Treatment for the Chiari I malformation is surgical. To date, no effective non-surgical treatment exists. While the natural history of Chiari I malformation is not known, it is generally accepted that patients with neurologic deficits referable to the Chiari, those with syringomyelia, and those who have symptoms that decrease quality of life and are thought to be due to the Chiari malformation (i.e. pain in the back of the head that is worse with coughing or Valsalva), are candidates for posterior fossa decompression. In the patient with a Chiari I malformation who has none of the above symptoms or findings, it is generally agreed that observation is the preferred treatment.
All surgical interventions for Chiari I involve creating more room in the posterior fossa and foramen magnum to relieve the hindbrain compression. The patient is positioned prone (face down) on the operating table with the head in a fixation device. Incision is made in the midline, and the muscles and soft tissues are separated. Bone is exposed from the occiput to the top of the C2 vertebra.
When a Chiari I malformation is severely symptomatic, causes neurological deficits, or is associated with syrinx formation, the treatment is surgical. All options for surgery have a common goal of expanding the posterior fossa allowing normalization of CSF through the 4th ventricle and foramen magnum. This goal can be accomplished in many ways, from simple bone removal, to duraplasty and tonsillar resection. So far, no one method has shown superiority over the others.
Some patients with symptomatic Chiari I malformation also have craniocervical instability or ventral brainstem compression that contributes to their signs and symptoms. Bone and connective tissue disorders— such as Ehlers-Danlos or Marfan syndromes— may exhibit instability of the craniocervical junction or ventral brainstem compression. Large retrospective studies suggest that craniocervical instability or ventral brainstem compression may occur in up to 20% of Chiari I patients.7,8 In these cases (in which there is instability, ventral brainstem compression/basilar invagination), posterior fossa decompression alone may not improve symptoms. These patients should be evaluated for occipitocervical fusion and, in some cases, resection of the upper-most part of the spine— the odontoid process— which is compressing the brainstem and upper spinal cord.
Basilar invagination is a developmental anomaly of the craniovertebral junction in which the odontoid bone in front of the brainstem abnormally protrudes through the opening at the base of the skull and pushes against the brainstem. This occurs in the setting of diseases that cause softening of the bones, such as Osteogenesis imperfecta, spondyloepiphyseal dysplasia, achondroplasia, Hurler’s syndrome, Hadju-Cheney, renal rickets and Paget’s disease.8
Basilar impression is similar to basilar invagination, but occurs in the setting of normal bone.
Ventral compression of the brainstem: A retroflexed odontoid (wherein the odontoid points backwards) may cause compression of the brainstem.2,3
Platybasia is a flattened angle between the floor of the front of the cranium (the anterior fossa) and the clivus, and represents an increase in the basal angle of the skull. On lateral skull radiographs, sagittal CT or MRI, a basal angle greater than 145 degrees represents platybasia.
Occipitocervical (craniocervical) fusion refers to a procedure in which the occipital bone is fused to the upper cervical vertebrae. This procedure typically involves the use of rigid hardware, typically titanium (instrumented fusion). The procedure can also involve a fusion construct (combination of bone, bone matrix, and sometimes bone morphogenic proteins) without hardware (non-instrumented fusion), following which patients usually undergo rigid cervical immobilization (cervical brace, halo vest, or custom Minerva brace) until the occiput and cervical bones completely fuse together.
SIGNS AND SYMPTOMS OF CHIARI MALFORMATION WITH INSTABILITY OR VENTRAL BRAINSTEM COMPRESSION
Presenting signs and symptoms include headache, neck pain, dysphagia, decreased gag reflex, hyperreflexia, weakness, numbness, gait problems, and dizziness.2,3 Some data suggest that problems of instability or ventral compression are more prevalent in pediatric patients with Chiari malformation, compared to adults.7 Some patients with associated craniocervical instability and Chiari malformation have hereditary disorders of connective tissue, such as Down syndrome, Noonan syndrome, or Ehlers-Danlos syndrome.2,3,6
Flexion/extension and sometimes supine/standing cervical spine radiographs are often needed to evaluate the degree of instability at the craniocervical junction.2,3,6 Craniocervical CT, including 3D reconstruction, is used to better define the anatomy and bone anomalies of the craniocervical junction. This can also be used to further define skull base abnormalities such as platybasia, basilar invagination, ventral compression, syringomyelia, and other anomalies of the craniovertebral lesions.2,3,6 It can also to be used to evaluate descent of the tonsils into the foramen magnum and the extent of ventral or dorsal cervicomedullary compression. Dynamic flexion/extension imaging can also help to determine the reducibility of the abnormality (i.e. whether the angle can be corrected with cervical traction reduction before or at the time of surgical fusion).1,2,3,6 Therefore, kyphosis of the craniocervical junction (kyphotic clivoaxial angle) occurring in hereditary disorders of connective tissue may be reduced, or normalized with preoperative or intra-operative traction11.
INDICATIONS FOR CRANIOCERVICAL JUNCTION FUSION
Patients considered for craniocervical fusion should be selected very carefully. Surgery should be considered a last option in those patients with severe pain, disabling neurological symptoms and findings on examination referable to the lower brainstem or upper spinal cord alongside congruent radiological findings (craniocervical instability, basilar invagination, atlas assimilation, basilar invagination, Klippel-Feil anomaly, Ehlers-Danlos syndrome, Down syndrome and other pathologies). Dynamic MRI may show ventral compression of brainstem in flexion, or with extension.1,2,3,6,9,10 If a transoral (through the mouth) or other ventral decompression is performed, craniocervical fusion is required.1,2,3,6
Degree of stability and reproducibility, as assessed on flexion/extension or supine/standing studies, is important in determining surgical treatment. In patients with reducible lesions, direct occipitocervical fusion can be performed at the time of posterior fossa decompression.1,2,3,6 When Chiari I malformation is associated with an irreducible ventral compressive lesion, a single stage transoral decompression with posterior occipitocervical decompression and fusion may be performed.1,2,3,6
Outcome depends on many factors including duration of compression prior to intervention, degree of compression, and preoperative neurological state. Excellent outcomes have been reported, with up to 97% showing symptomatic improvement in carefully selected cases. Syrinx resolution has been reported in up to 65% of patients undergoing adequate decompression.2 Patients who undergo occipitocervical fusion lose a certain degree of neck mobility. Specifically, there can be a 40- to 70-degree loss of neck rotation, and a small (up to 10-degree) loss of flexion/extension. As with any surgery, one of the most important factors in predicting outcome is patient selection. The need for an occipitocervical fusion in Chiari patients is rare. Thus, the decision to perform a fusion should be made with the utmost care, and the resultant loss of neck mobility should be weighed against the severity of the radiographic instability and the patient’s original symptoms.