Forebrain Anencephaly failure of anterior neuropore to close resulting in failure of brain and calvarium to form no swallowing center → polyhydramnios ↑ AFP most common cause of ↑ AFP is incorrect pregnancy dating Holoprosencephaly failure of prosencephalon to divide and form cerebral hemispheres represents a field defect with regional manifestations including absence of olfactory bulbs and tracts cyclopia, cleft palate, cleft lip associated with Patau syndrome (trisomy 13) and Edwards syndrome (trisomy 18) severe fetal alcohol syndrome cleft lip/palate fetal alcohol syndrome Posterior Fossa Dandy-Walker syndrome dilation of 4th ventricle absence of cerebellar vermis small cerebellar hemispheres posterior fossa cysts enlargement of posterior fossa associated with hydrocephalus and spina bifida Arnold Chiari I cerebellar tonsil herniation through foramen magnum often presents in adulthood with chronic headaches and ataxia associated with cervico-thoracic syringomyelia milder in severity in comparison to type II malformations Arnold Chiari II due to small posterior fossa, the cerebellar tonsils herniate through foramen magnum causing aqueductal stenosis and hydrocephaly may present with synringomelia, thoracolumbar spina bifida with myelomeningocele symptoms present from compression of medulla oblongata and CN IX, X, and XI reduced gag reflex vocal cord paralysis laryngeal stridor swallowing difficulty spastic dysphonia Spinal Cord Syringomyelia central canal of spinal cord enlarges nerves closest to central canal affected first spinothalamic tract: bilateral loss of pain and temperature sensation in affected area, touch not affected occurs most often at C8-T1 cape-like pattern loss often associated with Chiari I, less frequently with Chiari II