CMs are classified by the severity of the disorder and the parts of the brain that protrude into the spinal canal.
Type I involves the extension of the cerebellar tonsils (the lower part of the cerebellum) into the foramen magnum, without involving the brain stem. Normally, only the spinal cord passes through this opening. Type I — which may not cause symptoms — is the most common form of CM and is usually first noticed in adolescence or adulthood, often by accident during an examination for another condition. Type I is the only type of CM that can be acquired.
Type II, also called classic CM, involves the extension of both cerebellar and brain stem tissue into the foramen magnum. Also, the cerebellar vermis (the nerve tissue that connects the two halves of the cerebellum) may be only partially complete or absent. Type II is usually accompanied by a myelomeningocele — a form of spina bifida that occurs when the spinal canal and backbone do not close before birth, causing the spinal cord and its protective membrane to protrude through a sac-like opening in the back. A myelomeningocele usually results in partial or complete paralysis of the area below the spinal opening. The term Arnold-Chiari malformation (named after two pioneering researchers) is specific to Type II malformations.
Type III is the most serious form of CM. The cerebellum and brain stem protrude, or herniate, through the foramen magnum and into the spinal cord. Part of the brain’s fourth ventricle, a cavity that connects the upper parts of the brain and circulates CSF, may also protrude through the hole and into the spinal cord. In rare instances, the herniated cerebellar tissue can cause an occipital encephalocele, a pouch-like structure that protrudes out of the back of the head or the neck and contains brain matter. The covering of the brain or spinal cord can also protrude through an abnormal opening in the back or skull. Type III causes severe neurological defects.
Type IV involves an incomplete or underdeveloped cerebellum — a condition known as cerebellar hypoplasia. In this rare form of CM, the cerebellar tonsils are located further down the spinal canal, parts of the cerebellum are missing, and portions of the skull and spinal cord may be visible.
Another form of the disorder, under debate by some scientists, is Type 0, in which there is no protrusion of the cerebellum through the foramen magnum but headache and other symptoms of CM are present.
Are other conditions associated with Chiari malformations?
Individuals who have a CM often have these related conditions:
Hydrocephalus is an excessive buildup of CSF in the brain. A CM can block the normal flow of this fluid, resulting in pressure within the head that can cause mental defects and/or an enlarged or misshapen skull. Severe hydrocephalus, if left untreated, can be fatal. The disorder can occur with any type of CM, but is most commonly associated with Type II.
Spina bifida is the incomplete development of the spinal cord and/or its protective covering. The bones around the spinal cord don’t form properly, leaving part of the cord exposed and resulting in partial or complete paralysis. Individuals with Type II CM usually have a myelomeningocele, a form of spina bifida in which the ...
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