Chiari I malformation


The Chiari malformations were described in the early 1890s by Dr. Hans Chiari, a pathologist in Prague. Since that time, our understanding has progressed. Here we will discuss the 2 common anomalies: Chiari I and Chiari II. Chiari III and IV are rare.

The Chiari anomalies are not numbered to denote increasing severity but, rather, they represent four distinct and different anomalies of the hindbrain.

In general, Chiari malformations are thought to be congenital, and may be associated with more serious conditions such as brainstem dysfunction or syringomyelia.


Cerebellar tonsillar descent into the foramen magnum with resulting impairment of cerebrospinal fluid flow and/or brainstem composition.


Commonly occurs in children (40% under age 5, 25% age 5-10 and 30% age 10-15).Common presenting symptom is pain (60-70%):6,14,15 usually occipital or upper cervical headache exacerbated by Valsalva (straining, cough, sneezing). Other symptoms vary greatly and may include motor (40-74%) and sensory (50%) changes in the extremities, clumsiness (15%), and dysphagia (10%).6,7,14 Syringomyelia may coexist in 60-70% of Chiari patients and may be associated with progressive scoliosis in 30% of these patients.10,14


MRI is gold standard. Descent of the tonsils through the foramen magnum is measured from a line drawn from the basion to opisthion. Normal tonsillar position has been correlated with age.9 Normal when they extend into the foramen magnum up to 6mm in children 0-10 years old, 5mm in 10-29 years old, 4mm in 30 to 79 years old, and 3mm in those patients above 80 years old. In Chiari patients, the cerebellar tonsils are generally pointed, rather than normally rounded. Supratentorial mass lesions, hydrocephalus, or history of multiple lumbar punctures may all cause tonsillar descent, which is considered an acquired Chiari.4 Other possible findings: Cervical syrinx (50-70% of patients8), evidence of skull base abnormalities such as platybasia, basilar invagination, and ventral compression. Cine-MRI was associated with conflicting results regarding sensitivity and specificity.


Posterior fossa decompression should include a suboccipital craniectomy and C1 laminectomy. Treatment may vary in children compared to adults. Approximately 5-10% of surgeons choose to stop at this point; 45% open the dura and perform a duraplasty, leaving the arachnoid intact; and the other 45% open the arachnoid, lyse arachnoidal adhesions, and often shrink or resect the cerebellar tonsils. Currently, there is no alternative to surgical therapy for Chiari I. Adequate CSF flow from the foramen of Magendie should be visualized at the time of surgery.5 Intracranial pressure monitoring should precede decompression if there is a question of ICP elevation, as pseudotumor cerebri occasionally accompanies Chiari I. Rule out craniocervical instability with dynamic cervical spine films prior to surgery. Basilar invagination or ventral compression should alert the surgeon to the possibility of chronic craniocervical instability that might need a concomitant craniocervical fusion with or without transoral odontoid resection. Three indications for surgical treatment: (1) The presence of neurological deficits, especially if progressive. (2) The presence of syringomyelia. (3) Severe intractable symptoms that are debilitating to the patient. Make sure that the symptoms are typical for Chiari I (e.g., valsalva-induced occipital headaches and others).


Outcome data on Chiari I are quite difficult to interpret. They are also dependent on age of presentation and surgery. In a series of 71 adult patients, 80-90% of patients presenting with any symptoms improved initially but patients with foramen magnum syndromes and patients with central cord syndrome were more likely to relapse later (33% and 25% respectively).15 Syringomyelia should decrease after Chiari decompression. Re-exploration of the posterior fossa is preferred before consideration is given to shunt the syrinx directly to the subarachnoid space, pleura, or peritoneum. Symptoms that most consistently improve with surgery are scoliosis of less than 30 degrees in young adolescents, occipital headache, cervical pain, and sleep apnea.11Patients with predominantly brainstem and spinal cord symptoms are less likely to improve than patients with headaches alone.3


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  2. Bhadelia RA, Bogdan AR, Wolpert SM, Lev S, Appignani BA, Heilman CB. Cerebrospinal fluid flow waveforms: analysis in patients with Chiari I malformation by means of gated phase-contrast MR imaging velocity measurements. 1995;196:195-202.
  3. Dyste GN, Menezes AH, VanGilder JC. Symptomatic Chiari malformations. An analysis of presentation, management, and long-term outcome. J Neurosurg. 1989;71:159-168.
  4. Elster AD, Chen MY. Chiari I malformations: clinical and radiologic reappraisal. 1992;183:347-353.
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  7. Mampalam TJ, Andrews BT, Gelb D, Ferriero D, Pitts LH. Presentation of type I Chiari malformation after head trauma. Neurosurgery 1988;23:760-762
  8. Menezes AH. Chiari I malformations and hydromyelia–complications. Pediatr 1991;17:146-154.
  9. Mikulis DJ, Diaz O, Egglin TK, Sanchez R. Variance of the position of the cerebellar tonsils with age: preliminary report. 1992;183:725-728.
  10. Muhonen MG, Menezes AH, Sawin PD, Weinstein SL. Scoliosis in pediatric Chiari malformations without myelodysplasia. J Neurosurg.1992;77:69-77.
  11. Nagib MG. An approach to symptomatic children (ages 4-14 years) with Chiari type I malformation. Pediatr Neurosurg.1994;21:31-35.
  12. Naidich TP, McLone DG, Fulling KH. The Chiari II malformation: Part IV. The hindbrain deformity. 1983;25:179-197.
  13. Naidich TP, Pudlowski RM, Naidich JB, Gornish M, Rodriguez FJ. Computed tomographic signs of the Chiari II malformation. Part I: Skull and dural partitions. 1980;134:65-71.
  14. Nohria V, Oakes WJ. Chiari I malformation: a review of 43 patients. Pediatr 1990;16:222-227.
  15. Paul KS, Lye RH, Strang FA, Dutton J. Arnold-Chiari malformation. Review of 71 cases.J Neurosurg. 1983;58:183-187.
  16. Samuelsson L, Bergstrom K, Thuomas KA, Hemmingsson A, Wallensten R. MR imaging of syringohydromyelia and Chiari malformations in myelomeningocele patients with scoliosis. AJNR Am J Neuroradiol.1987;8:539-546
  17. Schmitt HP. “Inverse Chiari type II syndrome” in untreated hydrocephalus and its relationship to typical Arnold-Chiari syndrome. Brain Dev. 1981;3:271-275


Reviewed on 9/2019

Department of Medicine, Section of Medical Genetics Duke University Medical Center
Department of Neurological Surgery University of Wisconsin Hospitals and Clinics, Madison
Department of Neurosurgery Children’s of Alabama – University of Alabama
Department of Biological Engineering University of Idaho
Department of Neurological Surgery University of Michigan
Department of Neurosurgery Oregon Health Science Center
Department of Neurological Surgery Formerly: The Cleveland Clinic
College of Engineering The University of Akron
Department of Neurosurgery Metropolitan Neurosurgery Group
Department of Neurological Surgery