Cerebellar Tonsillar Ectopia (Chiari I): Symptoms, Causes & MRI Findings

Cerebellar Tonsillar Ectopia (Chiari I Malformation)

Definition: Downward displacement of the cerebellar tonsils ≥5 mm below the foramen magnum (some use ≥3 mm with symptoms).

Pathophysiology

Congenital mismatch between posterior fossa volume and cerebellar content → tonsillar herniation → CSF flow obstruction at foramen magnum → syrinx formation (hydrosyringomyelia) in ~30–50%.

Clinical Presentation

• Suboccipital headache: Valsalva-exacerbated (cough, sneeze, strain).
• Neck pain, stiffness.
• Brainstem/cranial nerve signs: Dysphagia, dysphonia, nystagmus, sleep apnea.
• Cerebellar signs: Ataxia, dysmetria, intention tremor.
• Syrinx-related: Cape-distribution sensory loss, weakness, scoliosis.
• Asymptomatic: Incidental finding in ~0.5–1% of brain MRIs.

MRI Findings

• Tonsillar position relative to McRae's line (basion-opisthion) or foramen magnum plane.
• Pointed/peg-shaped tonsils.
• CSF flow void at foramen magnum (cine MRI).
• Syrinx (T2 hyperintensity within cord).
• Associated: basilar invagination, scoliosis, tethered cord.

Management

• Asymptomatic/mild: observation, NSAIDs for headache, avoid Valsalva.
• Symptomatic/progressive: posterior fossa decompression (suboccipital craniectomy, C1 laminectomy, duraplasty).
• Syrinx often resolves after decompression.

Multidisciplinary team (neurosurgery, neurology, sleep medicine) optimizes outcomes.