Chiari malformations are a group of conditions that occur when the back part of the brain (hindbrain) and the skull do not form in the usual way. This abnormal development can change how the brain sits on the base of the skull, often resulting in part of the brain being pushed into the wrong position (herniation).

There are 4 types of Chiari malformations that are historically grouped together. Chiari malformation type I and II involve parts of the brain herniating into the spinal canal, whilst Chiari malformation type III and IV do not involve herniation into the spinal canal. For this reason, some modern literatures do not consider type III and IV to be true Chiari malformations.

Chiari malformation type I

• characterised by herniation of the cerebellar ‘tonsils’ (two small, rounded bulbs on the bottom of the brain) into the spinal canal.
• the most common type of Chiari malformation.

Chiari malformation type II

• characterised by herniation of both the cerebellar tonsils and part of the brain stem into the spinal canal.
• is associated with myelomeningocele, which is a severe form of spina bifida (birth defect where the spine and spinal canal do not close before birth).

Chiari malformation type III

• characterised by part of the brain passing through an abnormal opening at the back of the head and neck.
• more severe than type I and II, and is often associated with life-threatening complications in infancy.

Chiari malformation type IV

• characterised by underdevelopment of the brain and is often referred to as cerebellar hypoplasia.
• the most severe form, and is often life-limiting during infancy.

Below on this page is some information about this condition as well as links to useful resources for Australians living with a rare disease.

Synonyms: CM, hindbrain herniation.

Type I: Arnold Chiari malformation type 1, Chiari 1 malformation, adult Chiari malformation, Chiari malformation type I, Cerebellar tonsillar ectopia

Type II: Arnold Chiari malformation type 2, Chiari 2 malformation, Chiari malformation type II, paediatric Chiari malformation

Type III: Chiari malformation type 3

Type IV: Chiari malformation type 4, cerebellar hypoplasia, cerebellar aplasia, Chiari 4 malformation, isolated cerebellar agenesis, near total absence of cerebellum, subtotal absence of cerebellum

Universal rare disease classifications provide a common language for recording, reporting and monitoring diseases. Please visit the Rare Disease Classifications page for more information about these internationally recognised classifications.

Type 1 : ORPHA:268882 Arnold-Chiari malformation type I

ICD-11: LA07.4 Arnold-Chiari malformation type I

Type 2: No current Orphacode has been identified

ICD-11: LA03 Arnold-Chiari malformation type II

Type 3: No Orphacode or ICD:11 classification has been identified

Type 4: ORPHA:1398 Isolated cerebellar agenesis

No ICD-11 classification has been identified

Rare diseases typically display a high level of symptom complexity. There is often a wide range of symptoms and the symptoms may vary between individuals in terms of its presentation, severity, duration and impact.

Below are some of the symptoms associated with the different types of Chiari malformations:

Chiari malformation type 1 – usually causes no symptoms, but in some people, it may cause headaches at the back of the head (especially with coughing or straining), neck pain, dizziness or balance problems, numbness or weakness in the arms or legs, ringing in the ears, vision changes and trouble swallowing. These symptoms are referred to as Chiari syndrome.

Chiari malformation type 2 – symptoms typically occur during childhood and are similar to those symptoms listed for type 1. There may also be vomiting, irritability and seizures caused by increased fluid pressure on the brain (hydrocephalus). It is also associated with myelomeningocele, which is a severe form of spina bifida (birth defect where the spine and spinal canal do not close before birth).

Chiari malformation type 3 – symptoms often include seizures and mental and physical developmental delays. There are often life-threatening complications during infancy.

Please speak to your medical team to learn more about the symptoms of this condition.

Rare diseases are often serious and progressive, exhibiting a high degree of symptom complexity, leading to significant disability. Majority of the estimated two million Australians living with a rare disease meet the Australian Government’s definition for disability (in accordance to the Australian Public Service Commission and Australian Bureau of Statistics), and many experience severe and permanent disability impacts. If you or someone you care for is experiencing disability-related impacts from a rare condition, please speak with a health or disability professional for advice. Information about relevant disability support can be found at the RARE Portal’s Disability Support Information page.

People with Chiari malformation are usually born with the condition (congenital).

The cause of Chiari malformation type I is unknown. Some people may acquire Chiari malformation type I later in life through injury to the spine or head, or other conditions that affect cerebrospinal fluid (CSF) production (fluid that surrounds the brain to protect it).

Chiari malformation type II may be caused by low CSF pressure before birth (prenatal). This can occur when CSF leaks out of the neural tube due to its failure to close properly (myelomeningocele). The neural tube is part of an embryo that eventually forms the brain and spinal cord.

Chiari malformation type III and IV are caused when the parts of the brain (including cerebellum and brain stem) not forming properly before birth. The cause of these conditions are unknown.

A timely diagnosis is critical for better patient outcomes, the provision of the best possible care and treatment options, access to services and support, increased reproductive confidence and the ability to participate in clinical trials.

