Chiari type I malformation

General Features

  • Best diagnostic clue
    • Low-lying, pointed (not rounded) "peg-like" CBLL tonsils
    • Tonsillar sulci have vertical (not horizontal) orientation
    • Compressed/absent cisterna magna
  • Size
    • Age-related tonsil descent below "opisthion-basion line"
      • 1st decade (6 mm) (most pronounced at"" 4 y, then tonsils "retreat")
      • 2nd/3rd decades (5 mm)
      • 4th to 8th decades (4 mm)
      • 9th decade (3 mm)


  • Tonsils can normally lie below FM (5 mm in adults, slightly more in children < 4 y)
  • Unless tonsils ~ 5 mm and/or pointed, probably not Ch 1
  • Tonsillar impaction in FM without caudal herniation can also be symptomatic
    • Look for absent cisterna magna, posteriorly angled odontoid with compressed brainstem, short posterior arch C1, short supra occiput, syrinx

Radiographic Findings


  • 4th occipital sclerotome syndromes> 50%
    • Short clivus, craniovertebral segmentation/fusion anomalies, pro-atlas remnants, atlas assimilation, odontoid retroflexion
    • Small occipital enchondral skull: Basiocciput, exocciput, supra occiput
    • 1 Angulation of posteriorly tilted odontoid process (more common in females) => 1 symptoms
  • Suspect Ch 1 if
    • Cervical lordoses > 0 degrees
    • Thoracic kyphosis> 40 degrees
  • Suspect syrinx if enlarged cervical spinal canal on lateral film

CT Findings

    • NECT
    • Small bony PF ::>low torcular, effaced PF cisterns
    • "Crowded" FM
    • Lateral/3rd ventricles usually normal (89%)

MR Findings

  • TlWI
    • Sagittal: Pointed, triangular-shaped ("peg-like") tonsils 2: 5 mm below FM
    • Surrounding CSF in PM effaced
    • Short clivus::> apparent descent 4th ventricle, medulla
  • T2WI: Look for upper cervical cord edema, syrinx (15-75%)
  • Phase-contrast cine MR shows pulsatile systolic tonsillar descent, obstructed CSF flow through FM