Chiari II malformation is defined as the herniation of the vermis, medulla, and fourth ventricle into the spinal canal. It occurs in nearly 100% of patients with myelomeningocele (spina bifida) and is exclusive to this population. This is sometimes referred to as Arnold-Chiari malformation.
Symptoms of Chiari II malformation stem from dysfunction of the brainstem and lower cranial nerves. These can include stridor, dysphagia, sleep apnea, and respiratory distress. These symptoms can be severe and may require urgent or emergent neurosurgical treatment. A child with Chiari II malformation and any respiratory abnormality requires close observation. While Chiari II will become symptomatic most often before 2 years of age, symptoms may appear later. Therefore, an individual with myelomeningocele and Chiari II malformation should have established specialist care.6 Additionally, Chiari II may lead to syringomyelia, similar to Chiari I.3,4,7
MRI shows descent of the vermis, medulla and fourth ventricle into the upper cervical spine. There are numerous other abnormalities of the cerebrum, cerebellum, and brainstem that accompany hindbrain herniation in Chiari II.2,5
The complex anatomy and significant risk of neurological injury and deterioration necessitate treatment by an experienced surgeon. Chiari II symptoms are symptoms of hindbrain dysfunction.8 The first line of treatment for symptomatic Chiari II is always treatment of hydrocephalus. As all Chiari II is associated with myelomeningocele, and most individuals with myelomeningocele have hydrocephalus, Chiari II symptoms can be a sign of untreated or under-treated hydrocephalus. Before considering posterior fossa decompression for Chiari II symptoms, it is essential to assure that hydrocephalus is adequately treated. In most cases, this means placing or revising a VP shunt. Shunt taps and normal or small ventricular appearance on radiographs should not be used as proof of shunt function. Instead, surgical shunt exploration is preferred.1 “It is the shunt until proven otherwise.”
In rare cases when adequate hydrocephalus treatment does not relieve Chiari II symptoms, posterior fossa decompression may be necessary. Because the herniation of hindbrain structures may be substantially below the foramen magnum, decompression often does not require craniectomy. Cervical laminectomy may be sufficient. Indeed, the foramen magnum is often larger than normal in these patients. Cervical laminectomy is performed to the level of the lowest hindbrain tissue. Whether to open the dura and perform duraplasty is a topic of some controversy. Opening dura increases the risk, especially given that the confluence of sinuses or torcular Herophili may be extremely low-lying. On the other hand, not opening the dura may fail to address intradural obstructions to CSF flow. In either case, care must be taken to assure that the decompression includes the tissue overlying the fourth ventricle outlet. This can be somewhat challenging to localize on imaging. MRI with contrast may show the choroid plexus of the 4th ventricle, helping with the decision of exactly where to perform the decompression.
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. However, there is a subset of young infants and neonates with Chiari II symptoms who fail to improve despite adequate treatment of hydrocephalus and posterior fossa decompression. It is possible that these children have a disorganized and dysfunctional brainstem, and therefore, are unlikely to respond to any treatment. In these cases, tracheostomy and gastrostomy are often performed to prevent life-threatening respiratory compromise.