Chiari 1 malformation

Diagnostic Criteria

1. Elongated Inferiorly displaced "pointed" , peg-shaped cerebellar tonsils extend below below basion-opisthion line into upper cervical spinal canal
2. The "5 mm" criterion for tonsillar displacement below basion-opisthion line is flawed criterion
3. Tonsillar position is a morphometric distribution and changes with time
4. Tonsillar position also risk factor for syrinx (the lower the tonsils, the higher the risk)
5. "crowded" posterior fossa
6. Effaced retrocerebellar CSF spaces at foramen magnum/upper cervical level
7. Crowding" of posterior fossa 
8. Evaluate skull base, upper cervical spine
9. Short clivus, craniovertebral junctioni (CVJ) assimilation anomalies common
10. Normally located 4th ventricle (normal dorsally-pointed fastigium)

Consider
• Degree of tonsillar correlates with clinical severity
• Unless tonsils > 5 mm and pointed ± "crowded posterior fossa" probably not clinically significant

Image Interpretation Pearls

• • Don't use 5 mm cut-off point alone to diagnosis CM1 (with pathological and clinical implications)
• Look for findings of intracranial hypotension before making diagnosis of CM1

Imaging gallery

 Sagittal graphic demonstrates pointed peg-like tonsils extending below foramen magnum, elongating the normally positioned 4th ventricle. 

Sagittal T2WI in a 23-year-old male with classic Chiari 1 malformation shows low-lying pointed tonsils and hyperintensity in the upper cervical cord that may represent "presyrinx" state.

Sagittal T1WI MR shows normal 4th ventricle position and appearance. The fastigium is in normal position, helping to distinguish from Chiari 2 malformation. There is inferior displacement of the ectopic cerebellar tonsils through the foramen magnum with odontoid process retroflexion and clivus foreshortening. 

Axial T2WI MR confirms inferior displacement of ectopic cerebellar tonsils through the foramen magnum, producing foramen magnum crowding.

Sagittal T1WI MR (osteopetrosis) demonstrates severe cerebellar tonsillar ectopia, with extension of the elongated cerebellar tonsils into the upper cervical canal to the C2-C3 level. Hypointense marrow signal reflects diffuse sclerosis.

Axial T2WI MR (osteopetrosis) reveals characteristic crowding of the foramen magnum related to CM1, with extension of the ectopic cerebellar tonsils into the upper cervical canal.

Sagittal T2WI MR (asymptomatic CM1) demonstrates severe cerebellar tonsillar ectopia . The tonsils produce deformation of the upper cervical spinal cord and abnormal cord T2 prolongation reflecting edema and potentially an early presyrinx state. 

Axial T2WI MR (asymptomatic CM1 patient) reveals caudal extension of ectopic cerebellar tonsils into the foramen magnum, completely effacing the basilar cisterns and displacing the adjacent spinal cord .

Sagittal T2WI MR depicts marked cerebellar tonsillar ectopia with normal tectum and 4th ventricular position. The clivus is mildly foreshortened and dens retroflexed. There is central intramedullary edema in the cervical spinal cord without frank syringomyelia, a finding that has been described as a "presyrinx" state. 

Axial T2WI MR confirms displacement of the ectopic cerebellar tonsils through the crowded foramen magnum, producing foramen magnum crowding.