Chiari Malformation: Wading Through the Confusion

Myth 5: Chiari I Is a Feature of SIH
Myth 5: Chiari I Is a Feature of SIH
Meta-analyses are only as good as the quality of the primary data, which is mostly low to medium evidence in the case of Chiari malformation.

Scientific interest in Chiari malformation (CM), also known as Arnold-Chiari malformation, is high. Since the beginning of 2016 to the present, PubMed has indexed 309 published papers, or an average of 2.25 per week. However, as CM is a rare disease, most of the published studies are retrospective and based on small numbers. Meta-analyses are only as good as the quality of the primary data, which is mostly low to medium evidence in the case of CM. This is not doing much to address the widespread confusion around CM and its treatment.

Defining CM

The current definition of CM is excessively wide. In 1891, Hans Chiari described 3 forms of malformations with herniation of the hindbrain outside the cranium:

  • Type I: the cerebellar tonsils herniate through the foramen magnum into the spinal canal for 5 mm or more.
  • Type II: the hernia involves more of the brainstem and cerebellum, including the fourth ventricle, always in combination with a myelomeningocele.
  • Type III: consists of an encephalocele containing cerebellum.

Later, a type IV was added to the equation, consisting of a hypoplasia, or even aplasia, of the cerebellum, without any herniation.1

Some researchers have also proposed levels 0 and 1.5, but these forms are still not recognized by many. More recent authors speak of CM even when the herniation of the hindbrain is not a congenital anomaly, but the result of space-occupying intracranial processes such as hydrocephalus or tumors.

Only type I of malformation (CM-I) will be discussed in this article, as this is the most common form of CM and consists of the herniation of the cerebellar tonsils into the spinal canal, often associated with a smaller posterior fossa. Its symptomatology is varied because of the numerous nerve nuclei located in the brainstem: the vagal nucleus in the medulla (the vagus nerve supplies also the larynx with innervation), the eye motility nerves, the trigeminal sensitive and motor nuclei, the facial nerve nucleus, and the nerves of the tongue. The cerebellum plays a role not only in movement coordination and balance but also in cognition.

CM Symptoms

Neurological signs may be recognized at birth, later in childhood, or in adulthood. Most people with CM-I are asymptomatic2 and remain like that for a long time. In others, the compression of parts of the brainstem and cerebellum generates neck pain, dizziness, tinnitus, troubles with speech or swallowing, sleep apnea, nausea and vomiting, and incoordination of hand movements including fine motor functions.

Cephalalgia, the most frequent manifestation of CM-I, is a consequence of the impaired circulation of the cerebrospinal fluid (CSF) through the foramen magnum. It is severely exacerbated when coughing. The abnormal flow of the CSF within the medullary canal leads to its dilatation (hydromyelia) or to para-axial fluid-filled cavities in the spinal cord (syringomyelia). These may impair the function of the spinal cord, resulting in muscular weakness or sensorial deficits.3 Such neurological manifestations should suggest performing a magnetic resonance imaging scan of the brain and of the whole spine to detect a possible syringomyelia.

Numerous studies also point to a cognitive deficit in people with CM. A recent review found that there is relatively stronger evidence for an executive cognition deficit than for other alterations of cognition.4 However, the authors recommend further prospective controlled studies to reach a definitive conclusion.

Treating CM

The treatment of symptomatic CM-I is surgical, with the aim being to reduce compression and restore the normal flow of the CSF. The number of surgeries for CM-I is increasing, although most specialists in the field do not recommend prophylactic surgery in asymptomatic people with CM-I.5 Oliver Bozinov, MD, vice chairman of the Department of Neurosurgery at the University Hospital in Zurich, Switzerland, believes that many operations are performed too early, and possibly by less experienced specialists, which could lead to a higher rate of complications.

