The cerebellum is the part 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.
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WHAT ARE THE TYPES OF CHIARI MALFORMATIONS?
A number of types of Chiari Malformation have been identified, with the most common being Chiari I and Chiari II.
Chiari I malformations are the most common type in adolescents and adults, and many do not have any symptoms. 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).
Chiari malformations (type I) are generally congenital, however they usually don’t cause problems until adolescence or adulthood. Typical patients are aged 20-50 years.
Headaches (usually at the back of the head but can be anywhere) are the most common complaint, and neck pain is also common. Coughing, sneezing, straining, or neck extension can all make these symptoms worse. These headaches are also known as ‘impulse headaches’.
Patients often have a larger than usual fluid-filled canal in the middle of their spinal cord (hydromyelia or syringomyelia). This may cause symptoms such as hand and arm weakness and numbness. If left untreated, it can result in severe weakness and muscle loss in the hands, stiffness and rigidity of the legs, and problems walking.
Other symptoms may include poor balance, dizziness, diplopia (double vision), and repetitive downward eye movements (‘downbeat nystagmus’).
Chiari malformations are suspected on the basis of the symptoms and, sometimes, on abnormal findings on neurological examination. The diagnosis is confirmed by performing an MRI brain scan.
Chiari I Malformations that are asymptomatic (do not have symptoms) do not need treatment.
Chiari malformations that only cause headaches are initially treated with analgesia for pain control.
Sometimes surgery is required. This is usually only suggested for patients who have headaches which do not settle with analgesic medications, or those who experience other significant symptoms.
The usual surgical procedure of choice is a posterior fossa (or Chiari) decompression
WHAT IS CHIARI DECOMPRESSION?
A Chiari decompression is a type of craniotomy which is designed to make more room for the herniated cerebellum. This in turn relieves pressure on the brain. It can also restore the normal flow of cerebrospinal fluid (CSF) around the brain.
A Chiari decompression is performed under general anaesthesia. Some hair at the back of the head is shaved, and the skin is cleaned with antiseptic solution. Antibiotics are given, and compression devices are used on the calves to reduce the risk of deep vein thrombosis.
A 5-6cm incision is made at the back of the head and down to the upper part of the neck. The muscles attaching to the back of the skull and spinal bones are elevated. A small window of bone (measuring around 2.5cm diameter) is then removed from the base of the 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 manoeuvres 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 graft of tissue is sewn in place to create more room. If adhesions are found, they may be divided.
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.