A Chiari malformation (sometimes called an Arnold Chiari) means that the lower parts of your brain have been pushed downwards towards your spinal cord, so they are below the entrance to your skull.
Most people will have a type 1 Chiari malformation, which is the least serious form of the disease. This is where the lowest part of the back of the brain (the cerebellar tonsils) drops down into the top of the spinal canal.
Types 2 and 3 Chiari malformations are less common and more serious. They are associated with spina bifida (a birth defect involving problems with spinal development), and adults will also have hydrocephalus (a build-up of fluid in the brain).
The rest of this page focuses on type 1 Chiari malformations – the most common type.
How a type 1 Chiari happens
Usually, the lower parts of the brain are contained in a space within the skull, above the level of the foramen magnum (opening at the base of the skull). If you have a type 1 Chiari malformation, these brain parts are pushed downwards, because they are too big for the skull.
When parts of the brain are pushed out of the skull towards the spinal cord, this can:
cause pressure at the base of the brain
block the flow of cerebrospinal fluid (CSF) to and from the brain
CSF is a clear fluid that surrounds and protects the brain and spine, and also carries nutrients to the brain and removes waste.
Is it hereditary?
Chiari malformation can sometimes run in families, and it's possible that some children born with this disorder may have inherited a faulty gene that caused problems with their skull development in the womb. Researchers are currently trying to find the gene or genes that are responsible.
However, try not to worry – the risk of passing a Chiari malformation on to your child is very small. Even if children do inherit it, most will not develop symptoms.
What are the symptoms?
Many people with a type 1 Chiari malformation will not have any symptoms.
When symptoms do develop, they may include:
headaches, usually at the back of the head – these are brought on, or made worse by, exercise, straining, laughing or bending over
dizziness and balance problems
numbness or tingling in the arms or legs
blurred vision and involuntary movement of the eyes (nystagmus)
swallowing problems (dysphagia)
hearing loss and tinnitus
feeling sick and vomiting
Syrinxes and syringomyelia
Many people with a type 1 Chiari malformation will develop a condition called syringomyelia, where a tube-shaped cavity forms within their spinal cord and fills with fluid.
This fluid-filled cavity, known as a syrinx, can expand and elongate over time, pressing on, and damaging, the spinal cord. It can cause pain, numbness, weakness, stiffness and problems with bladder or bowel functions, and sometimes leads to paralysis.
Surgical treatments involve treating the underlying cause – in other words, operating on the Chairi malformation or treating hydrocephalus – or releasing the fluid using a system of drainage tubes and valves (see the "treatment" section below).
Reducing the fluid in a syrinx can stop symptoms getting worse and can sometimes improve them.
How common is it?
In the past, it was estimated that Chiari malformations occurred in about 1 in 1,000 births. However, the increased use of diagnostic imaging has shown that they may be much more common.
How is it diagnosed?
Sometimes, Chiari malformations are discovered by chance after a magnetic resonance imaging brain scan (MRI scan) is done to investigate a different condition.
If you see your GP with symptoms suggesting you may have a Chiari malformation, they should arrange for you to have an MRI scan to confirm this diagnosis. A brain scan will show up any abnormalities in the structure of your brain and spine.
How is it treated?
The symptoms caused by a type 1 Chiari malformation can usually be managed with medication. For example, painkillers can help relieve any headaches and neck pain.
You may need decompression surgery to reduce the pressure at the base of your brain. The surgeon will make an incision at the back of your head and remove a small piece of bone from the base of your skull. This will widen the space in the foramen magnum (opening to the skull).
They may also remove a small piece of bone from the top of your spine, to relieve the pressure on your brain and allow CSF to flow normally. This procedure is known as a spinal laminectomy.
Other treatment options include:
Endoscopic third ventriculostomy (ETV) – a small hole is made in the wall of one of the cavities of the brain, releasing the trapped fluid. For more information, read the Hydrocephalus Association's factsheet about ETV (PDF, 167kb).
Ventriculoperitoneal shunting – a small hole is drilled into the skull and a thin tube called a catheter is passed into the brain cavity to drain the trapped fluid and relieve the pressure. Read a factsheet on the MedlinePlus website about ventriculoperitoneal shunting.
Untethering – some children with a type 1 Chiari malformation have a tethered spinal cord, which means it has become abnormally attached within the bony spine. Untethering involves separating the spinal cord and releasing tension in the spine.
Most people who have surgery find that their symptoms improve afterwards. It should also prevent existing symptoms from getting worse.
However, as with all types of surgery, there is a small risk of complications, which you should discuss with your neurosurgeon.
Possible risks of decompression surgery include:
a stroke or haemorrhage (bleeding)
paralysis of the arms and legs
meningitis or other infections
memory loss or problems with thinking
seizures (although these are rare)
a risk to life
Sometimes, surgery leads to no improvement or symptoms getting worse.
Children in hospital
Get advice on how to prepare for your child's hospital stay and learn your rights about giving consent to treatment