbed (after meningitis or subarachnoid haemorrhage), so all four ventricles enlarge.

## Which acquired causes can block the cerebral aqueduct?

Pineal and tectal tumours, bleeding, infection and parasitic cysts such as neurocysticercosis.

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Educational information, not medical advice. For any symptom, see a health professional.

## Its names

**Official Latin (TA2)**
aquaeductus mesencephali TA2 5910

**Official Spanish (DeCS)**
Acueducto del Mesencéfalo DeCS D002535

**English (TA2)**
Aqueduct of midbrain

**Also called**
aqueduct of Sylvius · aquaeductus Sylvii

## Related

- [Third ventricle](https://durgical.com/en/anatomy/third-ventricle/)
- [Fourth ventricle](https://durgical.com/en/anatomy/fourth-ventricle/)

Sources: Terminologia Anatomica (TA2); [DeCS](https://decs.bvsalud.org/es/ths/resource/?id=2572&filter=ths_termall&q=Cerebral%20Aqueduct); [Neuroanatomy, Cerebral Aqueduct (Sylvian)](https://www.ncbi.nlm.nih.gov/books/NBK540988/); [13.3 Circulation and the Central Nervous System](https://openstax.org/books/anatomy-and-physiology-2e/pages/13-3-circulation-and-the-central-nervous-system) (OpenStax (Rice University), 2022) · adapted, [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anatomy/aqueduct-of-midbrain/