Arnold-Chiari malformation (Chiari Malformation type II or CMII) is found exclusively in pts with myelomeningocele.

Myelomeningocele occurs in 0.6 of 1000 live births.

Vocal cord paralysis

Respiratory distress

Apnea

Neurogenic dysphagia and pulmonary aspiration

Hydrocephalus and increased ICP

Congenital heart defects (37% of pts with myelomeningocele) including atrial septal defect, ventricular septal defect, anomalous pulmonary return, tetralogy of Fallot, bicuspid aortic valve, coarctation of the aorta, and hypoplastic left heart syndrome

Vocal cord paralysis

Respiratory distress

Apnea

Neurogenic dysphagia and pulmonary aspiration

Hydrocephalus and increased ICP

Congenital heart defects (37% of pts with myelomeningocele) including atrial septal defect, ventricular septal defect, anomalous pulmonary return, tetralogy of Fallot, bicuspid aortic valve, coarctation of the aorta, and hypoplastic left heart syndrome

Any symptoms of possible brainstem compression (stridor, hoarse voice, or difficulty swallowing) in a child less than 2 y with CMII must be urgently evaluated as a neurosurgical emergency.

Nearly 21% of children with myelomeningocele will development hindbrain, cranial nerve, or spinal cord compression by 3 mo of age, increasing to 33% by age 5 y.

Of pts with symptomatic CMII, 15% die by 3 y of age.

Mortality rate has improved with emergent surgical treatment in symptomatic pts.

CMII is characterized by herniation of the cerebellar vermis, brainstem, and fourth ventricle through the foramen magnum in the setting of myelomeningocele.

Commonly associated with hydrocephalus (90%) and syringomyelia (20-95%).

Other variable abnormalities associated with CMII include dysplasia of the corpus callosum, enlargement of mass intermedia, abnormalities of the white and gray matter, hippocampal dysplasia, elongation of the pons, beaking of the midbrain tectum, and defects of the falx and tentorium.

Symptomatic CMII is the leading cause of death in pts less than 2 y old with myelomeningocele.

Symptoms differ relative to age of onset, with neonates presenting as a neurologic emergency and older children presenting with more subtle findings of hyporeflexia weakness, or headache.

Pathophysiology not completely understood

Multiple theories of embryologic origin, including primary malformation or secondary abnormalities related to altered cerebral spinal fluid dynamics

Early closure of myelomeningocele, usually within 72 h after birth, proven to cause upward movement of hindbrain herniation but does not appear to prevent lifetime occurrence of symptomatic CMII.

Treatment of hydrocephalus more important than surgical decompression to prevent CMII symptoms, and, in any symptomatic CMII pt, physician must first rule out and treat hydrocephalus.

Of pts, 20% will need surgical treatment including brainstem decompression via posterior cervical laminectomies at all involved segments and possible duraplasty.

Unlike Chiari Malformation type I, suboccipital craniotomy often not necessary due to already enlarged foramen magnum.

Intrauterine myelomeningocele repair shows evolving benefits of possibly preventing CMII and decreasing overall severity.

Intrinsic brainstem dysfunction cannot be treated surgically.