Measure tonsillar descent below the foramen magnum, assess crowding and CSF flow, and find the syrinx — then look for the causes of a syrinx other than Chiari (tumour, trauma, tethered cord).
Orient first
- Tonsillar ectopia ≥ 5 mm below the basion–opisthion line is the usual threshold for Chiari I (verify; mild descent is common and often asymptomatic).
- Pointed (peg-like) tonsils and effaced CSF behind the cerebellum matter more than the number alone.
- A syrinx without Chiari needs contrast to exclude an intramedullary tumour.
Acquire the study
- MRI brain and whole spine: sagittal T1 and T2, axial T2 through the foramen magnum and syrinx, phase-contrast CSF flow study where available; contrast if no Chiari.
The manoeuvre
- Sagittal T1/T2: tonsillar tip distance below the basion–opisthion line in mm; shape (pointed vs rounded).
- CSF spaces at the foramen magnum: effaced cisterna magna and retrocerebellar CSF.
- Phase-contrast flow: reduced or absent flow behind the tonsils.
- Whole spine sagittal T2: syrinx extent in vertebral levels and maximal width in mm.
- Skull base: basilar invagination, short clivus, retroflexed odontoid.
- No Chiari: post-gadolinium T1 to exclude tumour; conus level for tethered cord.
What confirms it
- Tonsillar descent with pointed tonsils and effaced CSF, often with a syrinx, concordant with symptoms.
What licenses you to exclude it
- Mild rounded tonsillar descent with normal CSF flow and no syrinx is usually incidental.
The classic misread
- Diagnosing Chiari from intracranial hypotension (sagging brain) — look for pachymeningeal enhancement and venous distension.