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The evidence suggests that asymptomatic patients should be reassured that the risk of developing neurological signs is low and that a regular follow-up with magnetic resonance imaging is the best course of action. A recent meta-analysis examining the natural history of CM-I found that 93.3% of asymptomatic subjects, including some with syringomyelia, remained the same at the end of a follow-up of at least 10 years.2 Patients with mild symptoms may choose to be treated for headaches (including cough headache) or nausea and be followed up, as clinical improvement is often seen. Such a conservative approach is justified by the benefit of avoiding the potential complications of surgery.2

Surgery will be necessary in patients with severe manifestations: cranial nerve dysfunction, cerebellar symptoms, myelopathy, syringomyelia, or severe cough headache. Most neurosurgeons perform posterior fossa decompression by suboccipital craniectomy and laminectomy of the first cervical vertebra. To further reduce compression, often the dura is incised and a patch (of autologous tissue or synthetic) is used to widen the subdural space.

A recently published meta-analysis compared the 2 techniques: Although leaving the dura intact was associated with a shorter operative time, the postoperative improvement rate was 15% lower. Performing a duraplasty prolonged the surgery time and was significantly more often followed by CSF leakage and pseudomeningocele, but the clinical result was better. This approach offered a better chance of syringomyelia cure.6 Should the symptomatic syringomyelia persist, it may be drained by shunt to the subdural or to the pleural or peritoneal spaces. Other complications after surgery are less frequent but potentially severe: bacterial or aseptic meningitis, epidural hematoma, postoperative apnea, and other brain stem dysfunctions.7

As a surgical alternative in children, a mere incision of the dura without patching can give good results.8 Also, a minimally invasive subpial cerebellar tonsillectomy with reconstruction of the cisterna magna has been presented as a valid alternative to posterior fossa decompression.9 Dr Bozinov cautions against the risk for arachnoiditis and adhesions after tonsillectomy, which would worsen the postoperative prognosis. He also mentions that, despite proper posterior fossa decompression, in a few patients, the disease may still progress to hydromyelia.

In general, the prognosis after surgery is good, but it depends on the severity of the disease before operation. Some patients with long-standing neurological deficits and muscle atrophy may not improve. Ultimately, CM-I remains a challenge for neurosurgeons, and the approach should be carefully adapted to the individual characteristics of every patient.


  1. National Institute of Neurological Disorders and Stroke. Chiari Malformation Fact Sheet. Updated July 6, 2018. Accessed August 20, 2018.
  2. Langridge B, Phillips E, Choi D. Chiari malformation type 1: a systematic review of natural history and conservative management. World Neurosurg. 2017;104:213–219.
  3. Gad KA, Yousem DM. Syringohydromyelia in patients with Chiari I malformation: a retrospective analysis. AJNR Am J Neuroradiol. 2017;38:1833-1838.
  4. Rogers JM, Savage G, Stoodley MA. A systematic review of cognition in Chiari I malformation. Neuropsychol Rev. 2018;28:176-187.
  5. Wilkinson DA, Johnson K, Garton HJL, Muraszko KM, Maher CO. Trends in surgical treatment of Chiari malformation Type I in the United States. J Neurosurg Pediatr. 2017;19:208-216.
  6. Xu H, Chu L, He R, Ge C, Lei T. Posterior fossa decompression with and without duraplasty for the treatment of Chiari malformation type I-a systematic review and meta-analysis. Neurosurg Rev. 2017;40:213-221.
  7. Bhimani AD, Esfahani DR, Denyer S, et al. Adult Chiari I malformations: an analysis of surgical risk factors and complications using an international database. World Neurosurg. 2018;115:e490-e500.
  8. Kurzbuch AR, Jayamohan J, Magdum S. Surgical treatment of paediatric Chiari I malformation: foramen magnum decompression with dura left open, in 70 patients. Presented at: Syringomyelia-Chiari 2018 International Symposium; July 17-20, 2018; Birmingham, United Kingdom.
  9. Lou Y, Li H, Jin Y, Liu L. Minimally invasive sub-pial tonsillectomy (MIST) and reconstruction of the cysterna magna in the treatment of Chiari malformation type 1 with syringomyelia. Presented at: Syringomyelia-Chiari 2018 International Symposium; July 17-20, 2018; Birmingham, United Kingdom.