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Synonyms of Chiari Malformations
- Arnold-Chiari Malformation (ACM)
- Hindbrain Herniation
- Tonsillar Ectopia
- Chiari type 0 (Chiari malformation 0)
- Chiari type I (Chiari malformation I)
- Chiari type II (Chiari malformation II)
- Chiari type III (Chiari malformation III)
- Chiari type IV (Chiari malformation IV)
Chiari malformations are a group of complex brain abnormalities that affect the area in lower posterior skull where the brain and spinal cord connect. The underlying anatomy of Chiari malformations is thought to be present at birth (congenital), although in many cases they may not become clinically apparent until adulthood. In extremely rare cases, a Chiari malformation may be acquired during life. The exact cause of Chiari malformations are not known, but often the cavity near the base of the skull (posterior fossa) is narrow and abnormally small in relation to the size of the cerebellum, which this portion of the skull encloses. Researchers believe that in some cases the small posterior fossa may cause the developing brain, specifically the cerebellum and the brainstem, to be pushed downward. Part of the cerebellum (known as the cerebellar tonsils) may protrude (herniate) through the foramen magnum, which is the normal opening found in the occipital bone at the base of the skull. The tonsils may thus interfere with the flow of cerebrospinal fluid (CSF) to and from the skull and spinal canal, potentially leading to accumulation of cerebral spinal fluid in the subarachnoid spaces of the brain and spine. A Chiari malformation can also cause pressure on the brain and produce hydrocephalus (pressure due to excessive cerebrospinal fluid accumulation in the brain) and the spinal cord, potentially causing a wide variety of symptoms. In fact, no two cases of Chiari malformation are exactly alike and the associated symptoms are highly variable. The severity of Chiari malformations can vary dramatically as well. In some cases, affected individuals may not develop any symptoms (asymptomatic); in others, severe, potentially debilitating or life-threatening symptoms can develop.
Traditionally, Chiari malformations have been defined and classified by how much of the cerebellar tonsils protrude through the foramen magnum. A diagnosis of a Chiari malformation usually signifies that the cerebellar tonsils protrude below the foramen magnum (often cited as at least 5 millimeters, though this is controversial). However, researchers have determined that the length of tonsil descent in a Chiari malformation does not always correspond to the severity of symptoms or to the response to treatment. In fact, some individuals are classified as having Chiari malformation type 0, in which there is minimal or no descent of the cerebellar tonsils. These individuals still have symptoms associated with a Chiari malformation, most likely due to abnormalities in the flow of cerebrospinal fluid within the skull and spinal canal. Research is ongoing to understand the complex, underlying mechanisms that cause Chiari malformations.
Chiari malformations are named for Hans Chiari, an Austrian pathologist, who first identified type I-III in 1891. Julius Arnold further expanded the definition of Chiari malformation type II and some medical sources began using the name Arnold-Chiari malformation. Nowadays, some medical sources use Arnold-Chiari malformation as a broad term for all forms. Chiari malformations have also been known as congenital tonsillar herniation, tonsillar ectopia or tonsillar descent.
Chiari Malformations Resources
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