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The Nervous System

VARIATIONS OF CRANIUM BIFIDA

Cranium bifida occurs when the bony skull fails to form properly, thereby creating a skull defect usually in the occipital region.

It is due primarily to expectant mothers not taking enough folic acid during pregnancy.

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ARNOLD – CHIARI MALFORMATION

Chiari I malformation

• Ectopia of cerebellar tonsils (1 structure)
• Congenital, usually asymptomatic in childhood, manifests in adulthood with headaches and cerebellar symptoms.
• Associated with spinal cavitations (eg, syringomyelia).

Chiari II malformation

• Herniation of low-lying cerebellar vermis and tonsils (2 structures) through foramen magnum with aqueductal stenosis Ž> hydrocephalus
• Usually associated with lumbosacral meningomyelocele (may present as paralysis/sensory loss at and below the level of the lesion)

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Arnold - Chiary I

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• Cystic cavity (syrinx) within central canal of spinal cord (yellow arrows in A ). Fibers crossing in anterior white commissure (spinothalamic tract) are typically damaged first. Results in a “capelike,” bilateral, symmetrical loss of pain and temperature sensation in upper extremities (fine touch sensation is preserved).
• Associated with Chiari I malformation (red arrow in A shows low-lying cerebellar tonsils), scoliosis and other congenital malformations; acquired causes include trauma and tumors. Most common location cervical > thoracic >> lumbar. Syrinx = tube, as in “syringe.”

Syringomyelia

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Holoprosencephaly

• Developmental field defect characterized by failure of embryonic forebrain (prosencephalon) to separate into 2 cerebral hemispheres; usually occurs during weeks 3–4 of development. May be related to mutations in sonic hedgehog signaling pathway.
• Associated with other midline defects including cleft lip/palate (moderate form) and cyclopia (severe form).  risk for pituitary dysfunction (eg, diabetes insipidus). May be seen with Patau syndrome (trisomy 13) and maternal alcohol use. MRI reveals monoventricle A and fusion of basal ganglia (star in A ).

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Dandy-Walker syndrome

• Agenesis of cerebellar vermis leads to cystic enlargement of 4th ventricle that fills the enlarged posterior fossa.
• Associated with noncommunicating hydrocephalus, spina bifida.

The Nervous System