Fusion with Chiari Surgery
Introduction
Some patients with symptomatic Chiari I Malformation and/or syringomyelia have associated craniocervical instability or ventral brainstem compression that contributes to their signs and symptoms. In some cases, bone and connective tissue abnormalities (such as Ehlers-Danlos) lead to instability of the craniocervical junction and ventral brainstem compression. In this small percentage (less than 5%) of Chiari patients, posterior fossa decompression alone may not improve symptoms and may, in fact, worsen symptoms. These patients may require occipitocervical fusion and, in some cases, removal of compressive tissue anterior to the brainstem.
Definitions
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: When the abnormal angle of the odontoid is severe enough, it can cause compression of the brainstem.2,3
Platybasia indicates an increase in the basal angle of the skull. On lateral skull radiographs, sagittal CT or MRI, the angle of the line drawn along the plane of the sphenoid sinus and clivus is greater than 145 degrees.
Occipitocervial (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) but can also involve a fusion construct (combination of bone, bone matrix, and sometimes bone morphogenic proteins) without hardware (noninstrumented fusion). Patients usually undergo rigid cervical immobilization following this procedure (cervical brace, halo vest, or custom minerva) until the occiput and cervical bones completely fuse together.
Signs and Symptoms of Chiari malformation with instability or ventral compression
Presenting signs and symptoms include: headache, neck pain, dysphagia, decreased gag reflex, weakness, numbness, gait problems, hyperreflexia, C-2 numbness, facial numbness, torticollis 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
Diagnostic Studies
Flexion/extension and sometimes supine/standing cervical spine radiographs are performed 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 or other ventral compression.2,3,6 MRI is used to evaluate descent of the tonsils into the foramen magnum, syringomyelia, platybasia, basilar invagination and other ventral lesions. In some cases, dynamic MRI in flexion and extension is done to evaluate the extent of ventral and dorsal cervicomedullary compression with these maneuvers. This can help determine the reducibility of the abnormality (i.e. whether the angle can be corrected with cervical traction before or at the time of surgical fusion).1,2,3,6 Diagnostic criteria for Ehlers-Danlos syndrome and related hereditary disorders of connective tissue may be present in some patients with Chiari I malformation. In this group of patients, there appears to be a reduced basion-dens interval, enlargement of the basion-atlas interval, and reduced angles of clivus-axis, clivus-atlas and atlas-axis with upright position. These changes can often be reduced with traction.
Indications for craniocervical junction fusion
Unstable craniocervical junction, with or without bone or soft tissue abnormalities (atlas assimilation, basilar invagination, Klippel-Feil anomaly, Ehlers-Danlos syndrome, Down syndrome and other pathologies). In these situations, dynamic MRI shows ventral compression of brainstem in flexion, relieved with extension.1,2,3,6 If anterior decompression of lower brainstem/upper cervical cord is done to remove ventral compressive lesion (typically, transoral decompressive procedure), craniocervical fusion is required.1,2,3,6
Treatment
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 (through the mouth) decompression with posterior occipitocervical decompression and fusion are typically done.1,2,3,6 When no obvious radiographic instability or ventral compression exists, significant controversy exists regarding the extent of basilar invagination or cranial settling it would take for a surgeon to recommend a fusion along with the Chiari decompression.
Outcomes
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.