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Cranial Anatomy - Chiari Malformation

What is Chiari Malformation?

The Chiari (kee-ar-ee) malformations are congenital abnormalities of the posterior fossa (base of brain where the spinal column joins the skull). This usually causes a protrusion of the cerebellum through the bottom of the skull (foramen magnum) into the spinal canal. This results in a poor circulation of cerebrospinal fluid from the brain to the spinal cord.

Professor Hans Chiari was a German pathologist who first described these abnormalities of the brain at the junction of the skull with the spine in 1890. Another doctor, Arnold, later added to the type II description, hence the name “Arnold-Chiari Malformation.” Of the four classifications of Chiari, only two types of the malformation have practical importance, commonly referred to as Chiari type I and Chiari type II.

Type I Chiari malformation

The Chiari I malformations are a result of the smallest herniation of an otherwise normal hindbrain. The tonsils would normally be round but often become elongated as they protrude down the spinal canal. Diagnosis can be difficult because not all patients will have the classical sign of deeply herniated tonsils. The Chiari II is usually found in children with spina bifida or myelomeningocele. Not only is part of the cerebellum unusually low and lying below the bottom of the skull, but the brain stem can be malformed in several ways. This malformation is part of a much greater malformation of the nervous system. This type of Chiari malformation is correctly referred to as “Arnold-Chiari” malformation.

What symptoms can it cause?

Many people with Chiari I malformation have no symptoms. However, any of the following symptoms may occur, alone or in combination. Some of the symptoms are related to the development of a syrinx (a fluid filled cavity in the spinal cord).

  • Severe head and neck pain
  • An occipital headache felt at the base of the skull that is made worse by coughing, sneezing, or straining
  • Loss of pain and temperature sensation of the upper torso and arms (as a result of a syrinx)
  • Loss of muscle strength in the hands and arms (as a result of a syrinx)
  • Drop attacks – collapsing to the ground due to muscle weakness
  • Spasticity
  • Dizziness
  • Balance problems
  • Double or blurred vision
  • Hypersensitivity to bright lights

Type II Chiari malformation

This malformation is characterised by downward displacement of the medulla, fourth ventricle, and cerebellum into the cervical spinal canal, as well as elongation of the pons and fourth ventricle. This type occurs almost exclusively in patients with myelomeningocele. Myelomeningocele is a congenital condition in which the spinal cord and column do not close properly during fetal development, resulting in an open spinal cord defect at birth. Other abnormalities associated with myelomeningocele include hydrocephalus, cardiovascular abnormalities, imperforate anus as well as other gastrointestinal abnormalities, and genitourinary abnormalities.


The symptoms associated with a Chiari II malformation can also be caused by problems related to myelomeningocele and hydrocephalus. These symptoms include:

  • Alteration in the pattern of breathing, including periods of apnea (brief periods of cessation of breathing)
  • Depressed gag reflex
  • Involuntary, rapid, downward eye movements
  • Loss of arm strength

Type III Chiari malformation

This malformation includes a form of dysraphism with a portion of the cerebellum and/or brainstem pushing out through a defect in the back of the head or neck. These malformations are very rare and are associated with a high early mortality rate, or severe neurological deficits in patients that survive. If treatment is undertaken, then early operative closure of the defect is necessitated. Hydrocephalus, which is commonly present, must also be treated through shunting.

Additional severe birth defects are often present, which may require extensive treatment. Infants with Chiari III malformation may have life-threatening complications.

Type IV Chiari malformation

This malformation is the most severe form and the rarest. The cerebellum fails to develop normally. There may be other associated malformations of the brain and brainstem. Most babies born with this malformation do not survive infancy.

How is it evaluated?

There are several tests that can help diagnose and determine the extent of Chiari malformation and syringomyelia.

Brainstem auditory evoked potential (BAER): An electrical test to examine the function of the hearing apparatus and brainstem connections. This is used to determine if the brainstem is working properly.

Computed tomography scan (CT or CAT scan): A diagnostic test that creates an image by computer reconstruction of x-rays; it is particularly good at defining the size of the cerebral ventricles and showing an obvious blockage. It is less effective for analysis of the posterior fossa or the spinal cord.

Magnetic resonance imaging (MRI): A diagnostic test that produces three-dimensional images of body structures using magnetic fields and computer technology. It can provide an accurate view of the brain, cerebellum and the spinal cord, is very good at defining the extent of malformations, and distinguishing progression. The MRI provides more information than the CT scan when analyzing the back of the brain and spinal cord, and is usually the preferred test.

Myleogram: An x-ray of the spinal canal following injection of a contrast material into the CSF space; can show pressure on the spinal cord or nerves due to malformations. This test is performed less frequently now.

Somatosensory evoked potentials (SSEP): An electrical test of the nerves involved in sensation, which gives some information about peripheral nerve, spinal cord, and brain function.


Treatment of Chiari malformations and syringomyelia is very dependent on the exact type of malformation, as well as progression in anatomy changes or symptoms.

Chiari I malformations that are asymptomatic should be left alone. There is no indication for "prophylactic" surgery on these. If the malformation is defined as symptomatic, or is causing a syrinx, treatment is usually recommended.

Chiari II malformations are treated if the patient is symptomatic, and physicians have determined that there are no complications from hydrocephalus. In some patients, consideration of a tethered cord is also explored. In many infants who become symptomatic from a Chiari II malformation, the symptom onset and progression are severe and rapid, and this requires an urgent or emergency approach.



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