NOTE! This site uses cookies and similar technologies.

If you not change browser settings, you agree to it. Learn more

I understand

In order to make our website more comfortable and intuitive, we use cookies: they are small files of information needed to understand how users navigate in our website and make your browsing experience more enjoyable and more efficient in the future. Cookies do not store any personal information, and will not be stored any identifiable data. If you want to disable the use of cookies you have to customize the settings of your internet browser by removing all existing cookies and disabling their storage. To proceed without modifying the application of the cookies just continue the surfing.

Please visit AboutCookies.org for more information about cookies and how they affect your browsing experience.

Types of cookies used:

Technical cookie 
These cookies are essential for the website navigation; without some of these, technical issues could not work.

Performance cookie
These cookies collect informations about how visitors use the website: for example, which pages are most popular, and which pages have reported warnings or error messages. These cookies do not collect any personal information about the visitor, and they are used only to improve the website operation. By using our website, you agree that these cookies may be installed on your device.

Functionality Cookie
Cookies allow the website to remember the choices made by the user (for example, to remember the language choice) and provide custom functionality. These cookies can also be used to remember changes to the text size and other features of web pages that you can customize. They can also be used to provide services such as watching a video or sharing on social networks. The information gathered from these types of cookies can be anonymous and can't track your browsing activity on other websites. By using our website, you agree that these cookies may be installed on your device.

Google Analytics
This website uses Google Analytics, a web analytics service provided by Google Inc.
The information generated by the cookie about your use of the website (including your anonymous IP address) will be transmitted and stored in Google's servers in the United States. Google will use this information with the purpose of evaluating your use of the website, compiling reports on website activity for the operators and providing other services relating to website activity and internet usage. Google may also transfer this information to third parties, unless required by law, or where such third parties process the information on Google's behalf. Google will not associate your IP address with any other data held by Google. By using this website, you allow Google to process the data about you in the manner and purposes set out above.

Facebook 
This website uses plugins from the social network facebook.com, which is operated by Facebook Inc., 1601 S. California Ave, Palo Alto, CA 94304, USA (subsequently called "Facebook"). When opening a website that contains such a plugin, your browser will establish a direct connection to the Facebook servers. Facebook will transfer the content of the plugin directly to your browser, the latter of which will embed it in the website. This website hence does not have any influence on the amount of data that Facebook collects through this plugin and informs you according to its best knowledge. Through embedding the plugins Facebook receives the information that you have opened the respective website. If you are logged in to Facebook, Facebook can link this information to your Facebook account. If you interact with the plugins, for example by clicking the Like-button or commenting, your browser will submit this information directly to Facebook, which will save it. If you are not a member of Facebook, Facebook nonetheless might identify and save your IP address. Purpose and scope of the data collection as well as its distribution and usage of the data by Facebook as well as respective rights and preferences regarding privacy can be found in Facebook's privacy policy http://www.facebook.com/policy.php. If you are a member of Facebook and do not want Facebook to collect data through this website and connect it to your Facebook profile, you have to log out from Facebook prior to visiting this site.

 

Definition

CMI is part of a class of malformation diseases involving the base of the skull and the rhombencephalon. The most used definition is a descent of the cerebellar tonsils into the cervical canal due to a volumetric disproportion between the posterior skull base and the neural content.

Descent of the cerebellar tonsils as diagnosis

Classically, a diagnosis of CMI is proposed if tonsils are at least 5 mm below the foramen magnum. However, this conception has been variously questioned following the evidence of clinical CMI symptoms with only mild tonsils downward dislocation.

The real incidence is not clear but it is believed to affect approximately 1 out of 1500 patients. In almost 1/3 of patients CMI is associated to syringomyelia (see below).  

MacRae’s Line

There are different craniometric lines used to measure the descent of cerebellar tonsils. The most used is the MacRae’s line (joins the basion to the posterior margin of the foramen magnum on a sagittal midline MRI or CT scan).  

 

 

MacRae’s Line

Other types of CMs

CM type II is characterized by small posterior cranial fossa with downward dislocation of cerebellum, tonsils and brainstem. It is typically associated to mielomenigocele. Other associated malformations are: hydrocephalus, anegesia of the corpus callosum, scoliosis, syringomyelia. 

