Question:medium
Diagnosis of the MRI image:
Diagnosis of the MRI image:
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On midsagittal MRI, look for the cerebellar tonsils herniating like pegs below the foramen magnum.
Updated On: Jun 22, 2026
- Arnold Chiari malformation
- Corpus callosal agenesis
- Vein of Galen Malformation
- Dandy Walker Malformation
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The Correct Option is A
Solution and Explanation
Use the single decisive sign. On a midsagittal brain MRI, the one feature that nails the diagnosis is whether the cerebellar tonsils dip below the foramen magnum. Here the tonsils are pulled down as pointed pegs into the upper cervical canal - that is the signature of a Chiari (Arnold-Chiari) malformation.
What is happening anatomically. The posterior fossa is too small, so its contents are forced caudally. In Chiari I the tonsils herniate; in Chiari II the vermis, brainstem and fourth ventricle also descend and there is usually an associated lumbar myelomeningocele. Syrinx formation and hydrocephalus are common companions because CSF flow at the craniocervical junction is obstructed.
Quick contrast for the alternatives.
- Corpus callosal agenesis: the corpus callosum is missing, ventricles run parallel with a high third ventricle and 'sunburst' medial gyri.
- Vein of Galen malformation: a big midline vascular flow void behind the third ventricle, classically with neonatal heart failure.
- Dandy-Walker malformation: a large posterior-fossa cyst continuous with a dilated fourth ventricle and an absent/hypoplastic vermis.
None of these show downward tonsillar herniation.
The MRI therefore shows an Arnold Chiari malformation (Option A).
What is happening anatomically. The posterior fossa is too small, so its contents are forced caudally. In Chiari I the tonsils herniate; in Chiari II the vermis, brainstem and fourth ventricle also descend and there is usually an associated lumbar myelomeningocele. Syrinx formation and hydrocephalus are common companions because CSF flow at the craniocervical junction is obstructed.
Quick contrast for the alternatives.
- Corpus callosal agenesis: the corpus callosum is missing, ventricles run parallel with a high third ventricle and 'sunburst' medial gyri.
- Vein of Galen malformation: a big midline vascular flow void behind the third ventricle, classically with neonatal heart failure.
- Dandy-Walker malformation: a large posterior-fossa cyst continuous with a dilated fourth ventricle and an absent/hypoplastic vermis.
None of these show downward tonsillar herniation.
The MRI therefore shows an Arnold Chiari malformation (Option A).
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