Low Cerebellar Tonsils

Gregory L. Katzman, MD, MBA

DIFFERENTIAL DIAGNOSIS

Common

Less Common

Rare but Important

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Cerebellar tonsils may normally lie up to 5 mm below foramen magnum (FM)
Normal rounded tonsillar shape/configuration more important than precise measurement
Normal folia course horizontally, not vertically
Chiari 2 is not in differential diagnosis (herniated tissue is nodulus of vermis, not tonsils!)

Helpful Clues for Common Diagnoses

Tonsillar Ectopia
- Zero to 4.8 mm below foramen magnum
- Avoid terms “Chiari 0” or “Chiari 1/2”
Chiari 1
- Pointed “peg-like” cerebellar tonsils > 5 mm below foramen magnum
- Absent CSF space/flow behind tonsil
  - Sagittal phase contrast MR best
- Low torcular, effaced posterior fossa cisterns
- Folia orientation runs more vertically
- Look for syrinx, CVJ/skull base anomalies
Herniation Syndromes, Intracranial
- Tonsils impacted inferiorly into FM
- Posterior fossa CSF cisterns effaced
- Clinically associated with decreased mental status or obtundation

Helpful Clues for Less Common Diagnoses

Intracranial Hypotension
- Can be spontaneous or acquired
- “Slumping” midbrain, flattened pons, optic chiasm draped over dorsum sellae
- Diffusely enhancing thickened dura ± SDH
Basilar Invagination (Mimic)
- A mimic → tonsils are normal
- Primary often associated with bony malformations such as occipitalization of the atlas or Klippel-Feil; often familial
- Secondary from acquired bone diseases that cause “softening” & skull base flattening, such as osteogenesis imperfecta, osteomalacia, Paget

Helpful Clues for Rare Diagnoses

Brain Death
- Gyral swelling with complete central brain herniation → tonsils pushed downward
- No intracranial vascular flow
- Clinical diagnosis, legal criteria varies