CM type III: occipital encephalocele

CM type IV: agenesia of the cerebellum.

 

Pathophysiology

There have been proposed many theories to explain CMI, both congenital and acquired. 

The most accredited is that of underdeveloped posterior fossa which eventually pushes the tonsils outside the foramen magnum. This is seen not only when the posterior fossa is “small” but also in case of craniostenosis (where the whole skull’s volume is reduced) or in other conditions which thicken the bone of the skull (osteopetrosis, Paget disease). 

Other mechanism

- Increased intracranial pressure (in case of intracranial masses, hydrocephalus, haematoma) can force the tonsils from above. 

  • Intracranial hypotension provoke a pressure gradient between the intracranial and intraspinal compartment; consequently, tonsils tend to slide down. 
  • Rarely, a tethered cord pulls the brainstem and tonsils from below. 

- Many other malformation of the cranio-cervical junction are frequently associated to CMI (basilar invagination, platybasia, atlas assimilation, achondroplasia). 

- Genetic disorders of the collagen such as Ehlers-Danlos can be also associated to CMI. 

The majority of symptoms associated to CMI relate to the block of cerebrospinal fluid (CSF) flow at the level of the foramen magnum. This affects the intracranial compliance, the ability of the body to manage a volume increase in the intrathecal space without causing a pressure increase and explains many of the symptoms related to CMI. 

Association between CMI and syringomyelia

Syringomyelia is defined as a cavitation into the spinal cord which leads to debilitating myelopathy. The pathophysiology connecting CMI and syringomyelia resides in the obstruction of the cerebrospinal fluid flow at the level of the foramen magnum caused by the low-lying tonsils generating a pulsatile wave onto the spinal cord which favors the penetration of CSF into the cord through the perivascular spaces of Virchow. In this light, operating on CMI can lead to resolve syringomyelia in many patients (see below). 

 

Symptoms and signs

The most important complaint from almost all the CMI patients is a headache. This is sited in the occipito-nuchal region and is usually triggered by a cough, straining, running and prolonged or extreme neck flexion. 

Differential diagnosis: cough headache (typically this tends to last few seconds and is not associated to other neurological impairment).  

Frequent associated complaints

- Visual disturbances (diplopia, scotomas, retro-ocular pain, photophobia, visual filed alterations) 

- Oto-neurological symptoms (vertigo, tinnitus, sensation of pressure into the ears, hearing dysfunction) 

  • Numbness or tingling in hands or arms can be frequently referred
  • Fatigue, concentration and sleeping problems. 

In more severe forms of brainstem compression, patients can show swallowing difficulties, short breath, nystagmus,  the absence of gag reflex, facial hypoestesia, hemiatrophy of the tongue. 

Relationship with syringomyelia

When associated to syringomyelia, symptoms and signs can vary based on the extension, position and severity of the syringomyelia. In fact syringomyelia can either occupy the exact center of the spinal cord or conversely be eccentrically located. In the former case, syringomyelia spares the dorsal column/medial lemniscus, explaining why proprioception, vibration ad pressure sensation are intact whereas pain and temperature sensation is lost. 

The most typical complaint is dysesthesic pain to limbs or trunk; upper extremity weakness and atrophy and long-tract signs.

 

Radiological diagnosis

MRI is the gold standard to diagnose CMI. Typically, tonsils protrude through foramen magnum and appear triangular. Other signs, such as the absence of the cisterna magna and the disappearance of the CSF spaces around the brainstem, can also be useful.

Radiological diagnosis

It is mandatory to scan the whole cranio-spinal compartment in order to detect syringomyelia and other associated pathologies (intracranial hypertension, hydrocephalus, occipito-cervical malformations, tethered cord, scoliosis). It is worth noting that low-lying tonsils is just a radiological diagnosis and do not define the CMI clinical syndrome. The number of patients diagnosed with low-lying tonsils has increased thanks to the wide availability of MRI scanners. Other modalities such as phase-contrast MRI can support the demonstration of a altered flow of CSF at the level of the foramen magnum. CT scan and flexion-extension radiograms can help in defining skull-base deformities and instability. 

