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Topic

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foramen of Monro

The foramen of Monro, also known as the interventricular foramen, is a paired narrow channel that connects each lateral ventricle to the third ventricle. It serves as a critical conduit for cerebrospinal fluid (CSF) flow within the ventricular system and plays a central role in maintaining normal intracranial pressure dynamics.

Because of its small size and strategic location, the foramen of Monro is highly susceptible to obstruction by tumors, cysts, hemorrhage, or congenital anomalies, making it a key focus in neuroimaging and neurosurgical planning.

Synonyms

  • Interventricular foramen

  • Foramen interventriculare

Location

  • Situated at the junction of the lateral ventricles and the third ventricle

  • Located in the midline but present bilaterally

  • Positioned between the frontal horn and body of the lateral ventricle

  • Forms the anterosuperior entry into the third ventricle

  • Located just posterior to the columns of the fornix

  • Inferior to the body of the corpus callosum

Anatomical boundaries

  • Anterior:

    • Column of the fornix

  • Posterior:

    • Anterior thalamus

  • Superior:

    • Body of the fornix

  • Inferior:

    • Anterior thalamus and roof of the third ventricle

  • Medial:

    • Third ventricle

  • Lateral:

    • Lateral ventricle

Relations

Anteriorly:

  • Fornix (column)

Posteriorly:

  • Thalamus

Superiorly:

  • Corpus callosum (body)

  • Septum pellucidum

Inferiorly:

  • Roof of the third ventricle

Laterally:

  • Caudate nucleus (head)

Function

  • CSF transmission: Allows passage of cerebrospinal fluid from each lateral ventricle into the third ventricle

  • Pressure regulation: Maintains normal intraventricular pressure gradients

  • Ventricular system continuity: Essential for coordinated CSF circulation through the ventricular system

Clinical significance

  • Obstructive hydrocephalus: Even small lesions can block CSF flow and cause ventricular dilation

  • Colloid cyst: Classic lesion at the foramen of Monro causing intermittent or acute hydrocephalus

  • Intraventricular tumors: Central neurocytoma, subependymoma, or astrocytoma may obstruct the foramen

  • Intraventricular hemorrhage: Clot can acutely occlude the foramen

  • Congenital narrowing: Can lead to unilateral or bilateral hydrocephalus

  • Neurosurgical relevance: Key landmark during endoscopic third ventriculostomy and ventricular tumor surgery

MRI appearance

T1-weighted images:

  • Normal: Appears as a small CSF-filled channel with low signal intensity

  • Fornix and thalamus: Intermediate signal bordering the foramen

T2-weighted images:

  • Normal: High signal intensity consistent with CSF

  • Obstruction: Loss of normal CSF signal or distortion of the foramen

FLAIR:

  • Normal: Suppressed CSF signal (dark) within the foramen

Diffusion-weighted imaging (DWI):

  • Normal: No diffusion restriction

Post-contrast T1-weighted images:

  • Normal: No enhancement

CT appearance

Non-contrast CT:

  • Normal: CSF-density channel, often difficult to visualize directly

Post-contrast CT:

  • Normal: No enhancement

MRI images

MRI foramen of Monro axial anatomy image -img-00000-00000

MRI images

MRI foramen of Monro coronal anatomy image -img-00000-00000