The Chiari II Malformation (Arnold-Chiari Malformation)
Chiari II malformation is a type of Chiari malformation also known as Arnold-Chiari malformation. It is defined as the herniation of the vermis, medulla, and fourth ventricle into the spinal canal. Occurs in >90% of patients with myelomeningocele (spina bifida) and is exclusive to this population.
Signs and Symptoms
Symptoms of Chiari II malformation/Arnold-Chiari malformation include dysfunction of CN 9 and 10 leading to dysphagia and respiratory distress. May present as a neurosurgical emergency in up to 20%. A child with Chiari II malformation and any respiratory abnormality requires close observation. These symptoms appear from age 2 or 3 months up to 2 years).6 It is unusual for newborns to be symptomatic. Older children usually present with a spinal cord syndrome and/or syringomyelia, similar to Chiari I patients.3,4,7
With Arnold-Chiari malformation, the MRI shows descent into the upper cervical spine of the vermis, medulla and fourth ventricle, along with multiple other associated anomalies.2,5
The complex anatomy and significant risk of neurological injury and deterioration necessitate a referral to a pediatric neurosurgeon. Most Arnold-Chiari malformation patients have hydrocephalus and are usually shunted shortly after birth. Shunt malfunction can mimic Chiari II malformation symptoms!!! Prior to decompression, the shunt must be shown to be functional. Shunt taps and normal or small ventricular appearance on radiographs should not be used as proof of shunt function in these patients. Instead, surgical shunt exploration is preferred.1 "It is the shunt until proven otherwise." The bony decompression focuses on the hindbrain herniation and generally need not involve the posterior fossa or foramen magnum. Cervical laminectomy to the level of the lowest hindbrain tissue is sufficient. The dura is opened and dense arachnoid adhesions are dissected using the operative microscope. The procedure is generally considered over when the avascular floor of the fourth ventricle can be easily visualized. This often requires microsurgical dissection of the vermis off the fourth ventricular floor, and possibly placing a stent between the 4th ventricle and cervical subarachnoid space.
Prognosis and Outcomes
The key to good prognosis of Chiari II malformation is early recognition of symptoms and early treatment. Patients with less severe symptoms at diagnosis are much more likely to improve with surgical intervention. Previously, results of surgery for Chiari II malformation were less than satisfactory. However, these results were reported prior to the recognition of shunt malfunction without ventricular dilatation. Most recent series report better outcomes.
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