How should a suspected Chiari malformation be evaluated on a non‑contrast CT scan of the head?

Evaluation of Chiari Malformation on CT Scan of Head

CT scan is not the appropriate imaging modality for evaluating suspected Chiari malformation—MRI is required for proper diagnosis and assessment. 1

Why CT is Inadequate

CT cannot adequately visualize the key diagnostic features of Chiari malformation. The critical anatomical details needed for diagnosis—including cerebellar tonsillar position relative to the foramen magnum, CSF flow dynamics at the craniocervical junction, and associated findings like syringomyelia—are poorly demonstrated on CT imaging. 1, 2

• CT lacks the soft tissue resolution necessary to accurately measure tonsillar descent below the foramen magnum, which is the primary diagnostic criterion (≥5 mm herniation). 3, 4
• The craniocervical junction and posterior fossa structures require detailed visualization that only MRI can provide. 1, 5

Proper Imaging Protocol for Chiari Malformation

MRI of the brain without contrast, including sagittal T2-weighted sequences of the craniocervical junction, is the study of choice for suspected Chiari malformation. 1, 2

Essential MRI Components:

• Sagittal T2-weighted sequences of the craniocervical junction are mandatory to assess tonsillar position and relationship to the foramen magnum. 1, 2
• Optional phase-contrast CSF flow studies at the craniocervical junction can evaluate CSF flow obstruction, which is the primary mechanism causing symptoms. 1, 2, 5
• Complete spine imaging should be included to evaluate for syringomyelia, which occurs in 65% of symptomatic patients. 2, 4

Key Diagnostic Features on MRI:

• Cerebellar tonsillar herniation ≥5 mm below the foramen magnum (though some symptomatic patients have <5 mm descent). 3, 6, 4
• Obliteration of retrocerebellar CSF spaces (present in virtually all cases). 4
• Posterior fossa volume reduction and cranial base dysplasia. 3, 6, 4
• Associated syringomyelia (65% of cases) or hydrocephalus. 4, 7

Clinical Context Matters

The diagnosis requires correlation between imaging findings and characteristic symptoms—not imaging alone. 3, 8

Characteristic Chiari Symptoms to Assess:

• Valsalva-induced occipital headaches (worsened by coughing, straining) are the most characteristic symptom. 1, 5
• Lower cranial nerve dysfunction and brainstem signs. 4
• Cervical myelopathy symptoms (especially if syringomyelia present). 8, 4
• Meniere's-like syndrome with vertigo and hearing symptoms. 4

Critical Pitfall

Avoid diagnosing Chiari malformation based on incidental CT findings alone. If tonsillar descent is suspected on CT performed for another indication (such as trauma evaluation), this requires confirmation with dedicated MRI before any clinical significance can be assigned. 1, 3 Many patients have incidental tonsillar ectopia without symptoms, and surgical intervention based on inadequate imaging unnecessarily exposes patients to operative risks. 3

When CT May Have Limited Role:

• In emergency settings where acute hemorrhage or mass effect must be excluded rapidly, CT may be performed first. 1
• However, definitive evaluation of Chiari malformation still requires subsequent MRI. 1