Chiari II Malformation (also known as  Arnold-Chiari Malformation, ACM, and hindbrain herniation)

Before describing Chiari II, here are some descriptions of some of the structures involved.

Cerebellum (or hindbrain)- small part of the brain located at the back of the head under the main part of the brain (cerebral hemispheres)

Cerebellar tonsils-bottom part of the cerebellum

Posterior Fossa-bony compartment at the back of the skull, just above the neck containing the cerebellum, brainstem and fourth ventricle.

Cranial Nerves-twelve pairs of nerves which emerge directly from the brain, rather than the spine. They are concerned with sense of smell, taste, vision, hearing and balance, and sensation and movement in the face. They are also responsible for eye movements, facial expression, and tongue movements. The Tenth Cranial Nerve, the Vagus, serves many internal organs, controlling gut movements, and heart rate along with other nerves. The Olfactory Nerve (smell) and Optic Nerves (vision) begin in the main part of the brain, all the others come from the brainstem.

Brainstem- small part of the brain in the posterior fossa, which controls many basic functions via the Cranial Nerves, and the sensory and motor nerve pathways between the brain and body.

Foramen Magnum- large hole in the base of the skull which allows the spinal cord to pass from the skull to the spinal canal.

Chiari II is a developmental condition of the cerebellum, strongly associated with the myelomeningocoele (MMC) form of spina bifida. It is always present from birth, although symptoms may not develop until adulthood, if at all. Nowadays, it is usually detected on the midterm (20 week) antenatal scan, and is a clear indicator that the baby is likely to have open spina bifida. At this time, the posterior fossa looks small and crowded, the cerebellum looks long and thin rather than round, and wrapped around the spinal cord (the so-called ‘Banana Sign on ultrasound). The cerebellar tonsils bulge (herniate) into the Foramen Magnum and spinal canal. This may block the flow of CSF around the brain and spinal cord, and lead to hydrocephalus.

Chiari II often progresses through the pregnancy, and may be reversed by prenatal surgery for open spina bifida.( Prenatal surgery, to close or cover a spina bifida lesion before birth, can sometimes be carried out in certain circumstances. There are significant risks for both the mother and fetus, the benefits are variable and impossible to predict, and the long term effects are unknown at the present time.)

Chiari II can result in either physical or cognitive (thinking, learning and behaving) effects. For information on cognitive effects, see information sheet Spina Bifida and Cognition.

Physical effects.

Many people with Chiari II experience few or no physical effects. The difficulties arise from the crowding together of the brainstem and the cerebellum.  Occasionally, newborn babies have problems with breathing or swallowing, and can need support such as ventilation and tube feeding. Reflux of stomach contents, and vomiting occur sometimes.  Chiari II can be associated with low muscle tone (floppiness) throughout the body, (for which body bracing and specialist seating may be needed), .It may also cause high muscle tone if there is significant compression. There may also be difficulty controlling body temperature

If there are persistent difficulties with swallowing, reflux and gut movement, feeding liquid food through a tube directly into the small bowel can help.

 A condition called syringomyelia can also be associated with Chiari II. In this condition, a cavity filled with CSF forms in the very centre of the spinal cord. This can create pressure on the spinal cord from the middle, affecting the way the nerves function.

As the brain and spinal cord are joined, tethered cord can lead to Chiari II symptoms developing in later childhood or adulthood. These symptoms can include

  • headaches (usually at the back of the head, made worse by bending over or by sneezing, coughing, or straining)  often called impulse headaches
  • neck pain
  • dizziness and balance problems
  • unusual feelings in the arms or legs (numbness or tingling)
  • muscle weakness and paralysis
  • problems with eye movements, such as nystagmus (involuntary rapid movements)
  • swallowing problems
  • hearing loss and tinnitus

(For more information on tethered cord, see Information sheet Tethered Cord 

Treatment for Chiari II

There are three main treatments, the Neurosurgeon will consider what is causing the symptoms to worsen to decide which treatment will be best.


If the Chiari II is being forced further downwards by raised intracranial pressure and blockage to the CSF flow, a shunt in the ventricles above the cerebellum can relieve the pressure.


If tethered cord is pulling downwards on the cerebellum, this can be relieved by detethering the spinal cord.


Decompression surgery involves removing a little of the bone in the skull near the cerebellum, to create a little  more space and relieve the crowding Tethered cord should be considered before decompression surgery.



Note-These diagrams illustrate the main features we would need to demonstrate on our own illustration. The small one at the top has clear labelling of the structures described in the introduction

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