Snapshot A 9-month-old child presents with with a head circumfrence that is two standard deviations above the norm, frontal-bossing, and translucent skin. An MRI is ordered. Introduction Congenital structural defect of the cerebellum occuring during brain development including noncommunicating hydrocephalus most commonly associated with type II spina bifida meningomyelocele elongated 4th ventricle with kinking of the brainstem displacement of the brainstem and cerebellum into the spinal cord CSF outflow obstruction small posterior fossa Three primary types of malformation are type I most common minimal brainstem herniation, but usually no compression or CSF outflow obstruction may not cause symptoms usually identified by accident type II aka Arnold-Chiari malformation usually accompanied by a myelomeningocele form of spina bifida that occurs when the spinal canal and vertebral column do not close before birth causing the spinal cord to protrude through an opening in the back can result in partial or complete paralysis below the spinal opening type III most severe form usually results in lethal neurological defects Arnold-Chiari syndrome is often associated with other pathology including hydrocephalus, syringomyelia, and spinal curvature Presentation Symptoms range widely depending on the degree of cerebellar and brain stem herniation dizziness muscle weakness numbness visual defects or blindness headache poor motor coordination poor balance Physical exam neurological exam deficits motor deficits mental disability may be noted others as above IMAGING CT/MRI used to identify anatomic abnormality and determine severity of herniation Differential Other developmental defects (neural tube defects), intrauterine trauma, cerebral palsy, Treatment Prevention no preventive measures are available at this time prevent neural tube defects with adequate B12/folate in mothers diet Medical management symptom control usually indicated for pain Surgical intervention indicated for correction of functional disturbances or halt the progression of herniation and damage repeat operations may be required as child continues to grow Complications Permanent paralysis Death as neonate or toddler Prognosis Range widely depending on type and severity of anatomic abnormality Some children with type I function normally, while type III has low survival rate