Please speak to your medical team to learn more about the available diagnostic pathways for this condition.

Please speak to your medical team to learn more about the possible treatment or management options for your condition. Treatment will depend on an individual’s specific condition and symptoms. It is also important to stay connected to your medical team so that you can be made aware of any upcoming clinical trial opportunities.

Healthcare professionals involved in the care of individuals with Chiari malformations may include general practitioners (GP), paediatricians, surgeons, pain specialists and neurologists. The need for different healthcare professionals may change over a person’s lifetime and extend beyond those listed here.

Clinical care for rare diseases often involves a multidisciplinary team of medical, care and support professionals. Please note that the information provided here is as a guide and that RVA does not necessarily monitor or endorse specific clinics or health experts.

For many rare diseases, palliative care services may be relevant and useful. Palliative care services are available for people (adults, children and their families) living with a life-limiting illness and is not only for end-of-life care. It can also help at any stage of illness from diagnosis onwards, and will look different for different people. Palliative care services provide assistance, support, resources and tools to help people manage their illness and the symptoms, ease pain, and improve comfort and quality of life. If this is relevant to you and you wish to find out more information about palliative care and how it can help you, please visit:

• Australian Institute of Health and Welfare: Palliative care Overview
• Australian Government Department of Health, Disability and Ageing: Palliative care
• Palliative Care Australia

If you know of any relevant care guidelines, please let us know via the Contribute page.

Individuals living with rare diseases may have complex medical issues and disabilities, which are not always visible. It is often useful to refer to their medical history as well as personal information such as a medical card, doctor’s letter, or if available, a rare disease passport, for relevant information.

If you know of any relevant emergency management guidelines or information relevant to emergency care, please let us know via the Contribute page.

There are specific considerations around participating in rare disease research, including clinical trials. It is important to be mindful of issues such as data privacy, research ethics, consent and differences in research regulations between Australia and other countries. For more information, please visit the RARE Portal’s Considerations for Participating in Health and Medical Research page.

If you are interested in finding clinical trials for your condition, please visit the following websites; however, there may not be any clinical trials available:

• Australian Clinical Trials
• ClinicalTrials.gov

It is best to discuss your interest in research, including clinical trials,  with your medical team to determine suitability and eligibility.

Australian Organisation:
Brain Foundation
Website: https://brainfoundation.org.au/
The Brain Foundation is a national charity that funds Australian research into brain and neurological disorders, including Chiari malformation. It also provides plain-language information and patient resource directories, helping people understand their condition and where to find further support.

International Organisation:
Conquer Chiari (United States of America)
Website: https://conquerchiari.org/

American Syringomyelia & Chiari Alliance Project (ASAP) (United States of America)
Website: https://asap.org/

Please note that RVA does not monitor or endorse each group/organisation’s operational governance and activities. When engaging with a group, please consider the information on the RARE Portal’s Finding Helpful Peer and Community Supports page.

Chiari malformation varies between individuals, and each person’s experience is unique.

If you would like to share your personal story with RVA, please visit the Rare Voices Australia: Share Your Story page. RVA will consider your story for publishing on our website and inclusion on the RARE Portal.

For information on available government and social services that provide support for individuals with a rare disease, please visit the National and State Services pages.

People living with a rare disease often face unique challenges such as diagnostic delays, misdiagnoses, limited treatment options, and limited access to rare disease specialists and support. These challenges may impact people’s emotional wellbeing and quality of life. Many find it helpful to seek mental health and wellbeing support to cope with ongoing stress and uncertainty. Connecting with people who have shared experiences through a support group may also be helpful. Information about relevant mental health and wellbeing support can be found at:

• Mental Health and Wellbeing Support for Australians Living with a Rare Disease
• The National and State Services pages underneath the ‘Mental Health’ sections listed

Further information on Chiari malformation be found at:

• Healthdirect: Chari malformation
• National Organization for Rare Disorders (NORD): Chiari malformations
• National institute for neurological disorders and stroke: Chiari Malformations
• Genetic and Rare Diseases (GARD) Information Center: Chiari type I malformation
• Genetic and Rare Diseases (GARD) Information Center: Chiari type II malformation

Orphanet: Arnold-Chiari malformation type I

Orphanet: Isolated cerebellar agenesis

Human Phenotype Ontology (HPO): Arnold-Chiari malformation type I

Human Phenotype Ontology (HPO): Arnold-Chiari malformation type II

Online Mendelian Inheritance in Man, OMIM^®: #118420 Chiari Malformation Type I

Online Mendelian Inheritance in Man, OMIM^®: #118420 Chiari Malformation Type II

This page has been developed by Rare Voices Australia (RVA)’s RARE Portal team. If you would like to see more information added to this page, please reach out via the Contact form. If you would like to share relevant resources for this condition, please visit the Contribute page.

If you are aware of any additional information that may benefit stakeholders with an interest in this page, or if you notice any broken links or inaccurate information, please let us know via the Contribute page.