Congenital hindbrain malformation where the cerebellar tonsils herniate downward through the foramen magnum (typically >5 mm descent).
Chiari I is relatively common (≈0.5–3.5% of the population) and can cause severe occipital headaches (often triggered by coughing/straining) and progressive neurologic deficits from brainstem compression or syringomyelia. It is frequently tested because of its classic imaging findings and associations with syrinx and scoliosis.
Children: often diagnosed during scoliosis evaluation or when investigating occipital headache; many have a cervicothoracic syrinx on MRI.
Adults: chronic suboccipital headache worsened by Valsalva (cough/strain) and neck pain; exam may reveal cerebellar ataxia, dysmetria, or dissociated sensory loss.
Imaging: sagittal MRI shows cerebellar tonsils >5 mm below the foramen magnum with a small posterior fossa; often accompanied by a spinal syrinx.
Suspect Chiari I in patients with Valsalva-exacerbated occipital headache, neck pain, cerebellar signs, or unexplained scoliosis.
Confirm diagnosis with MRI (brain + cervical spine) – look for tonsillar herniation >5 mm and any syrinx.
If asymptomatic or mild (minimal symptoms, no syrinx), manage conservatively (observe, analgesics/physical therapy).
If symptomatic (persistent headaches, neurologic deficits) or syrinx present, refer for posterior fossa decompression.
Condition
Distinguishing Feature
Intracranial hypotension (CSF leak)
MRI shows diffuse brain sagging and pachymeningeal enhancement, not an isolated tonsillar herniation.
Benign tonsillar ectopia
Tonsils descend <5 mm on MRI; patient is asymptomatic with no CSF flow block.
Raised ICP from mass lesion
MRI reveals a tumor or hydrocephalus causing downward pressure on the brain (look for ventricular dilation or mass).
Conservative: if asymptomatic or mild (observe, manage pain with NSAIDs/muscle relaxants, avoid straining).
Surgical: posterior fossa decompression (suboccipital craniectomy ± C1 laminectomy, often with duraplasty) to restore CSF flow and relieve compression.
"Chiari I – think Cough headache" (cough/sneeze-induced headache).
Mnemonic: "Syrinx and Scoliosis" → check for Chiari I.
Rapid deterioration or new lower cranial nerve deficits.
Suspect Chiari I with characteristic symptoms (occipital/Valsalva headache, neck pain, scoliosis).
Obtain MRI of brain and cervical spine for tonsillar descent and syrinx.
If tonsils <5 mm descent and no syrinx, consider other diagnoses.
If >5 mm descent (Chiari I) and no symptoms, observe with follow-up imaging.
If >5 mm descent with significant symptoms or syrinx, refer for surgical decompression.
Monitor post-treatment; promptly address any red-flag signs.
Sagittal MRI image question showing cerebellar tonsils herniating >5 mm below the foramen magnum (diagnostic of Chiari I).
Vignette: young adult with Valsalva-induced occipital headaches and MRI-confirmed cerebellar tonsil descent.
Vignette: adolescent with scoliosis and dissociated sensory loss; MRI reveals Chiari I malformation.
Case 1
A 20-year-old woman has chronic occipital headaches worsened by coughing. Neurologic exam shows left-hand dissociated sensory loss. MRI reveals the cerebellar tonsils extending 7 mm below the foramen magnum.
Case 2
A 14-year-old boy with idiopathic scoliosis reports occasional hand numbness. MRI of the brain and spine shows cerebellar tonsils 6 mm below the foramen magnum and a cervical spinal cord syrinx.
Sagittal MRI of Chiari I malformation showing tonsillar herniation
