What is a Chiari Malformation?
The cerebellum is the portion of the brain located low down at the back of the head. It has two small areas at the bottom called the cerebellar ‘tonsils’. Normally, the cerebellum (and nearby brainstem) sits entirely within the skull.

In Chiari Malformations, there is a descent of the cerebellar tonsils (and sometimes the brain stem) through the hole on the base of the skull (the foramen magnum) and into the spinal canal. In other words, there is a herniation of the brain into the spinal canal.

What are the types of Chiari Malformations?
A number of types of Chiari Malformation have been described, with the most common being Chiari I and Chiari II.

Chiari I malformations are the most common type in adolescents and adults, and many are asymptomatic. In Chiari I malformations, the cerebellar tonsils have descended at least 4mm into the upper spinal canal.

Chiari II malformations cause problems in infancy and/or childhood. In addition to the cerebellar tonsils herniating into the spinal canal, the brainstem also descends partially through the foramen magnum. Chiari II malformations are frequently associated with spina bifida and/or hydrocephalus (a build-up of fluid within the brain).

What are the symptoms of Chiari Malformations?
This patient information guide deals with Chiari I malformations, the type seen in adolescents and adults.

Chiari malformations (type I) are generally congenital, but usually don’t cause problems until adolescence or adulthood. Typical patients are aged 20-50 years. Headaches (usually at the back but can be anywhere or everywhere) are the most common complaint, and neck pain is also frequently reported. These symptoms commence or worsen with coughing, sneezing, straining, or neck extension. They are also known as ‘impulse headaches’.

Patients frequently also have a larger than usual fluid-filled canal in the middle of their spinal cord (hydromyelia or syringomyelia). This may cause symptoms of spinal cord dysfunction, including hand and arm weakness and numbness. If left untreated, it can result in severe weakness and muscle loss in the hands, stiffness and rigidity (spasticity) of the legs, and problems walking.

Other symptoms may include poor balance, dizziness, diplopia (double vision), and repetitive downward eye movements (‘downbeat nystagmus’).

How is the diagnosis made?
Chiari malformations are suspected on the basis of the symptoms complained of and, sometimes, on abnormal findings on neurological examination. The diagnosis is confirmed by performing an MRI brain which reveals the abnormality and its extent.

What are the treatment options?
Asymptomatic Chiari I Malformations do not require any treatment.

Chiari malformations causing only headaches are initially treated with analgesia for pain control.

Surgery is reserved for patients with headaches which do not settle with analgesic medications, significant other symptoms, or the development of abnormal findings on neurological examination. The surgical procedure of choice is a posterior fossa (or Chiari) decompression

What is a Chiari decompression?
A Chiari decompression is a specific type of craniotomy designed to make more room for the herniated cerebellum, and to relieve pressure on the brain. It also may permit restoration of the normal flow of cerebrospinal fluid (CSF) round the brain and sometimes results in an improvement in any associated hydrocephalus, hydromyelia, or syringmyelia.

A Chiari decompression is performed under general anaesthesia Some hair at the back of your head is shaved, and the skin is cleaned with antiseptic solution. Antibiotics are given, and compression devices are used on your calves to reduce the risk of blood clot formation in your legs (deep venous thrombosis).

A midline incision is made at the back of your head, and extends down to the upper part of your neck. The incision is typically 5 or 6cm in length. The muscles attaching to the back of your skull and spinal bones are elevated. A small window of bone ( measuring around 2.5cm diameter is then removed from the base of your skull using a fine high-speed drill. This is part of the occipital bone and adjoining foramen magnum. This gives the cerebellum more room, and decompresses the brainstem.

Because the tonsillar herniation frequently extends through the spinal canal formed by the first neck bone (cervical vertebra, C1), the posterior arch of the C1 bone is also removed.

In many cases the above maneuvers are enough and nothing further is required. In some situations, however, such as when there is a tight band of tissue constricting the lining of the brain (dura), or where it is thought that there is scarring (adhesions) around the cerebellum and brain stem, the dura is opened and a patulous graft of tissue is sewn in place to create more room. If adhesions are found, they may be divided.

What is the prognosis?
The main aim of surgery is to prevent ongoing deterioration.

Most patients (around 80%) experience a significant improvement in their headaches and/or neck pain after surgery.