Arnold Chiari Malformation or Chiari II (CMII) is uniquely associated with myelomeningocele. Chiari II malformation is defined as the caudal herniation of the vermis, brainstem, and fourth ventricle through the foramen magnum . The CM II was later found to be also associated with other abnormalities throughout the brain, skull, and spine. Hans Chiari attributed the hindbrain herniation to hydrocephalus. Cleland, helps to explain the CNS malformations in the posterior fossa but fails to account for the cranial and supratentorial anomalies frequently associated with CM II and myelomeningocele. Lichtenstein, Peinfield, and Coburn believed that it is a tethered spinal cord secondary to the myelomeningocele that pulls the brainstem and cerebellum inferiorly, resulting in elongation and herniation of the posterior fossa structures. Combining features of each of the previously mentioned theories is McLone and Knepper’s unified theory of CM II. In this theory, both the open neural tube defect and incomplete spinal occlusion allow CSF to drain through the central canal and it is therefore not maintained in the ventricular system. On ultrasound a “banana sign” is produced by the abnormal shape of the cerebellum combined with an absent cisterna magna. This finding is better seen on prenatal ultrasonography than the neural tube defect itself and can aid in prenatal diagnosis.
Given that CM II is universally associated with myelomeningocele, the first sign is an open neural tube defect. Once born, a symptomatic CM II is the most common cause of death in patients with myelomeningocele who are younger than 2 years of age. They present with respiratory and brainstem findings which should be evaluated urgently. In this age group inspiratory stridor from vocal cord paresis is often the most noticeable sign of brainstem dysfunction. Older children present with spinal cord and cerebellar dysfunction. Signs and symptoms of cervical myelopathy are the hallmark finding in older children, with upper-extremity weakness and spasticity being the most common finding.  The first treatment option to be considered should be cerebrospinal shunt revision because the most common symptom in this population is hydrocephalus or a failed shunt. If this fails, surgical decompression with removal of the lamina of the first and second cervical vertebrae is required. There have been advances in fetal intrauterine myelomeningocele repair but, as of now, intrauterine myelomeningocele repair has not been shown to improve lower extremity motor and sensory function.  However, this technique may reduce the severity of, or completely prevent, the CM II.