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
- Molecular/Developmental: Underdevelopment of the posterior fossa (small volume) forces the normally growing cerebellum downward.
- Hydrodynamic Theory (Gardner’s): CSF pressure pulse wave from the ventricles causes syringomyelia and herniation.
- 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.