Arnold-Chiari Malformation

Arnold-Chiari Malformations (ACM) are a heterogeneous group of structural defects in the cerebellum, brainstem, and craniocervical junction, characterized by the downward displacement of hindbrain structures through the foramen magnum into the cervical canal.

CLASSIFICATION (CLELAND-CHIARI)

Chiari Type I (Adult Type)

• Pathology: Downward displacement of cerebellar tonsils (>5mm) through the foramen magnum.
• Associations: Syringomyelia (30-50%), basilar invagination, and scoliosis. Usually no myelomeningocele.
• Presentation: Often asymptomatic until adolescence; presents with suboccipital headache (exacerbated by Valsalva), neck pain, and progressive syringomyelia symptoms (dissociated sensory loss).

Chiari Type II (Classic Arnold-Chiari)

• Pathology: Downward displacement of the cerebellar vermis, brainstem (pons and medulla), and fourth ventricle.
• Associations: Nearly always associated with Myelomeningocele (MMC) and obstructive hydrocephalus (due to aqueductal stenosis).
• Radiological Signs: "Beaking" of the tectum, low-lying torcular, and scalloping of the petrous bone.

Chiari Type III

• Pathology: Occipital or high cervical encephalocele containing herniated cerebellar and brainstem tissue.
• Prognosis: Rare and carries a high mortality/severe neurological deficit rate.

Chiari Type IV

• Pathology: Cerebellar hypoplasia or agenesis without herniation. Now considered a distinct entity from the Chiari spectrum by many authorities.

PATHOPHYSIOLOGY

1. Molecular/Developmental: Underdevelopment of the posterior fossa (small volume) forces the normally growing cerebellum downward.
2. Hydrodynamic Theory (Gardner’s): CSF pressure pulse wave from the ventricles causes syringomyelia and herniation.
3. Unified Theory (McLone & Knepper): Failure of primary neurulation leads to lack of distension of the ventricular system, resulting in a small posterior fossa.

CLINICAL FEATURES (TYPE II - PEDIATRIC)

• Infants (Brainstem dysfunction):
  • Stridor (vocal cord paralysis - CN X).
  • Weak cry, swallowing difficulties, and chronic aspiration.
  • Opisthotonos and apneic spells.
• Older Children:
  • Progressive hydrocephalus signs.
  • Ataxia, spasticity, and hand weakness.
  • Scoliosis (often secondary to an underlying syrinx).

INVESTIGATIONS

• MRI Brain and Spine (Gold Standard):
  • To visualize the degree of tonsillar/vermicular herniation.
  • To rule out Syringomyelia (syrinx) or Hydromyelia.
  • To assess for corpus callosum agenesis or hydrocephalus.
• Cine-MRI: To evaluate CSF flow dynamics at the craniocervical junction.
• Sleep Study: If central apnea is suspected.

MANAGEMENT

• Surgical Decompression:
  • Posterior Fossa Decompression (PFD): Suboccipital craniectomy + C1 (±C2) laminectomy to create space.
  • Duraplasty: Opening the dura and patching it to expand the CSF space.
  • Tonsillar Resection: In severe Type I cases.
• Management of Comorbidities:
  • Hydrocephalus: Ventriculoperitoneal (VP) shunt. Note: Shunt malfunction can worsen Chiari symptoms.
  • MMC Repair: Immediate postnatal (or fetal) closure of myelomeningocele.
• Follow-up: Periodic monitoring for syrinx progression or recurrence of symptoms.

PROGNOSIS

• Type I: Excellent prognosis if decompressed before permanent cord damage occurs.
• Type II: Dependent on the severity of the myelomeningocele and brainstem dysfunction. Early decompression for stridor/apnea is life-saving but carries guarded outcomes.