Chiari I Malformation¶

Definition¶

Chiari I deformity is defined as caudal displacement of the cerebellar tonsils below the foramen magnum by ≥5 mm. Unlike Chiari II (associated with myelomeningocele), Chiari I is not associated with open spinal dysraphism and is often an incidental finding or presents in adolescence/adulthood with headache, syringomyelia, or brainstem symptoms.

Imaging Findings¶

MRI (Modality of Choice)¶

Sagittal T1 — Cerebellar tonsils extending ≥5 mm below the foramen magnum (McRae line — the line connecting the basion to the opisthion)
Tonsillar morphology — Pointed (peg-like) tonsils are more clinically significant than rounded tonsils
Crowding at the foramen magnum — Effacement of the CSF spaces around the cervicomedullary junction
Syringomyelia (syrinx) — Present in 30–70% of Chiari I patients; typically cervicothoracic
CSF flow study (cine MRI) — May demonstrate impaired CSF flow at the foramen magnum, which helps determine surgical candidacy

Key Measurements¶

Tonsillar herniation ≥5 mm = Chiari I
3–5 mm = borderline (assess tonsillar morphology and symptoms, presence of syrinx)
<3 mm = normal variant

Clinical Pearl

Tonsillar position of 3–5 mm below the foramen magnum is a borderline finding. In this range, pointed (peg-like) tonsillar morphology is more clinically significant than rounded tonsils. A cine CSF flow study showing impaired flow at the foramen magnum supports the diagnosis in borderline cases and helps determine whether surgical decompression is indicated.

Associated Findings¶

Syringomyelia (30–70%)
Scoliosis (especially in children with syrinx)
Basilar invagination or platybasia
Klippel-Feil syndrome
Atlantoaxial assimilation

Management¶

Asymptomatic — Observation with serial MRI
Symptomatic — Posterior fossa decompression (suboccipital craniectomy with C1 laminectomy ± duraplasty) to restore CSF flow at the foramen magnum

Key Points¶

Cerebellar tonsils ≥5 mm below the foramen magnum defines Chiari I
Syringomyelia is present in 30–70% — always evaluate the cervicothoracic cord
Peg-like tonsillar morphology is more significant than rounded tonsils
Cine MRI evaluates CSF flow dynamics at the foramen magnum
Posterior fossa decompression is the treatment for symptomatic patients

References¶

Loftus JR, Wassef C, Ellika S. Chiari I deformity: beyond 5 mm below the foramen magnum. RadioGraphics. 2024;44(9):e230227. Available from: https://pubs.rsna.org/doi/full/10.1148/rg.230227
Elster AD, Chen MY. Chiari I malformations: clinical and radiologic reappraisal. Radiology. 1992;183(2):347-53. Available from: https://pubmed.ncbi.nlm.nih.gov/1561334/
Bauer DF, Niazi T, Qaiser R, et al. Congress of Neurological Surgeons systematic review and evidence-based guidelines for patients with Chiari malformation: diagnosis. Neurosurgery. 2023;93(4):723-6. Available from: https://pubmed.ncbi.nlm.nih.gov/37646512/
Pindrik J, McAllister AS, Jones JY. Imaging in Chiari I malformation. Neurosurg Clin N Am. 2023;34(1):67-79. Available from: https://pubmed.ncbi.nlm.nih.gov/36424066/
Haughton V, Mardal KA. Spinal fluid biomechanics and imaging: an update for neuroradiologists. AJNR Am J Neuroradiol. 2014;35(10):1864-9. Available from: https://www.ajnr.org/content/35/10/1864
Kular S, Karsonovich T. Chiari Malformation Type 1. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2025. Available from: https://www.ncbi.nlm.nih.gov/books/NBK554609/
Gaillard F, et al. Chiari 1 malformation. Reference article, Radiopaedia.org. Available from: https://radiopaedia.org/articles/chiari-1-malformation-1