Chiari 2 malformation

Definition
• Complex hindbrain malformation
• Virtually 100% associated with neural tube closure defect (NTD), usually lumbar myelomeningocele (MMC)


Diagnostic criteria 

1. Crowded posterior fossa, widened tentorial incisura, tectal beaking, inferior vermian displacement
2. Cascade or waterfall of cerebellum/brainstem downward
3. Uvula/nodulus/pyramid of vermis → sclerotic peg
4. Cervicomedullary kink (70%)
5. Towering cerebellum → compresses midbrain, associated beaked tectum
6. 4th ventricle elongated with no posterior point (fastigium)
7. Lacunar skull: Focal calvarial thinning with scooped-out appearance

PATHOLOGY
• Secondary to sequelae of CSF leakage through open spinal dysraphism during gestation (4th fetal week)
• Methylenetetrahydrofolate reductase (MTHFR) mutations → abnormal folate metabolism
• Spine- and brain/skull-associated anomalies common

DIAGNOSTIC CHECKLIST
• Towering cerebellum, downward vermian displacement, ± brainstem compression diagnostic for Chiari 2 especially if MMC present


Best imaging tool
○ Multiplanar MR for initial brain, spine evaluation
○ Follow-up brain CT or MR to assess hydrocephalus
○ Cervical spine MR for progressive brainstem or spinal symptoms
  
Consider
• Brain/spinal axis MR to detect presence of Chiari 2, assess severity, look for complications

Image Interpretation Pearls
• Low torcular herophili indicates small posterior fossa
• CT or MR showing towering cerebellum, downward vermian displacement, ± brainstem compression diagnostic of Chiari 2

Imaging gallery

Sagittal graphic of the posterior fossa and upper cervical spine demonstrate characteristic findings of Chiari 2 malformation, including callosal dysgenesis , tectal beaking, small posterior fossa, vermian ectopia , and medullary kinking . 

Sagittal T1WI MR reveals characteristic Chiari 2 malformation findings. Note the tectal beaking , vermian displacement through the foramen magnum, large massa intermedia, and dysplastic corpus callosum.

Sagittal T2WI MR confirms characteristic findings of Chiari 2 malformation, including tectal beaking , vermian displacement through the foramen magnum , "towering" cerebellum , large massa intermedia, and dysplastic corpus callosum . 

Axial T2WI MR shows characteristic posterior fossa crowding at the foramen magnum, reflecting small dimensions of the posterior fossa combined with cerebellar ectopia and vermian displacement through the foramen magnum.

Sagittal T2WI MR of the brain shows characteristic Chiari 2 findings, with very small posterior fossa size, prominent tectal beak , and vermian/tonsillar extension  through the foramen magnum. The corpus callosum  is severely dysplastic and the ventricles small following CSF shunting. 

Axial T2WI MR depicts enlargement of the occipital horns of the lateral ventricles (colpocephaly) related to callosal dysgenesis in conjunction with beaked tectum 

Axial T2WI MR at the posterior fossa level demonstrates the classic axial image manifestation of the towering cerebellum extending through the wide tentorial incisura. 

Axial T2WI MR performed at the foramen magnum level depicts characteristic crowding of the inferior posterior fossa. Both of the cerebellar tonsils , as well as vermis , are inferiorly displaced into the foramen magnum.

Sagittal T2WI MR depicts marked vermian ectopia and prominent medullary kink positioned lower than typically seen at the C4 level. Note also the dysplastic beaked tectum and prominent massa intermedia . 

 Sagittal T2WI MR shows crowded posterior fossa with vermian ectopia , large cervicothoracic cord syrinx  as well as sizable focal cervicomedullary syrinx (syringobulbia). Note the hyperintense gliotic changes of scarred vermis.