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Arnold-Chiari malformation

Arnold-Chiari malformation is a malformation of the brain. It consists of a downward displacement of the cerebellar tonsils and the medulla through the foramen magnum, sometimes causing hydrocephalus as a result of obstruction of CSF outflow.


Some sources use "Chiari malformation" to describe four specific grades of the condition, reserving the term "Arnold-Chiari" for type II only. Other sources use "Arnold-Chiari" for all four types. This article uses the latter convention.

One advantage of using "Arnold-Chiari" is that the term "Chiari's syndrome" can refer to Budd-Chiari syndrome, a hepatic condition also named for Hans Chiari.


The average age at diagnosis is about 34 and it is more common in women.

Some characteristics are visible prenatally.


The incidence of Arnold-Chiari Malformation (Chiari I malformation) defined as tonsilar herniations of 3 to 5 mm or greater is approximately 1 in 1,200. The incidence of symptomatic Chiari is less but unknown.

A prevalence of approximately 1 in 1000 has been described.

Average age at presentation is 41 years (range : 12 - 73 years). Slight female preponderance (f:m = 1,3:1). Average duration clearly related to Chiari malformation is 3,1 years (range 1 month to 20 years).

History and classification

The Austrian pathologist Hans Chiari in the late 1800s described seemingly related anomalies of the hindbrain, the so called Chiari malformations I, II and III. Later, other investigators added a fourth (Chiari IV) malformation. The scale of severity is rated I - IV, with IV being the most severe. Types III and IV are very rare.

  • The most common form of Arnold-Chiari Malformation is Type I, which is generally asymptomatic during childhood, but often manifests with headaches and cerebellar symptoms.
  • Type II is usually accompanied by a myelomeningocele leading to partial or complete paralysis below the spinal defect. Abnormal development of the cerebellar vermis and medulla occur, and they both descend into the foramen magnum. Hydrocephalus is also nearly always present.
  • Type III causes severe neurological defects. It is associated with an encephalocele.
  • Type IV involves a failure of brain development.

Other conditions sometimes associated with Chiari Malformation include hydrocephalus, syringomyelia, spinal curvature, and connective tissue disorders such as Ehlers-Danlos syndrome and Marfan Syndrome.


The brainstem, cranial nerves, and the lower portion of the cerebellum may be stretched or compressed. Therefore, any of the functions controlled by these areas may be affected. The blockage of Cerebro-Spinal Fluid (CSF) flow may also cause a syrinx to form, eventually leading to syringomyelia. Chiari is often associated with major headaches, sometimes mistaken for migraines. Chiari headaches usually include intense throbbing in the back of the head. Chiari also includes extreme muscle soreness and low energy levels. It also can cause insomnia cycles of sleep deprivation followed by inabilities to remain awake cycling between them. 15 - 30 % of patients with adult Chiari malfomation are asymptomatic.


Once symptomatic onset occurs, a common treatment is decompression surgery, in which a neurosurgeon usually removes the first and part of the second and sometimes third cervical vertebrae and part of the occipital bone of the skull to allow the flow of spinal fluid and may be accompanied by a shunt. This treatment is well recognized and accepted with many studies published (involving a total of hundreds of patients) in well-respected peer-reviewed medical journals showing that about 80% of patients obtain improvement.Some authors advocate performing a transoral clivus-odontoid resection in cases with ventral brain-stem compression, as they feel these patients may potentially deteriorate with posterior fossa decompression alone.

A small number of neurological surgeons believe that detethering the spinal cord as an alternate approach relieves the compression of the brain against the skull opening (foramen magnum), obviating the need for decompression surgery and associated trauma. However, this approach is significantly less documented in the medical literature, with reports on only a handful of patients. It should be noted that the alternative spinal surgery is also not without risk.


An Austrian pathologist, Hans Chiari, first described these hindbrain malformations in the 1890s. A colleague of Professor Chiari, Dr. Julius Arnold, later contributed to the definition of the condition, and students of Dr. Arnold (Schwalbe and Gredig) suggested the term "Arnold-Chiari malformation" to henceforth refer to the condition.

Some sources credit the characterization of the condition to Cleland or Cruveilhier.


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