Physical examination and patient’s history are crucial to formulate diagnosis and eventually propose treatment. Moreover, it has been showed that the degree of tonsillar descent does not correlate to severity of symptoms nor to response to treatments.

 

   

 

Management strategies

Asymptomatic patients do not need surgical treatment. Mild complaints could be treated conservatively with pain killers and physical therapy. 

In case of scarce response to medical treatment or when severe neurological impairment appears, surgery should be proposed.

Surgical technique

The most used technique is the posterior fossa decompression with duroplasty (removal on the dura and enlargement with an autologous or heterologous substitute). Coagulation of tonsils, plugging of the obex, arachnoid dissection and other more invasive technique have been progressively abandoned. 

The necessity to create a duroplasty is still under debate. What has emerged from many surgical series is that opening the dura yields higher risks of CSF leaks, infection and pseudomeningocele. On the other hand, leaving the dura intact is reported to increase the risk of symptoms recurrence due to formation of scars and fibrosis. 

CMI and syringomyelia

In case of CMI and syringomyelia, posterior fossa decompression is still the procedure of choice with the goal of eliminating the mechanism of formation of the syringomyelia. There is no consensus about the indication to surgery in asymptomatic patients with CMI and syringomyelia. Many authors advise surgery in these patients to prevent spinal cord damage.  

Syrinx expansion

In case of syrinx expansion despite good posterior fossa decompression, it can be considered a direct approach to syrinx through the positioning of a shunt.  

 

References

Xu H, Chu L, He R et al (2016) Posterior fossa decompression with and without duraplasty for the treatment of Chiari malformation type I-a systematic review and meta-analysis. Neurosurg Rev Jun 1. [Epub ahead of print]

Spena G, Bernucci C, Garbossa D et al (2010): Clinical and radiological outcome of craniocervical osteo-dural decompression for Chiari I-associated syringomyelia. Neuros Rev 33(3):297-303

Oldfield EH (2001) Syringomyelia. J Neurosurg 95(Suppl 1):153–155

Noudel R, Jovenin N, Eap C et al (2009) Incidence of basioccipital hypoplasia in Chiari malformation type I: comparative morphometric study of the posterior cranial fossa. Clinical article. J Neurosurg Nov;111(5):1046-1052

Nishikawa M, Sakamoto H, Hakuba A et al (1997) Pathogenesis of Chiari malformation: a morphometric study of the posterior cranial fossa. J Neurosurg 86(1):40-47

Krishna V, McLawhorn M, Kosnik-Infinger L et al (2014) High long-term symptomatic recurrence rates after Chiari-1 decompression without dural opening: A single center experience. Clin Neurol Neurosurg 118: 53– 58

Förander P, Sjåvik K, Solheim O et al (2014) The case for duraplasty in adults undergoing posterior fossa decompression for Chiari I malformation: A systematic review and meta-analysis of observational studies. Clin Neurol Neurosurg 125: 58–64

Fenoy AJ, Menezes AH, Fenoy KA (2008) Craniocervical junction fusions in patients with hindbrain herniation and syringohydromyelia. J Neurosurg Spine 9(1):1-9

Behari S, Kalra SK, Kiran Kumar MV et al (2007) Chiari I malformation associated with atlanto-axial dislocation: focussing on the anterior cervico-medullary compression. Acta Neurochir (Wien)149(1):41-50

Bejjani GK (2001) Definition of the adult Chiari malformation: a brief historical overview. Neurosurg Focus 11(1):E1

Alperin N, Loftus JR, Oliu CJ et al (2014) Magnetic resonance imaging measures of posterior cranial fossa morphology and cerebrospinal fluid physiology in Chiari malformation type I. Neurosurg 75(5):515-52

 

Authors

 

Giannantonio Spena, MD

Scientific Team - UpSurgeOn
Neurosurgeon Consultant
University of Brescia
"Spedali Civili" Hospital Brescia (Italy)

Federico Nicolosi, MD

Scientific Team - UpSurgeOn
Neurosurgery Resident
University of Milan
"Spedali Civili" Hospital Brescia (Italy)