Chiari Malformation

Chiari malformation is a congenital (i.e. present at birth) structural problem at the base of the skull that affects the brain and spinal cord. In fact it is not a malformation of the brain at all. More accurately, the lower portion of the skull, the portion known as the posterior fossa, is too small to accommodate the lower portion of the brain. As a result, a portion of the cerebellum called the tonsils is forced to protrude into the spinal column. This results in compression of the brain stem and spinal cord and interferes with the flow of spinal fluid. It is the compression of the brain stem and spinal cord that is primarily responsible for the symptoms. We have learned that compression that is sufficient to cause symptoms can occur even when the cerebellar tonsils protrude by as little as 1 millimeter.


Despite the fact that Chiari malformation is usually present at birth, the symptoms of the malformation are usually not experienced until adulthood, usually when the patient is in their 20s or 30s. Symptoms can develop sooner or later than this. Chiari malformation is often difficult to diagnose because the symptoms can be vague and so numerous that many doctors just can’t make sense of them.

The symptoms of Chiari malformation include:

  • Headache – This is the most common symptom of Chiari malformation. The headache is often pressure like and usually begins in the back of the head often radiates behind the eyes. Coughing, laughing, bending forward, sneezing or looking up can worsen the headache. Headache may be confused with migraine.
  • Neck Pain – often pressure like and radiating down the spine and across the shoulders.
  • Dizziness/vertigo – often worsened by extension of the neck
  • Vague pains throughout the body
  • Impaired balance
  • Clumsiness
  • Chronic nausea
  • Foggy thinking, poor memory and concentration
  • Frequent or urgent urination
  • Irritable bowel syndrome
  • Auditory Symptoms – ringing in the ears, decrease in the ability to hear or sensitivity to sounds
  • Difficulty swallowing
  • Changes in the voice – This can include hoarseness or the inability to regulate the voice when shouting or singing.
  • Visual Symptoms – including double vision, sensitivity to light, blind spots.

Types of Chiari Malformation (CM)

Type I involves the extension of the cerebellar tonsils (the lower part of the cerebellum) into the foramen magnum.There are usually no other associated malformations of the brain. This is the only form of Chiari malformation to present in adult life.

Type II involves the extension of both cerebellum and brainstem tissue into the foramen magnum. Type II is always accompanied by a myelomeningocele — a severe form of spina bifida- and therefore is recognized at birth. We do not treat Chiari II malformation.

Type III is the most serious form of CM. It is quite rare and involves the protrusion or herniation of the hind part of the brain through a defect in the skull. This diagnosis is also evident at birth. We do not treat Chiari III malformation.

Some people describe a Chiari 0 malformation. This is simply a Chiari I malformation in which the cerebellar tonsils do not protrude by the standard 5 mm. These are the patients that are frequently misdiagnosed because of the failure of the current diagnostic criteria to accurately reflect the underlying neurological problem. Many of our patients would fall into this category.

Testing and Diagnosis

The diagnostic process begins with a complete medical history and physical examination. These are followed by a complete neurological evaluation that will assess your specific symptoms and neurological functioning. The patient with a Chiari 1 malformation may have an entirely normal examination. In such cases, it becomes more difficult to determine if Chiari is indeed the problem and if surgery is indeed the best solution. In such cases, the judgment of an experienced neurosurgeon becomes very important. When the neurological examination is abnormal, the pattern of abnormal findings can make it very easy to pinpoint the problem to the cervical spinal cord and brain stem.

Patients may have difficulty with coordination, balance, eye movements, sensation and strength. The reflexes may be too active or not active enough. Reflexes that are never present except when the spinal cord is not functioning normally- pathological reflexes- may also be detected.

Magnetic Resonance Imaging (MRI) is best diagnostic tool for detecting Chiari malformation. This test provides the doctor with detailed anatomical information regarding the structures at the base of the brain. The criteria for diagnosing a Chiari malformation however have been very arbitrarily determined. This results in some doctors adhering to rigid standards – 5mm of tonsil herniation- before making the diagnosis of Chiari, even in the face of typical patient symptoms and obvious neurological findings. Hence many patients with Chiari go undiagnosed and untreated for years. We have developed some MRI imaging strategies that demonstrate the critical problem – brain stem compression- more clearly. These MRI techniques allow us to identify some Chiari patients who would otherwise go undiagnosed. Our imaging techniques involve the use of 3-D imaging of the brain and cine imaging – a movie of the brain- that allows us to determine if the brain stem is being compressed by the pulsating arteries that surround it. Examples of this imaging can be found on our web site.

Incidence of Chiari Malformation

The incidence of Chiari 1 malformation is not known because we have yet to fully understand the problem and arrived at a meaningful MRI standard for making the diagnosis If the 5mm standard of tonsil herniation is used, the incidence would seem to be less than 1% of the population. Brain stem compression due to tonsil herniation less than 5 mm – “Chiari 0”- is likely to be very much more common than that. For reasons not yet understood, Chiari malformation is much more common in women than in men. (3:1 women:men)


Some people with Chiari malformation have no symptoms and do not require treatment.

Some symptoms of Chiari malformation such as headache and dizziness may be effectively treated by medication. Medication alone, however, will not correct the root cause of the symptoms: the brain stem compression. As a rule, when the Chiari malformation causes symptoms that seriously impair the patients quality of life, it is best to consider surgery.

Surgery for Chiari malformation is basically about creating more room for the brain. Surgery relieves compression of the brain stem and spinal cord and improves circulation of the spinal fluid. 


View Chiari Operative Images
Please note: the following images are of a graphic nature.

View Chiari MRI Images

View Chiari Ultrasound Videos
Please note: the following video clips are of a graphic nature.

  • Video1 demonstrates an intra-operative ultrasound view showing the cerebellum and brain stem (side view). The colored pulsating structures are arteries which contribute to the compression of the brain stem.
  • Video2 demonstrates an intra-operative ultrasound (cross section view) showing the brain stem and cerebellar tonsils. The colored structures are the cerebral arteries which are embedded between the tonsils and the brain stem. The constant pulsation of the arteries is directed against the brain stem contributing to the compression.

View Chiari Surgical Videos
Please note: the following video clips are of a graphic nature.

  • Segment 1 demonstrates an intra-operative view taken through the operating microscope and showing the region of interest in a Chiari decompression surgery. An artery runs over the brain stem and under the cerebellar tonsils. This view is taken before the micro-surgical decompression. Note the pulsations of the artery.
  • Segment 2 demonstrates an intra-operative view taken through the operating microscope and showing the region of interest in a Chiari decompression surgery. An artery runs over the brain stem. The cerebellar tonsils have now been shrunk. The artery is now free and is no longer transmitting its pulsations to the brain stem. Note that the pulsations of the artery are now directed side to side rather than up and down (as in segment 1).


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