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Chiari malformations are a group of conditions characterised by abnormal displacement of the hindbrain (cerebellum, medulla oblongata, pons) into the spinal canal or through a defect in the skull.
Chiari malformations are estimated to occur in 1 in 1000 births and with a slight female predominance (3 female: 1 male).
There are four types of Chiari malformations.
Type 1 is the most common and mildest form of Chiari malformation.
People with type 1 malformations tend to present during late childhood or adulthood.
It is characterised by a downward displacement of the cerebellar tonsils, of more than 5mm, through the foramen magnum.
They can occur due to either congenital or acquired structural defects in the skull base.
This means that there is reduced space for the cerebellum in the posterior fossa, which causes the cerebellar tonsils to protrude through the foramen magnum.
As a result, nearby neurological structures can become compressed, which then causes the neurological signs and symptoms described later in this article.
Type 2 Chiari malformation (also known as Arnold-Chiari malformation) involves a greater range of brain tissue and an even shallower posterior fossa than is seen in type 1.
There is downward displacement of the cerebellar tonsils, as well as the 4th ventricle, brainstem and inferior vermis. Because there is more brain matter involved with this type of malformation, the clinical presentation tends to be more severe than type 1.
People with this type of malformation present earlier, typically during early childhood or even infancy.
Additionally, type 2 Chiari malformation is often associated with myelomeningocele, a birth defect where the vertebrae and spinal canal do not close properly before birth.
Type 3 Chiari malformation is much rarer than type 1 and 2.
In type 3, there is herniation of the posterior fossa’s contents through a posterior encephalocele. This is a rare congenital type of neural tube defect in which a portion of the skull has not formed properly, which enables parts of the brain and meninges to protrude through it.
It is usually associated with a poor prognosis.
Type 4 Chiari malformation is incredibly rare and, sadly, is non-compatible with life.
It occurs when there is an incomplete or undeveloped cerebellum.
This section will focus on the clinical presentation of type 1 and type 2, as they are the most common presentations.
Both types present with a similar set of symptoms but have a different symptom severity (people with type 2 malformations present with more severe symptoms).
Signs and symptoms common to both types include:
Sub-occipital headaches (often worsened by Valsalva manoeuvres such as coughing, straining or sneezing)
Lhermitte’s sign (an electric shock-type sensation that, upon neck flexion, radiates down the spine and sometimes into limbs)
Dizziness and balance problems
Weakness of arms and legs
Numbness/tingling in the arms and/or legs
Loss of temperature sensation
People with Chiari malformations can also develop other neurological conditions as a result of their structural abnormality.
The main conditions associated with this form of malformation are briefly described below.
This is an excessive build-up of cerebrospinal fluid (CSF) in the brain.
A Chiari malformation can block the normal flow of CSF fluid and cause obstructive hydrocephalus.
A form of neural tube defect where there is incomplete closure of the backbone and the membranes around the spinal cord.
Patients with type 2 Chiari malformations are likely to have a myelomeningocele defect in their back.
This defect can often cause impairments such as muscle weakness, paralysis and bladder/bowel dysfunction.
A disorder where a CSF-filled tubular cyst, or syrinx, forms within the spinal cord’s central canal.
The growing syrinx destroys the centre of the spinal cord, resulting in pain, weakness and sensory loss.
Ultrasound scans can be useful for prenatal screening.
However, the main investigation used for this condition is MRI.
Management is only offered if a patient is symptomatic, as surgical treatment can help ease the severity of symptoms and halt progression.
For symptomatic type 2 Chiari malformations, surgery is often offered during early childhood as it can help significantly reduce childhood mortality and morbidity rates.
Surgical treatments are mainly aimed at decompressing the neurological brainstem structures, in addition to relieving the pressure within the skull and spinal canal.
The main surgical treatments offered include:
Posterior decompression of the foramen magnum
Ventriculoperitoneal shunt (indicated for hydrocephalus)
Syringo shunt (sometimes indicated for syringomyelia – e.g. syringo-subarachnoid shunt)
Hull York Medical Student
Mr Konstantinos Lilimpakis
Neurosurgical Clinical Fellow
Chiari Malformation Fact Sheet | National Institute of Neurological Disorders and Stroke [Internet]. nih.gov. 2018 [cited 7 July 2018]. Available from: https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Fact-Sheets/Chiari-Malformation-Fact-Sheet
Jones J. Chiari malformations | Radiology Reference Article | Radiopaedia.org [Internet]. Radiopaedia.org. 2018 [cited 7 July 2018]. Available here: [LINK]
Khare S, Seth D. Annal of Indian Academy of Neurology. 2015. April; 18(2): 154-156. DOI: 10.4103/0972-2327.150622 Available here: [LINK]. Accessed 29/04/2019.
Neurosurgery Department at Great Ormond Street Hospital for Children. Encephalocele. Available here: [LINK]. Accessed 29/04/2019.
Chiari Malformation 1. Case courtesy of A.Prof Frank Gaillard, Radiopaedia.org. From the case rID: 2592. Accessed 29/04/2019.
Chiari Malformation 2. Case courtesy of A.Prof Frank Gaillard, Radiopaedia.org. From the case rID: 16048. Accessed 29/04/2019.
Chiari Malformation 1 with Syrinx. Case courtesy of A.Prof Frank Gaillard, Radiopaedia.org. From the case rID: 15819. Accessed 29/04/2019.