Arnold-Chiari Malformation

What happens in a Chiari malformation

The cerebellum sits at the back of the skull, above the foramen magnum, the large opening where the brainstem joins the spinal cord. In a Chiari malformation, the space at the back of the skull is too small or the brain is pushed down, so the lower parts of the cerebellum (the tonsils) sit below this opening. This can:

• press on the brainstem and upper spinal cord
• block the normal pulsing flow of cerebrospinal fluid (CSF) around the brain and spine
• lead to fluid cysts in the spinal cord (syringomyelia) or fluid buildup in the brain (hydrocephalus)

Types of Chiari malformation

Type I is covered in more detail on our Chiari malformation page, and type II on the Chiari II malformation page.

Symptoms

Type I

Many people have no symptoms. When present:

• headache at the back of the head and upper neck, typically short and triggered by coughing, sneezing, laughing, straining or bending. This "Valsalva headache" is the hallmark symptom
• neck pain
• dizziness, unsteadiness and poor coordination
• tingling, numbness or weakness in the hands and arms, or loss of pain and temperature sensation in a cape-like pattern (from syringomyelia)
• difficulty swallowing, hoarseness or choking
• ringing in the ears, hearing changes or blurred vision
• sleep apnea
• scoliosis in children

Type II in babies

• breathing problems, pauses in breathing or noisy breathing (stridor)
• weak cry, feeding and swallowing difficulty, choking
• arm weakness
• signs of hydrocephalus if a shunt fails

Causes

Most type I malformations are thought to arise during development, when the bony space at the back of the skull forms too small. Some run in families or occur with connective tissue disorders such as Ehlers-Danlos syndrome, or with basilar invagination. Acquired cases can result from conditions that lower pressure in the spinal canal, such as repeated lumbar punctures or a spinal fluid leak. Type II results from the open spinal defect of myelomeningocele, which lets CSF escape during fetal development and pulls the brain downward.

Diagnosis

• MRI of the brain and cervical spine: shows how far the tonsils extend and whether there is crowding. Imaging of the whole spine looks for syringomyelia and tethered cord.
• Cine MRI: measures CSF flow at the skull base.
• Neurological examination: reflexes, strength, sensation, coordination and swallowing.
• Sleep study if sleep apnea is suspected.
• In type II, diagnosis is often made on prenatal ultrasound or fetal MRI along with spina bifida.

Many type I malformations are found incidentally on scans done for other reasons. Tonsillar position alone does not determine whether symptoms or treatment are needed.

Treatment

Watchful waiting

People with no symptoms, or mild symptoms not clearly due to the Chiari, are usually monitored with periodic check-ups and sometimes repeat MRI. Headache treatment and avoiding heavy straining can help.

Posterior fossa decompression surgery

For typical symptoms, a syrinx, or progressive neurological problems, a neurosurgeon removes a small piece of bone at the back of the skull and sometimes part of the first vertebra, often opening the outer membrane (dura) and patching it to make more room. This relieves pressure and restores CSF flow. Cough headaches improve in most patients, and syrinxes often shrink. Recovery takes several weeks.

Type II

Treatment focuses on spina bifida closure (sometimes before birth), managing hydrocephalus with a shunt or endoscopic surgery, and decompression for brainstem symptoms. Shunt function must be checked first when symptoms worsen.

Outlook

Most people with type I lead normal lives, and many never need surgery. After decompression, symptoms such as Valsalva headaches usually improve, while long-standing numbness or weakness may only partly recover. Children with type II need lifelong multidisciplinary care for spina bifida and its complications.

Frequently asked questions

What is an Arnold-Chiari malformation?

It is a condition where the lower part of the brain extends through the opening at the base of the skull into the spinal canal. This can press on the brainstem and block spinal fluid flow. The term originally described type II, associated with spina bifida, but is often used for all Chiari types.

What does a Chiari headache feel like?

A Chiari headache is usually a sharp or pressure-like pain at the back of the head and upper neck, triggered by coughing, sneezing, laughing or straining. It typically lasts seconds to minutes. Many other headaches are not caused by an incidental Chiari found on a scan.

Is Chiari malformation serious?

It ranges from harmless to serious. Many type I cases cause no problems. Others cause headaches, nerve symptoms or syringomyelia that need surgery. Type II, which occurs with spina bifida, can affect breathing and swallowing in infancy and needs specialist care.

Can Chiari malformation get worse?

Most incidental type I malformations stay stable for years. Symptoms can worsen if a syrinx develops or enlarges, or with conditions that alter spinal fluid pressure. Regular follow-up is advised, and new neurological symptoms should be reported promptly.

Is Chiari malformation hereditary?

Most cases are not inherited, but some families have several affected members, and it is more common with certain genetic connective tissue disorders. Relatives without symptoms do not usually need screening unless advised by a specialist.