Surgical interventions



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. 


  1. Hydrocephalus: A minority of patients with Chiari I malformation will also have hydrocephalus. If there is any uncertainty as to the coexistence of hydrocephalus, intracranial pressure monitoring is recommended prior to posterior fossa decompression.
  1. Craniocervical stability: Chiari I malformation often occurs with other anomalies of the craniocervical junction. Flexion and extension radiographs or MRI should be performed prior to posterior fossa decompression if there is concern that there may be instability. If this is not recognized, the posterior fossa decompression may increase the instability, causing worsening of symptoms, and possibly requiring further surgery.
  1. Acquired tonsillar descent: In rare circumstances, the tonsillar descent is secondary to other problems, namely: pseudotumor cerebri, occult CSF leak, brain tumor, or multiple lumbar punctures. If there is clinical suspicion, these problems should be investigated/ruled out. 


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.

  1. Bony decompression: The simplest option is bony decompression only. After exposing bone, a portion of the occiput and the posterior arch of the C1 vertebra are removed. The dura and underlying brain are not disturbed. This operation is performed by a minority (but a growing number) of surgeons. Most believe that expansion of the dura is also needed.
  1. Bony decompression with duraplasty: This technique is probably the most common method of Chiari I decompression. Bone removal is performed as above, after which the dura is opened. Some surgeons take great care not to disturb the underlying arachnoid membrane, while others open this layer as well. Leaving the arachnoid intact may prevent the CSF from leaking out, thus presumably decreasing the incidence of aseptic meningitis and pseudomeningocele. A graft is then sewn to the dura, effectively expanding the posterior fossa. Materials used for grafting include autologous tissue (fascia from patient’s own tissues), bovine pericardium, and a variety of synthetic materials. The best material for duraplasty is not known.
  1. Tonsil resection or shrinkage: In addition to opening the bone and dura, some surgeons prefer to address the tonsillar descent directly. This can be done by either applying bipolar electrocautery to the tonsillar tissue, causing it to shrink, or by resecting the tonsils. In either case, the goal is to ensure that there is no obstruction of CSF flow out of the 4th ventricle.
  1. Other options: Several other surgical strategies are employed to assist in a Chiari I decompression. Some centers use ultrasound to visualize the movement of the tonsils with the cardiac and respiratory cycles, or to visualize the flow of CSF before and after decompression. This can be used as an aid to determine whether dural opening or tonsillar shrinkage is necessary. In addition, various combinations of the above procedures are sometimes used. For example, some surgeons will open the dura, shrink the tonsils, and close the dura without a graft. Others use endoscopy to shrink the tonsils through a small skin incision. 


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. 


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. 


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. 


  1. Behari S, Kalra SK, Kumar K, Salunke P, Jaiswal AK, Jain VK: Chiari I malformation associated with atlanto-axial dislocation: focusing on the anterior cervicomedullary compression. Acta Neurochir 149:41-50, 2007
  2. Fenoy AJ, Menezes AH, Fenoy KA: Craniocervical junction fusions in patients with hindbrain herniation and syringohydromyelia. J Neurosurg Spine 9:1-2, 2008
  3. Greenlee J, Garell C, Stence N, Menezes AH: Comprehensive approach to Chiari malformation in pediatric patients. Neurosurg Focus 6(6): Article 4, 1999
  4. Guo F, Wang M, Long J, Wang H, Sun H, yang B, Song L: Surgical management of Chiari malformation: analysis of 128 cases. Pediatr Neurosurg 43:375-381, 2007
  5. Iskandar B, Oakes W: The Chiari Malformations, in Albright A, Pollack I, Adelson P (eds): Principles and Practice of Pediatric Neurosurgery, ed 1st. New York, NY: Thieme Medical Publishers, 1999
  6. Kim LJ, Rekate HL, Klopfenstein JD, Sonntag VK: Treatment of basilar invagination associated with Chiari I malformations in the pediatric population: cervical reduction and posterior occipitocervical fusion. J Neurosurg 101:189-195, 2004
  7. Milhorat TH, Bolognese PA, Nishikawa M, McDonnell NA, Francomano CA: Syndrome of occipitoatlantoaxial hypermobility, cranial settling, and Ciari malformation type I in patients with hereditary disorders of connective tissue. J Neurosurg Spine 7:601-609, 2007.
  8. Smith JS, Sharrfey CI, Abel, MF, Menezes AH: Basilar invagination. Neurosurgery 66:A39-A47, 2010
  9. Bollo RJ1, Riva-Cambrin J, Brockmeyer MM, Brockmeyer DL. J Neurosurg Pediatr. 2012 Aug;10(2):134-41. doi: 10.3171/2012.3.PEDS11340. Epub 2012 Jun 22.Complex Chiari malformations in children: an analysis of preoperative risk factors for occipitocervical fusion. Format: Abstract
  10. Klekamp, J. Neurosurg Focus. 2015 Apr;38(4):E12. doi: 10.3171/2015.1.FOCUS14783.Chiari I malformation with and without basilar invagination: a comparative study.
  11. Henderson FC Sr, Henderson FC Jr, Wilson WA, Mark AS, Koby M. Utility of the clivo-axial angle in assessing brainstem deformity: pilot study and literature review. See comment in PubMed Commons below Neurosurg Rev. 2018 Jan;41(1):149-163. doi: 10.1007/s10143-017-0830-3. Epub 2017 Mar 3.



Revised 02/2018