Question

Diagnosis of the MRI image:

• A. Arnold Chiari malformation
• B. Corpus callosal agenesis
• C. Vein of Galen Malformation
• D. Dandy Walker Malformation

Hint:

On midsagittal MRI, look for the cerebellar tonsils herniating like pegs below the foramen magnum.

Answer 1

The Correct Option is A

Step 1: Read the image. The picture is a midsagittal MRI of the brain. The key finding is downward herniation of the cerebellar tonsils through the foramen magnum into the upper cervical canal (peg-like tonsils extending below the foramen magnum). This is the hallmark of an Arnold-Chiari (Chiari) malformation.

Step 2: Recall the entity. In Chiari I/II malformation, the posterior fossa is small and the cerebellar tonsils (and in Chiari II also the vermis, brainstem and fourth ventricle) are displaced caudally below the foramen magnum. It may be associated with syringomyelia, hydrocephalus and (in Chiari II) a myelomeningocele.

Step 3: Confirm the correct option. The sagittal MRI showing tonsillar descent through the foramen magnum is diagnostic of Arnold-Chiari malformation; option A is correct.

Step 4: Exclude the distractors. Corpus callosal agenesis shows absence of the corpus callosum with parallel/widely-spaced lateral ventricles, a high-riding third ventricle and a radial 'sunburst' gyral pattern - not tonsillar herniation; B is wrong. Vein of Galen malformation appears as a large, rounded, midline flow-void/vascular dilatation in the region of the great cerebral vein (often with high-output cardiac failure in a neonate); C is wrong. Dandy-Walker malformation shows cystic dilatation of the fourth ventricle with vermian hypoplasia/agenesis and an enlarged posterior fossa - the cerebellum is splayed open, not herniated downward; D is wrong.

Final answer: Option A - Arnold Chiari malformation.