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Nervous System Disorders

Chiari I Malformation (adulthood)!!navigator!!

= CEREBELLAR TONSILLAR ECTOPIA

= herniation of cerebellar tonsils below a line connecting basion with opisthion (= level of foramen magnum)

◊Frequently isolated hindbrain abnormality of little consequence without supratentorial anomalies!

Proposed causes:

  1. small posterior fossa
  2. disproportionate CSF absorption from subarachnoid spinal space
  3. cerebellar overgrowth

Associated with:

  1. syringohydromyelia (20–30%)
  2. hydrocephalus (25–44%)
  3. malformation of skull base and cervical spine:
    1. basilar impression (25%)
    2. craniovertebral fusion: eg, occipitalization of C1 (10%), incomplete ossification of C1-ring (5%)
    3. Klippel-Feil anomaly (10%)
    4. platybasia

    ◊NOT associated with myelomeningocele!
  • benign cerebellar ectopia: <3 mm of no clinical consequence; 3–5 mm of uncertain significance; >5 mm clinical symptoms likely
  • no symptoms in childhood (unless associated with hydrocephalus / syringomyelia)
  • ± cranial nerve dysfunction / dissociated anesthesia of lower extremities in adulthood
  • downward displacement of cerebellar tonsils + medial part of inferior lobes of cerebellum >5 mm below level of foramen magnum
  • inferior pointing peglike / triangular tonsils
  • obliteration of cisterna magna
  • elongation of 4th ventricle, which remains in normal position
  • slight anterior angulation of lower brainstem

Chiari II Malformation (childhood)!!navigator!!

= ARNOLD-CHIARI MALFORMATION

= most common + serious complex of anomalies involving hindbrain, spine, mesoderm ← posterior fossa too small

HALLMARK is dysgenesis of hindbrain with

  1. caudally displaced 4th ventricle
  2. caudally displaced brainstem
  3. tonsillar + vermian herniation through foramen magnum

No association with: basilar impression / C1-assimilation / Klippel-Feil deformity

  • newborn: respiratory distress, apneic spells, bradycardia, impaired swallowing, poor gag reflex, retrocollis, spasticity of upper extremities
  • teenager: gradual loss of function + spasticity of lower extremities

Skull film:

  • Lückenschädel (most prominent near torcular herophili / vertex) in 85% = dysplasia of membranous skull disappearing by 6 months of age
  • scalloping of clivus + posterior aspect of petrous pyramids (from pressure of cerebellum) in 70–90% leading to shortening of IAC
  • small posterior fossa
  • enlarged foramen magnum + enlarged upper spinal canal secondary to molding (in 75%)
  • absent / hypoplastic posterior arch of C1 (in 70%)
  • Supratentorial
  • obstructive hydrocephalus (duct of Sylvius dysfunctional but probe patent); may not become evident until after repair of myelomeningocele (in 50–98%)
  • nonvisualization of aqueduct (in up to 70%)
  • colpocephaly (= enlargement of occipital horns + atria) ← maldeveloped occipital lobes
  • dysgenesis of corpus callosum (in 80–90%): hypoplasia / absence of splenium + rostrum
  • absence of septum pellucidum (40%)
  • interdigitation of medial cortical gyri ← hypoplasia + fenestration of falx (in up to 100%)
  • “bat-wing” configuration of frontal horns (on COR views) = frontal horns point inferiorly with blunt superolateral angle ← prominent impressions by enlarged caudate nucleus
  • “hourglass ventricle” = small biconcave 3rd ventricle ← large massa intermedia
  • wide prepontine + supracerebellar cisterns
  • stenogyria = multiple small closely spaced gyri separated by shallow sulci within cortex of normal thickness ← dysplasia (in up to 50%)

Location: medial occipital lobe (on SAG image)

  • Cerebellum
    • “cerebellar peg” = protrusion of vermis + hemispheres through foramen magnum (90%) → craniocaudal elongation of cerebellum
    • hypoplastic poorly differentiated cerebellum (poorly visualized folia on sagittal images) ← severe degeneration
    • elongated / obliterated vertically oriented thin-tubed 4 th ventricle with narrowed AP diameter exiting below foramen magnum (40%)
    • obliteration of CPA cistern + cisterna magna by cerebellum growing around brainstem
    • dysplastic tentorium with wide U-shaped incisura inserting close to foramen magnum (95%)
    • “tectal beaking” = fusion of midbrain colliculi into a single beak pointing posteriorly and invaginating into cerebellum
    • V-shaped widened quadrigeminal plate cistern ← hypoplasia of cingulate gyri
    • “towering cerebellum” = “pseudomass” = cerebellar extension above incisura of tentorium
    • triple peak configuration = corners of cerebellum wrapped around brainstem pointing anteriorly+ laterally (on axial images)
    • flattened superior portion of cerebellum ← temporoparietal herniation
    • vertical orientation of shortened straight sinus
  • Spinal cord
    • medulla + pons displaced into cervical canal
    • “cervicomedullary kink” = herniation of medulla posterior to spinal cord (up to 70%) at level of dentate ligaments
    • widened anterior subarachnoid space at level of brainstem + upper cervical spine (40%)
    • AP diameter of pons narrowed
    • upper cervical nerve roots ascend toward their exit foramina
    • syringohydromyelia
    • lumbar myelomeningocele (>95%)
    • low-lying often tethered conus medullaris below L2

OB-US:

  • hydrocephalus
  • “banana” sign = cerebellum wrapped around posterior brainstem + obliteration of cisterna magna ← small posterior fossa + downward traction of spinal cord

Chiari III Malformation!!navigator!!

most severe rare abnormality; probably unrelated to type I and II Chiari malformation

  • low occipital / high cervical meningomyeloencephalocele

Prognosis: survival usually not beyond infancy

Chiari IV Malformation!!navigator!!

extremely rare anomaly probably erroneously included as type of Chiari malformation

  • agenesis of cerebellum
  • hypoplasia of pons
  • small + funnel-shaped posterior fossa

Outline